process discovery mendeleev

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The process of discovery: Mendeleev and

the periodic lawDon C. Rawson

a

aIowa State University, Ames, Iowa, 50010, U.S.A.

Version of record first published: 22 Aug 2006.

To cite this article: Don C. Rawson (1974): The process of discovery: Mendeleev and the periodic law,Annals of Science, 31:3, 181-204

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A N N A L S O F S C I E N C EA N I N T E R N A T I O N A L R E V I E W O F T H E H I S T O R Y O F S C IE N C E

A N D T E C H N O L O G Y S I N C E T H E R E N A I S S A N C EVol . 31 M a y , 1 9 7 4 N o . 3

T H E P R O C E S S O F D I S C O V E I ~ u M E N D E L E E V A N D T H EP E R I O D I C L A W

B y D o N C . R A w s o N *CONTENTS

PAGE1. INTRODUCTION 1812. PRESENTIMENTS AND FALLACIES: MENDELEEV'S PI~-CURSORS 1833. ALGEBRAIC PROGRESSIONS AND ' PRIMARY MATTER ' 1874. EVALUATION OF MENDELEEV'S PRECURSORS 1905. MENDELEEV'S INTEREST IN THE PROBLEM 1906. RECOGNITION OF THE LAW 1947. CONFIRMATION 203

1. IntroductionI N t h e c e n t u r y t h a t h a s p a s s e d s i n ce D . I . M e n d e l e e v a n n o u n c e d h isd i s c o v e r y o f t h e P e r i o d ic L a w o f t h e E l e m e n t s i n 1 86 9, c h e m i s t s a n dh i s t o ri a n s o f sc i e n ce al ik e h a v e a c k n o w l e d g e d h is w o r k i n c h e m i c a l c la s si -f i c a ti o n a s a f u n d a m e n t a l c o n t r i b u t i o n t o o u r s t o r e o f s c i en t if i c k n o w l e d g e .H o w e v e r , t h o s e w h o h a v e w r i t te n a b o u t t h e m e t h o d o f h is d i s c o v e r y h a v eu s u a l l y o n l y b r i e fl y a n d in c o m p l e t e l y e x p l a i n ed i t, w i t h o u t a d e q u a t e l yr e la t in g h is w o r k t o t h a t o f o t h e r c h e m i s ts w h o p r e c e d e d h i m a n d w i t h o u tt r a c i n g e x p l i c it l y t h e s t e p s b y w h i c h M e n d e l e e v a r r iv e d a t h i s c o n c lu s i o n s.T h e p u b l i c a t i o n w i t h i n t h e p a s t f e w y e a r s o f m a n y a r c h iv a l m a t e r ia l sd e a l i n g w i t h M e n d e l e e v ' s w o r k h a s p r o v i d e d n e w i n f o r m a t i o n , w h i c h i se x t r e m e l y u s e f u l in e x p l a in i n g t h e m e t h o d o f h i s d i s c o v e r y , b o t h i nr e g a r d t o t h e e x t e n t t o w h i c h h e r el ie d o n t h e a c c o m p l i s h m e n t s o f h isp r e c u r so r s a n d t h e p e n e t r a t i n g i n q u i r y o f h is o w n .

A g e n e r a l m e t h o d o f s c ie n t if ic d i s c o v e r y i s n o t e a s i l y d ef in e d . O no c c a s i o n , s u d d e n i n t u i t i o n m a y e n a b l e a s c i e n t i s t t o d i s c e r n r e l a t i o n s h i p si n d a t a o v e r w h i c h h e h a s l a b o r i o u s l y p u z z l ed . U s u a l l y , h o w e v e r ,t h e d i s c o v e r y o f s o m e t h i n g s o f u n d a m e n t a l a s a n a t u r a l l aw i s a n a c c u m -u l a t i v e p r o c e s s . I n M e n d e l e e v ' s c a se t h e e v i d e n c e i n d i c a t e s t h a t h i sd i s c o v e r y o f t h e P e r i o d ic L a w f o l lo w e d t h e c o u r se t h a t h e h i m s e l f l a t e rd e s c r i b e d a s t h e u s u a l p r o c e s s in t h e d i s c o v e r y o f a n y n a t u r a l l a w : ( 1)presentiments t h a t t h e l a w e x i s t s; ( 2) recognition o f t h e s i g n i fi c a n c e o f

*Assis tant Professor of His to ry, Io wa Sta te Univers i ty , Ames, I owa 50010, U.S.A.A.S. N


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1 8 2 D o n C . R a w s o nt h e l a w ; a n d ( 3 ) confirmation o f t h e l a w b y e x p e r i m e n t a t i o n . 1 I f o n ea c c e p t s t h e v a l i d i t y o f t h i s s e q u e n ce , h e m a y a l so a g r e e th a t a l th o u g ha l l t h r e e s t e p s a r e e s s e n t i a l, t h e s e c o n d - - t h e r e c o g n i t i o n o f t h e s ig n if i-c a n c e o f t h e l a w - - i s p r o b a b l y t h e m o s t c r u c i a l, f o r t h i s i s t h e ' b r e a k -t h r o u g h ' f r o m s i m p l y a c c u m u l a t i n g a n d a r r a n g in g p a r t i c u la r d a t a t od e r i v i n g g e n e r a l i z a t i o n s .F o r t h e h a l f - c e n t u r y p r i o r t o M e n d e l e e v ' s w o r k , v a r i o u s c h e m i s t sh e l d th e p r e s e n t i m e n t t h a t t h e e l e m e n t s w e r e r e la t e d m a t h e m a t i c a l ly ,p r o b a b l y a c co r d in g to th e i r p r o p e r t i e s a n d a t o m i c w e i g h ts . M e n d e l e e va c k n o w l e d g e d t h a t a t t h e t i m e h e b e g a n w o r k i n g o n a s y s t e m o f t h ee l e m e n t s h e w a s f a m i l ia r w i t h t h e w o r k o f s e v e r a l o f t h e s e c h e m i s t s ,n a m e l y , G l a d s t o n e , K r e m e r s , L e n s s e n , D u m a s , a n d P e t t c n k o f e r ; a n d h er e m a r k e d t h a t t h e ir i n v e s ti g a ti o n s e n c o u r a g e d h i m to s e e k t h e la wg o v e r n i n g t h e r e l a t io n s h i p o f t h e e l e m e n t s . ~ T h e q u e s t i o n t h e n a r is e s :w h y w a s i t M e n d e l e e v a n d n o t t h e y w h o r e co g n i z e d a n d e x p l i c a te d t h el a w u n d e r l y i n g t h e s e ch e m i c a l r e l a ti o n s h i p s? T h e re a s o n s a r e n o d o u b tm a n i f o ld , b u t t w o s t a n d f o r e m o s t .F i r s t , a s M e n d e l e e v ' s p r e c u r s o r s t h e o r i z e d o n t h e p r o b l e m o f s y s t e m a t i -z in g th e e l e m e n t s , t h e y o f t e n b e c a m e o v e r l y in t r ig u e d b y a p p a r e n t , b u tm i s l e a d i n g , n u m e r i c a l r e l a t i o n s h i p s , w h i c h i m p e d e d t h e i r s e a r c h f o rt h e a c t u a l u n i f$ i n g p ri n c ip l e . I n p a r t i c u l a r , t h e y w e r e d r a w n to t h en o t io n t h a t t h e a t o m i c w e i g h t s o f a n a l o g o u s e l e m e n t s i n c r e a s ed b y r e g u l a ri n c r e m e n t s , o f te n f o rm i n g ' t r i a d s ' o f e l e m e n t s. I n s o m e i n s ta n c e st h e y a l s o b e l i e v e d t h a t t h e i r n u m e r i c a l r e l a t io n s h i p s p r o v i d e d e v i d e n c et h a t a ' p r i m a r y m a t t e r ' s e r v e d a s t h e b a s i c s t r u c t u r a l u n i t f o r i n c r e a s in g l yc o m p l e x e l e m e n t s. A l t h o u gh M e n d e l e e v p r o f it e d f r o m t h e p r e s e n t i m e n t st h a t t h e e l e m e n t s w e r e r e l a t e d a c c o r d i n g t o t h e i r a t o m i c w e i g h t s , h e d i dn o t b e c o m e b o u n d t o f a u l t y a s s u m p t i o n s i n v o l v in g i n g e n io u s n u m e r i c a lr e l a t i o n s h i p s .S e c o n d , b y t h e l a t e 1 8 6 0 s, M e n d e l e e v h a d a t h i s d i s p o s a l m o r e a c c u r a t ev a l u e s f o r m a n y a t o m i c w e i g h t s t h a n w e r e a v a i l a b l e e v e n a d e c a d ee a r li e r a n d , m o r e i m p o r t a n t , a c l e a r e r d e f i n it i o n o f a t o m i c w e i g h t s , w h i c hh a d b e e n c o m m o n l y c o n f u s e d w i t h e q u i v a l e n t w e i g h ts . T h u s f re e f r o ms e v e r a l b a s i c m i s c o n c e p t i o n s a n d u t i l i z i n g s o u n d e r d a t a , h e p r o c e e d e df r o m t h o s e presentiments h e s h a r e d w i t h h i s p r e c u r s o r s t o a s t e p - b y - s t e pc o m p a r i s o n n o t o n l y o f analogous e l e m e n t s w i t h i n g r o u p s ( su c h a s t h ea l k a li m e t a ls ) , a s h is p r e c u r s o r s h a d d o n e ; b u t a l so o f dissimilar e l e m e n t s ,a n i n q u i r y t h e o t h e r s h a d o n l y t o u c h e d u p o n . T h r o u g h t h is c r e a ti v ep r o c e s s M e n d e l e e v f in a l ly r e l a t e d a ll o f t h e e l e m e n t s i n a s y s t e m , i n w h i c h

1 D. I. Mendeleev, Periodicheskii zakon: Osnovnye stat'i, Moscow, 1958, pp. 314-315,reprinted from Chapter 15 of Oenovy khimii , 8th edition.Zhurnal russkago fiziko-khimicheskago obshchestva, 1869, It 60-77; Mendeleev, Perio.dicheskii zakon, p. 314.


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T h e P r o c es s o f D i s c o v e r y : M e n d e l ee v a n d t he P e r i o d ic L a w 183t h e i r p r o p e r t i e s a r e a p e r i o d i c f u n c t i o n o f t h e i r a t o m i c w e i g h t s . T h i sw a s t h e p o i n t o f recogni t ion , a n d N I e n d el ee v p r o m p t l y a n n o u n c e d t h ep r i n c ip l e o f p e r i o d i c i t y a s a g e n u i n e l a w o f n a t u r e . 3 C o n f i r m a t i o n o ft h e l a w c a m e l a t e r t h r o u g h t h e e x p e r i m e n t a t i o n o f o t h e r c h e m i s t s .2 . Pre se n t ime n t s and Fa l lac i e s: Mend e l eev ' s Precur so r s

T h e i n t r ig u e o f n u m b e r s , w h i c h c a p t i v a t e d M e n d e l e e v ' s p r e c u rs o r s ,a p p e a r e d i n s e v e r a l p e r s i s t e n t c o n c e p t s b u t p r o b a b l y m o s t s t r i k i n g l yi n t h e c o m p e l li n g n o t i o n o f t h e ' t r i a d s '. F i r s t p o s t u l a t e d b y J o h a n nW o l f g a n g D S b e r e i n e r , a p i o n e e r in s t o ic h i o m e t r i c s t u d i e s a t t h e U n i v e r s i t yo f J e n a , i n 1 8 17 , i t a w a i t e d e l a b o r a t i o n a g e n e r a t i o n l a t e r b y J . H . G l a d -s t on e , P e t e r K r e m e r s , a n d E r n s t L e n s s en . I n e s se n c e i t s t a t e d t h a ti b r v a r io u s t h r e e - m e m b e r g r o u p s o f a n a l o g o u s e l em e n t s , t h e a t o m i c(o r e q u i v a le n t ) w e i g h t o f t h e m i d d le m e m b e r i s t h e m e a n o f t h e o t h e r t w o .I n i ti a l ly , D S b e r e i n e r s u g g e s t e d t h i s re l a t i o n s h i p f o r c a l c i u m - s t r o n t i u m -b a r i u m ( 2 7 . 5 -5 0 - 7 2 . 6 ); 4 t h e n , i n 1 8 2 9, h a v i n g a c c u m u l a t e d m o r e d a t a ,h e a d d e d c h l o r in e - b r o m i n e - i o d in e ( 3 5 -4 7 - 7 8 .3 8 - 1 2 6 .4 7 ) , l i t h i u m - s o d i u m -p o t a s s i u m ( 1 5 . 2 5 - 4 6 .5 4 - 7 8 . 3 9 ), a n d s u l p h u r - s e l e n i u m - t e l l u r i u m ( 3 2 . 2 4 -7 9 . 26 - 1 2 9. 2 4) . A l t h o u g h t h e w e i g h t s o f t h e m i d d l e m e m b e r s o n l ya p p r o x i m a t e d t h e t h e o r e t ic a l m e a n s, D 5 b e r e i n e r w a s s o s t r u c k b y th e s er e l a t io n s h i p s t h a t h e c o n c l u d e d t h e y r e f l e c te d a g e n e r a l p r in c i p l e o fn a t u r e - a p r i n c ip l e , w h i c h , h o w e v e r , h e c o u l d n o t y e t i d e n t i fy . 5N o r c o u l d o t h e r s w h o l a t e r p u r s u e d t h e n o t i o n o f t h e t r i a d s p r o v i d ea p r i n c i p le t o e x p l a i n t h e s e e m i n g l y s i g n if ic a n t n u m e r i c a l r e la t i o n s h i p st h e y d e t e c t e d . T h i s w a s t r u e o f J o h n H a l l G l a d s t o n e , l e c t u r e r i n c h e m i s t r y

a I t s h o u l d b e a c k a o w l e d g e d t h a t a t a p p r o x i m a t e l y t h e t i m e o f M e n d e l e ev ' s d i s c o v e rys e v e r a l o t h e r c h e m i s t s - - A . E . B ~ g u y e r d e C h a n e o u r t o i s , J . A . 1%. N e w l a n d s , W i l l ia mO d l i n g, G u s t a v u s I- Ii nr ic h s, a n d L o t h a r M e y e r - - w e r e w o r k i n g i n d e p e n d e n t l y o n s y s t e m so f c l a ss i f i c a ti o n i n v o l v i n g p e ri o d i ci t y . A l t h o u g h t h e s e m e n e x h i b i t e d c o n s i d e r a b l e o ri g in a -l i ty a n d i n s i g h t , n o n e o f t h e m s e e m s t o h a v e r e c o g n i z e d t h e s i g n if i ca n c e o f t h e p r i n c i p let o w a r d w h i c h h i s i n v e s t i g a t i o n w a s l e a d i n g a s c o m p l e t e l y a s d id M e n d e l e e v . I n a n yc a s e, i t i s n o t m y p u r p o s e t o a r g u e t h e q u e s t i o n o f p r e c e d e n c e i n th e d i s c o v e r y o f t h eP e r i o d ic L a w n o r t o d i m i n i s h t h e i m p o r t a n c e o f M e n d e l e e v ' s c o n t e m p o r a r i e s , b u t t o e x p l a i nt h e p r o c es s o f M e n d e l e e v ' s o w n i n v e s ti g a t i o n . S i nc e M e n d e l e e v w a s u n a c q u a i n t e d w i t ht h e w o r k o f t h e s e m e n a t t h e t i m e o f h i s d i s c o v e r y , I h a v e l i m i t e d t h e p r e s e n t s t u d y t oc h e m i s t s w h o c o n t r i b u t e d t o t h e p a r t i c u la r c o n t e x t i n w h i c h M e n d e l e e v p u r s u e d h i si n q u i r y , t h a t i s, G l a d s t o n e , K r e m e r s , L e n s s e n , D u m a s , a n d P e t t e n k o f e r . F o r t h e m o s tr e c e n t s t u d i e s o n t h e g e n e r a l q u e s t i o n o f c o n t r i b u t o r s t o c h e m i c a l c l a s s i fi c a t i o n a n d t h ep a r t i c u l a r q u e s t i o n o f p r e c e d e n c e , s e e J . W . v a n S p r o n s o n , The Periodic Syste~r* of ChemicalElements: A History of the First Hundred Years, A m s t e r d a m , 1 96 9; a n d H e i n z C a s s e b a u ma n d G e o r g e B . K a u f f m a n , ' T h e P e r i o d i c S y s t e m o f C h e m i c a l E l e m e n t s : T h e S e a r c h f o rI t s D i s c o v e r e r ' , Isis, 1 9 7 1 , 6 2 , 3 1 4 - 3 2 7 .4 An n. der Phys. (Gilbert), 1 8 1 7 , 5 6 , 3 3 1 - 3 3 4 .5Ann. der Phys. und Chemic (Poggendorff), 1 8 29 , 1 5 , 3 0 1 - 3 0 7 . D 6 b e r e i n e r ' s o c c a s i o n a ld o u b l i n g o f e q u i v a l e n t w e i g h t s , e. g . l i t h i u m - s o d i u m - p o t a s s i u m , r e fl e c ts t h e u n c e r t a i n t yo v e r c h e m i c a l c o m b i n a t i o n s ti ll e x t a n t a m o n g c h e m i s t s a t t h i s t im e .


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1 84 D o n C . R a w s o na t S t . T h o m a s H o s p i t a l in L o n d o n a n d F e l lo w o f t h e R o y a l S o c i e t y ,w h o r e p e a t e d D 6 b e r e i n e r ' s g r o u p i n g i n 1 8 53 , a l t h o u g h w i t h m o r e a c c u -r a t e l y d e t e r m i n e d w e i g h t s a n d a f i r m e r d e f ir d t i o n o f e q u i v a l e n c y : 6

L i = 6 -5 C a = 2 0N a = 2 3 S r = 4 3 . 8K = 3 9 .2 B a = 6 8 . 5C1 = 3 5 . 5 S = 1 6B r = 8 0 S e = 3 9 . 5I = 1 2 7 . 1 T e - - 6 4 . 2

T h e s a m e d e f i c i e n c y a p p e a r e d w h e n P e t e r K r e m e r s o f K 6 1 n p u b l i sh e da n e x t e n d e d t a b l e o f t r i a d s i n 1 85 6: ~

L i = 6 . 5 S = 1 6N a = 2 3 C r = 2 6 . 7K - - 3 9 " 2 S e ~ - 3 9 . 5C a - - 2 0 S = 1 6S r = 4 3 .8 S e = 3 9 . 5B a - - 6 8 .5 T e = 6 4 . 2H g ~- 1 0 0 C r = 2 6 . 7P b = 1 0 3 .7 M o = 4 6A g - - 1 0 8 . 1 V = 6 8 " 6C1 = 3 5 . 5 P = 3 1B r - - 8 0 A s = 7 5I = 1 2 7 . 1 S b - - 1 2 0 . 3

a d do w e v e r , K r e m e r s d i d a s i g n if ic a n t c o m p o n e n t t o t h e w h o l e q u e s t i o no f r e l a t i o n s h i p s , w h e n h e s u g g e s t e d a b i - d i r e c t i o n a l s c h e m e o f ' c o n j u -g a t e d t r i a d s ' , i n w h i c h c e r t a i n e l e m e n t s s e r v e a s m e m b e r s o f t w o t r i a d s .F o r e x a m p l e :

L i = 6 .5 N a = 2 3 K = 3 9 .2M g = 1 2 Z n = 3 2 .6 C d = 5 6C a = 2 0 S r = 4 3 . 8 B a = 6 8 . 5

B o t h h o r i z o n t i a l l y a n d v e r t i c a l ly , K r e m e r s o b s e r v e d , t h e w e i g h t o f t h em i d d l e m e m b e r is a p p r o x i m a t e l y th e m e a n o f t h e o t h e r tw o . s A l t h o u g hK r e m e r s a p p a r e n t l y d i d n o t a p p r e c i a t e t h e i m p o r t a n c e o f t h i s s te p ,h e h a d b r o k e n n e w g r o u n d b y c o m p a r i n g n o t o n l y analogous e l e m e n t sw i t h i n i n d e p e n d e n t g r o u p s , b u t a l so g r o u p s o f dissimilar e l e m e n t s . T h i sb i - d i re c t io n a l m o d e o f c o m p a r i s o n l a t e r p r o v e d t o b e o n e o f t h e k e y s t o~ e n d e l e e v ' s r e c og n i t i o n o f t h e P e r i o d ic L a w .

Phil. Mag. and Jour. Sci., 1853, (Ser 4) , 5 , 314-320.Ann. der Phys. und Chem. (Poggendor~), 1856, 99 , 58-63 .8 Ibid., 1857, 100 , 261-270.


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T h e P r o c e ss o f D i s c o v e r y : M e n d e l e e v a n d th e P e r i o d i c L a w 1 8 5M e n d e l e e v a l so p r o f i t e d f r o m t h e i n v e s t i g a t i o n o f t h e t r i a d s b y a n o t h e r

c h e m i s t w h o j o i l le d t h e s e a r c h f o r n u m e r i c a l r e l a t i o n s h i p s - - E r n s tL e n s s e n , a y o u n g a s s i s t a n t t o P r o f e s s o r K a r l R e i n i n g / u s F r e s e n i u s a tW i e s b a d e n . L e n s s e n e x t e n d e d t h e n o t i o n o f t h e t r i a d s t o i n c lu d e a llo f t h e k n o w n e l e m e n t s b y d i v i d in g t h e m i n to t w e n t y t r i a d s , a c c o r d in gt o t h e i r m e t a l li c o r n o n - m e t a l l i c p r o p e r t i e s , p la c i n g h y d r o g e n s e p a r a t e l ya n d m e r c u r y i n e a c h o f t w o t r i a d s , o n e m e t a l l i c a n d o n e n o n - m e t a l l i c 2

r H ,1

1 . L i N a K 1 1 .6 . 9 5 2 3 . 0 3 9 . 1 1( a i + K ) / 2 = 2 3 - 03

2 . C a S r B a 1 2 .2 0 4 3 . 6 7 6 8 . 5 9( C a + B a ) / 2 = 4 4 . 2 9

3 . M g Z n - C d 1 3.1 2 3 2 . 5 5 5 . 7( M g + C d ) / 2 = 3 3 . 8

4 . M n F e - C o 1 4.2 7 . 5 2 8 2 9 . 5( M n + C o ) / 2 = 2 8 . 55 . L a C e D i 1 5.4 7 4 7 . 3 4 9 . 6( L a + D i ) / 2 = 4 8 . 3

6. Y - - - - - E b T b 16 .32 .2 ? ?

7 . A1 -No= T h 17 .1 3 . 7 ? 5 9 . 5(A 1 + T h ) / 2 = 3 6 . 6

8 . B e Z r U 1 8 .7 3 3 . 6 6 0( B e + U ) / 2 = 3 3 . 5

9 . C r N i C u 1 9 .2 6 . 8 2 9 . 6 3 1 . 7( C r + C u ) / 2 = 2 9 . 3

1 0. H g P b A g 2 0.1 0 0 1 0 3 . 6 1 0 8( H g + A g ) / 2 = 1 0 4

9 Ann. der Chem. und P harm. , 1857, 103, 121-131.

C - N O6 7 8( C + 0 ) / 2 = 7F B S i9"5 11 15( F + S i ) / 2 = 1 2 " 2C 1 B r I1 7 . 7 4 0 6 3 . 5(C 1 + I ) / 2 = 4 0 . 6S - S e T e1 6 3 9 . 7 6 4 - 2( S + T e ) / 2 - - 4 0 .1P A s S b1 6 3 7 . 5 6 0( P + S b ) / 2 = 3 8T i S n T a2 5 5 9 9 2 . 3( T i + T a ) / 2 = 5 8 . 7M o V W4 6 6 8 . 5 9 2( M o + W ) / 2 = 6 9R h R u P d5 1 . 2 5 2 " 1 5 3 " 2( R h + P d ) / 2 = 5 2 .2I r - - - - P t O s9 8 . 5 9 9 9 9 . 4( I r + O s ) / 2 = 9 8 . 9A u H g B i9 8 . 4 1 0 0 1 0 4( A u + B i ) / 2 = 1 0 1 . 2


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1 8 6 D o n C . R a w s o nI n a d d it io n 1 L e n s s e n c o m b i n e d t h e s e t r i a d s i n t o g r o u p s o f t h r e e t r i a d s

e a c h ( h y d r o g e n s e r v in g a s a ' t r i a d ') , t h e m e a n e q u i v a l e n t w e i g h ts o fe a c h m i d d l e t r ia d l y in g a p p r o x i m a t e l y h a l f w a y b e t w e e n t h e m e a nw e i g h t s o f t h e o t h e r t w o t r i a d s i n t h e g r o u p .

T r i a d M e a n e q u i v a l e n tw e i g h t

1 2 33 3 3 ( 2 3 + 4 4 ) / 2 = 3 3 . 52 4 44 2 86 ?5 4 79 2 9 . 58 3 3 . 5 ( 2 9 . 5 + 3 7 ) / 2 = 3 3 . 37 3 7H 1

11 7 (1 + 12 ) /2 = 6.512 121 5 3 81 4 4 0 ( 3 8 + 4 0 ) / 2 = 3 91 3 4 018 52 .11 6 6 1 ( 5 2 . 1 + 6 9 ) / 2 = 6 0 . 617 6919 992 0 1 0 1 ( 9 9 + 1 0 4 ) / 2 = 1 0 1 . 51 0 1 0 4

S e v e r a l o f t h e t r i a d s in L e n s s e n ' s s c h e m e w e r e t h o s e p r e v i o u s l y u s e db y D S b e r e i n e r , G l a d s t o n e , a n d K r e m c r s ; t h e o t h e r s L e n s s e n a d d e dh i m s e lf . I n s o m e c a s e s t h e y d i d n o t f i t t o g e t h e r a d e q u a t e l y , e i t h e ra c c o r d i n g t o t h e i r e q u i v a l e n t w e i g h t s o r t h e i r c h e m i c a l p r o p e r t ie s .F o r e x a m p l e , L e n s s e n p l a c e d f lu o r in e w i t h b o r o n a n d s il ic o n ( G r o u p1 2) , w h e r e in a t o m i c w e i g h t i t c o r r e s p o n d e d o n l y r o u g h l y a n d i n p r o p e r -t ie s a o t a t a ll . E v i d e n t l y L e n s s e n r e a li z e d t h a t s o m e o f h is t r i a d s d idn o t d i s p l a y v e r y p re c i se m a t h e m a t i c a l r e l a t i o n s h i p s , a n d h e s u g g e s t e dt h a t t h e s e m i g h t a c t u a l l y b e ' d i a d s ' , e a c h w i t h a n ' a t t a c h e d ' m e m b e r .T h i s , h o w e v e r , p r o v i d e d n o r e a l e x p l a n a t i o n f o r t h e d i s c re p a n c i e s , e i t h e ri n e q u i v a l e n t w e i g h ts o r in p r o p e r t ie s . M o r e o v e r , L e n s s e n ' s s y s t e m le f tn o r o o m fo r u n d i s c o v e r e d e l e m e n t s . W h a t L e n s s e n d i d c o n t r i b u t e w a s a n


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The Process of Discovery: Mendeleev and the Periodic Law 187att emp t to include all of the known elements in a single system; wh at helacked for his system, as 1Vfendeleev later pointed out, was an underlyingprinciple.3. Algebraic Progressions and ' Primary Matter '

In 1857, Jean-Baptiste Dmnas, the prominent ~'rench chemist,who for years dominated scientific life at the Sorbonne, added a newtheoretical comp onen t to the search for a system of the elements, althoughit led no closer to a principle. In a len gth y exposition, Du ma s reject edthe notion of triads, even to the limited exte nt t hat Gladston e hademplo yed it, devising instead a complex algebraic set of increments,which he insisted could account for the relationship of the elements.Prom pted by the homologous series apparent in organic compounds,whose members could be related algebraically, Dumas proposed thefollowing sche me: x0

F = 19 = 19 aC1 = 35- 5=1 9+1 6-5 a4 dBr = 80 =19+ 33 428 a + 2d+d 'I = 1 2 7 = 1 9 + 3 3 + 5 6 4 1 9 a + 2 d 4 2 d ' + d "N = 14 = 14 aP = 31 =1 4+ 17 a+ dAs = 75 =14 +17 444 a4 d4 d 'Sb =119 =14+ 17+88 a 4 d+2d 'B i =207 =14+ 17417 6 a 4 d+4d 'C = 6 = 6 aB = l l =6 4 5 a+ dSi = 21 =6 41 5 a4 3dZr = 66 =6+ 60 adl 2d0 = 8 = 8 aS = 16 = 8+ 8 a+ dSe = 40 =8 +3 2 a+ 4dTe = 64 =8 +5 6 a4 7dMg = 12 = 12 aCa = 20 =1 2+ 8 a+ dSr = 44 =1 2+ 32 a+ 4dBa = 68 = 12 45 6 a+ 7dPb =104 =24+8 0 2a +10 d 11

lo Comptes Rend us, 1857, 45, 709-731, expanded and somewhat modified by Dumasin Ann. de chem. et de phys., 1859, 105, 129-210.11 In order to hav e avoided the an omalous 2a for lead, Du mas would need to huvecalculated lead as 12+ 80 + 12, or a+ 10d+ d' ; bu t d' first app eari ng for the .fifth element ofthe group 8nd equal to a would have been as anomalous as the 2a. Eviden tly lead didnot fi t properly into Dumas's algebraic scheme.


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188 Don C. RawsonDumas also showed certain numerical relationships between dissimilar

elements, a comparison thus far touched on only by Kremers:Increment

N = 14 F ~ 19 5P = 31 C1 = 35"5 5As = 75 Br = 80 5Sb =122 I = 127 5o = 8 M g = 1 2 4S =1 6 Ca = 20 4Se =39.75 Sr = 43.75 4Te =64 .5 Ba = 68.5 4Os =99"5 Pb = 103.5 4

In these tables Dumas no t o nly stated complex numerical relationshipsbut also implied a convertibility of the elements. For example, accordingto his equations, chlorine may actually be composed of fluorine--thebasic member of the halogen fam ily --to which has been added a qua ntit yof atomic material; bromine and iodine result from additional incrementsof the material. For several years, Dumas had alread y hinted at thepossibility of chemical convertibility, a notion he derived from Prout'shypothesis, first advanced in 1815. Using rather inadequate atomicweight determinations, Prout had attempted to show that the atomicweights of the various elements are exact multiples of the weight ofhydrogen, his hypothesis being that these elements are all compositesof hydro gen--t he ' primary matter 'While many chemists found a certain appeal in Prout's hypothesis,it was not generally accepted. As for Dumas , he began to give credenceto P rou t in 1840 while engaged in exhaustive atomic weight determ inati onsof carbon, in which he found its empirical weight to be an exact multipleof tha t of hydrogen (H = 1.00; C = 12.00). Upon examin ing the acceptedweights of other elements, Dumas observed that although not all wereexact multiples of hyd rog en --t ha t is, whole numb ers --m any of the excep-tions were strikingly close to being either quarter- or half-numbers. Inhis articles on numerical relationships, he listed twenty-two elementsos havi ng atomic weights of whole numbe rs (Br--80 , Ag=108 , etc.),eight as half-numbers (Mg=27.5, C1=35.5, etc), and five as quarter-numbe rs (A1--13.75, Zn=3 2.75, etc.). This amo unt ed to an extensionof Prout's hypothesis, the weights being multiples of either the whole,half-, or quarter-weight of hydrogen. Thus, although Dumas rejectedthe notion of triads, he revived another fascinating but unsu bstant iatedconcept in his intricate assumption of a ' primary matte r '


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The Process of Discovery: Mendeleev and the Periodic Law 1 8 9D u m a s b y n o m e a n s s e t t l e d t h e q u e s t io n o f t h e r e l a t i o n s h i p o f t h e

e l e m e n t s . H i s sp e c u l a t i o n s b r o u g h t a s w i f t r e s p o n s e f r o m M a x y o nP e t t e n k o f e r i n M u n i c h , w h o l a t e r g a i n e d i n t e r n a t i o n a l r e n o w n f o r h i sa p p l i c a t i o n o f c h e m i s t r y t o t h e f i el d o f h y g i e n e b u t w h o a t t h i s t i m e w a sc o n c e r n e d w i t h t h e t h e o r e t i c a l p r o b l e m o f c h e m i c a l c o m b i n a t i o n . 1=L i k e D u m a s , h e r e j e c te d t h e n o t i o n o f t r ia d s , s t a t i n g t h a t t h e n u m e r i c a lr e l a t io n s h i p s f o u n d i n t h e m w e r e o n l y c o i n c id e n t a l . W h i l e t h e a t o m i c( or e q u iv a l e n t) w e i g h t o f t h e m i d d l e m e m b e r o f t h e a n a l o g o u s e l e m e n t sc h l o r i n e - b r o m i n e - i o d i n e w a s in d e e d t h e m e a n o f t h e o t h e r t w o , P e t t e n -k o f e r n o t e d t h a t t h e a t o m i c w e i g h t o f t h e m i d d l e m e m b e r o f fl uo r in e -c h l o r i n e - b r o m i n e , e l e m e n t s j u s t a s a n a l o g o u s a s th o s e i n t h e o t h e r t r i a d ,w a s not t h e m e a n o f t h e o t h e r tw o . T h e s a m e d i s c r e p a n c y h e ld i f o n eg r o u p e d m a g n e s i u m - c a l c i u m - s t r o n t i u m , i n s t e a d o f c a l c i u m - s t r o n t i u m -b a r i u m .

B u t P e t t e n k o f e r a ls o r e j e c te d Dumas's s y s t e m f o r b e i n g t o o c o m p l e x .I n i t s p la c e h e s u b s t i t u t e d a s c h e m e o f h i s o w n , w h i c h d i s p l a y e d a g r e a t e rr e g u l a r i t y o f w e i g h t s w i t h i n g r o u p s o f e l e m e n t s . D i f f e re n c e s i n e q u i v a -l e n t w e ig h t s , h e m a i n t a i n e d , w e r e u s u a l ly m u l t i p l e s o f 8 , a l t h o u g h s o m e -t i m e s o f 5 , 1 8 , o r 2 2 . T h u s :

L i = 7 C = 61 6 = 8 x 2 5 = 5 x lN a - - 2 3 B = 1 11 6 = 8 x 2 1 0 = 5 x 2K = 3 9 S i = 2 1

M g = 1 2 8 = 8 x l N = 1 4C a = 2 0 1 = 3 22 4 = 8 x 3S r = 4 4 A s = 7 52 4 = 8 x 3B a = 6 8 S b = 1 2 9

l s = 1 8 x ] = [ ( 2 x 5 ) + S ] x ][ b l a n k ] la

5 4 = 1 8 3 = [ ( 2 x 5 ) + S ] x 3

0 - - 8 C r = 268 = 8 x l 22S = 1 6 M o - - 4 824 = 8 x 3 22S e = 4 0 V = 7 024 = 8 x 3 22T o = 6 4 W = 9 2

H g = 1 0 0A g = 1 0 8 8 = 8 x 1

,2 An n. der Chem. und Pharm ., 1 8 5 8 , 1 0 5 , 1 8 7 - 2 0 2 .l a P e t t e n k o f e r o f fe r e d n o e x p l a n a t i o n f o r t h e i n c r e m e n t b e t w e e n p h o s p h o r u s a n da r s e n i c b e i ng 4 3 a n d , t h e r e f o r e, n o t f i t t i n g h is s c h e m e ; h e s i m p l y s t a t e d t h a t t h i s g r o u p o ff o u r e l e m e n t s i s d i v i d e d i n t o t w o s e c t io n s o f n a t u r a l l y p a i r e d e l e m e n t s .


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1 90 D o n C . R a w s o nA l t h o u g h P e t t e n k o f f e r c l ai m e d t h a t h is s y s t e m w a s si m p le r t h a n t h a t

o f D u m a s , i t w a s s t il l b a s e d o n t h e s a m e l in e o f r e as o n i n g - - -r e g u l a ri n c r e m e n t s - - a n d h a d l it tl e a d v a n t a g e o v e r it .4. Evaluation of Mendeleev's Precursors.

I n r e v i e w i n g t h e w o r k o f t h e s e m e n , o n e f i n d s t h a t d e s p i t e t h e i rl i v e l y i n t e r e s t i n t h e n u m e r i c a l r e l a t i o n s h i p o f t h e e l e m e n t s d u r i n g t h e1 8 50 s a n d e a r l y 1 86 0s , t h e y o n l y p a r t i a l l y r e a l iz e d t h e i r h o p e s f o rs y s t e m a t i z a t i o n a n d d i d n o t d i s c o v e r a n u n d e r l y i n g p ri n c ip l e . I n e a c hc a s e t h e i n t r i g u e o f n u m b e r s l e d t h e s e i n v e s t i g a t o r s t o s e e k m o r e p r e c is ea n d a e s t h e t i c a l l y s a t is f y i n g r e l a t i o n s h i p s t h a n t h e i r d a t a c o u l d s u p p o r t .N o t o n l y d i d t h e a p p a r e n t s ig n if ic a n c e o f D S b e r e i n e r ' s t r i a d s p r o v e m i s -l e ad i n g, b u t D u m a s ' s a c c e p t a n c e o f t h e n o t i o n o f p r i m a r y m a t t e r a n d h isa t t e m p t t o u p h o l d P r o u t ' s h y p o t h e s i s i n t r o d u c e d a n a d d i t io n a l f a ll a cy .C o m p o u n d e d b y t h e p r e v ai li n g a m b i g u i t y o v e r t h e m e a n in g a n d u s a g eo f a t o m i c w e i g h t s v e r s u s e q u i v a l e n t w e i g h t s , t h e s e a c c u m u l a t i v e b i a s e st e n d e d t o o b s c u r e t h e p r i n c i p l e s o d i l i g e n t l y s o u g h t .

N e v e r t h e l e s s , t h e s e i n a d e q u a c i e s s h o u ld n o t b e e x a g g e r a t e d . W i t ho n l y p a r t i a l d a t a a n d a l i m i t e d r e s e r v o i r o f t h e o r y , t h e s e c h e m i s t s w e r ep r o b i n g a d i f f ic u l t f ie l d o f c h e m i c a l i n t e r p r e t a t i o n . D e s p i t e t h e i r f a l l a c i o u sl e a d s, t h e y d i d c o r r e c t ly p o r t e n d t h e c l a ss i fi c at io n o f t h e e l e m e n t s , p a r -t i c u l a r l y a s L e n s s e n e n d e a v o u r e d t o i n c l u d e a ll o f t h e e l e m e n t s i n a s in g les y s t e m a n d b o t h K r e m e r s a n d D u m a s t o o k t h e f al te r in g b u t h i gh l ys i g n if ic a n t s t e p o f a t t e m p t i n g t o e s t a b l is h r e l a t i o n s h i p s n o t o n l y w i t h i ng r o u p s o f a n a l o g o u s e l e m e n t s b u t b e t w e e n d is s im i l ar g r ou p s . A l t h o u g ht h e s e c h e m i s t s f el l s h o r t o f f in d i ng a s a t i s f a c t o r y s o l u t io n t o t h e p r o b l e mo f r e la t io n s h ip s , t h e y a c c e n t u a t e d t h e p r e s e n t i m e n t s t h a t b o t h a s y s t e ma n d a p r i n c ip l e e x i s t ed , t h u s p r o v i d i n g a n i n c e n t i v e f o r f u r t h e r w o r k b yo t h e r s , i n c l u d i n g ~ [ e n d e l e e v .5. Mendeleev's Interest in the Problem

~ [ e n d e l e e v ' s w o r k o n t h e r e l a t i o n s h i p o f t h e e l e m e n t s d e r i v e d f r o mb o t h t h e o r e t i c a l a n d p r a c t ic a l c o n c e r n s. N o t lo n g a f t e r a c c e p t i n g t h ec h a i r o f i n o r g a n ic c h e m i s t r y a t S t . P e t e r s b u r g U n i v e r s i t y i n 1 86 5, t h ey o u n g c h e m i s t s e t t o w o r k w r i t in g a t e x t b o o k o n t h i s s u b j e c t , hi s f a m o u sFundamentals of Chemistry (Osnovy/chimii). A t t h e e n d o f 1 8 6 8 , h e h a dc o m p l e t e d t h e f ir s t v o l u m e , w h i c h d e a l t w i t h g e n e r a l q u e s t i o n s o f c h e m i s t r y ,a s w e l l a s c e r t a in c o m m o n e l e m e n t s : h y d r o g e n , o x y g e n , n it r o g e n , a n d t h eh a l o g e n s ; a n d h e h a d b e g u n t h e s e c o n d v o l u m e , d e v o t i n g t h e f i rs t t w oc h a p t e r s t o t h e a l k a l i m e t a l s . 13 A t t h i s p o i n t h e f a c e d t h e d e c i s i o n o f h o w

14 D. I . Mend eleev , Sochineniia, Leningra d-Moscow , 1934-1956, 25 vols., vol. xiv, p. 6.Volumes 13 and 14 are reprints of Parts I and I I of Osnovy khimii.


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Th e Process o f Discovery: Mendeleev and the Per iodic L aw 19]to organize the remaining material, particularly the order in which todiscuss the rest of the elements. This practical problem had impor tan ttheoretical ramifications, as Mendeleev realized, for whatever systemhe might devise needed a rationale to support it; and if the system wereto include all of the elements, it must involve a universal principle.

As Mendeleev pondered the problem, he consulted the investigationsof Gladstone, Kremers, Lenssen, Dumas, a nd Pettenkofer, a nd like themhe surmised that the elements were related numerically according totheir atomic weights. But unlike them he did not subscribe to thenotion of triads or of a unified system of elements based on composites of' primary ma tte r '. In order to explain this reluctance, one must con-sider Mendeleev's att ent ion to the persuasive work of the Belgian chemist,Jean-Servais Stas, who, by the mid-1860s, had discredited both of thesenotions.While professor of chemistry a t the Royal Military School in Brussels,Stas engaged in an extended project to determine more precisely thaneven had Berzelius (whose tables were then widely accepted) the atomicweights of a number of common elements. In ]860, Stas publishedthe results of over a decade of rigorous experimentation, which showedthat for the halogens and alkali metals, as well as nitrogen, sulphur,lead, and silver, the atomic weights were neither whole numbers nor evenmultiples of one-half or one-fourth, as Dumas main tained. In hisconclusions, Stas specifically refuted Prout's hypothesis th at all elementswere composites of a pr ima ry matte r. 15 When the Swiss chemistCharles Marignac, in an att emp t to uphold Prout's hypothesis, challengedStas's work on the grounds that he had not demonstrated the constancyof the atomic weights of these elemen ts in different chemical combinations--for example, the ratio of silver to chlorine might be different in AgC1than in AgC1Od--Stas spent several additional years proving that indeedthe a tomic weight of each element was constant. With cve]x moreprecise values, he again refuted Prout's hypothesis, showing that ifH = 1.000, then:

N = 14.009Li = 7.004Na = 22.980K = 39.O4OC1 = 35.368Br = 79.750I = 126.533Ag = 107-660

is Bull. Aca d. sci. lettres et beaux-art s de Belgique (Classe des sciences), 1860, 10,2 0 8 - 3 3 6 .


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192 Don C. RawsonThe publication of these re sults in 186616 convinced all but a few diehardsth at Prout' s hypothesis was unsound. Mendeleev acknowledged theexcellent quality of Stas's work in the first edition of Fundamentals ofChemistry, agreeing that Prout's hypothesis was untenable, as wereDumas's one-half and one-quarter atomic weights. 17

Stas's investigation also tended to discredit the notion of the triads,inasmuch as those who employed them relied on approximate values,generally rounded off to integers. However, this was not the questionto which Stas explicitly addressed himself, nor did Mendeleev rejectthe triads solely because of Stas's work. He also recognized th at whenthe triads were extended to include all of the elements, as in Lenssen'ssystem, they resulted in extremely unnatural groupings according totheir properties. It is true t ha t Mendeleev did refer to triads in the firstvolume of Fundamentals of Chemistry and the initial chapters oi thesecond volume, written before his discovery of the Periodic Law: he notedtha t th e atomic weight of sodium was the mean of lithium and potassium,as well as tha t bromine was the mean of chlorine and iodine. Bu t heindicated that he did this in order to show parallels between groups ofelements, not to designate triads within groups; and he did not pursuethe notion of the triads. In fact, he suggested th at the newly discoveredelements o f rubi dium (1861), caesium (1860), and th all ium (1861) belongedto the alkali metal group, just as fluorine belonged to the halogens, inboth cases extending the groups to more th an th ree members, is Thus,by the time ~Iendeleev began to work intensively on the problem ofclassification, he had discarded any lingering notions of either triads orprimary matter, notions th at had so seriously encumbered his precursors.Free from these biases, Mendeleev also took advantage of otherdevelopments in chemical theory in the 1860s, particularly in regardto a clearer definition of atomic weights. As long as confusion over thedistinction between atomic weights and equivalent weights continued,he could never have devised a system of relationships based on a singlesequence of elements, let alone a periodic sequence. For example, aslong as calcium was used according to its equivalent weight of 20 insteadof its atomic weight of 40, it could not follow potassium, whose atomic,as well as equivalent, weight was 39. However, during the 1860s,Mendeleev, along with many other chemists, converted to a defined andconsistent use of atomic weights.

1 , A n n . d e c h e m . e t d e p h y s . , 1 8 6 6 , 9 9 2 1 5 - 2 4 3 .IT M e n d e l e e v , S o c h i n e n i i a , v o l . x i v , p . 2 4 5 .i s I b i d . , v o l . xi ii , p . 7 5 8 ; v o ]. x i v , p . 5 3. L a t e r , i n h i s a r t i c l e a n n o u n c i n g h i s d i s c o v e r yo f t h e P e r i o d i c L a w , M e n d e l e e v c o m m e n t e d t h a t o n e o f t h e m a j o r f l aw s in L e n s s e n ' s s y s t e mr e s u l t e d f r o m h i s a t t e m p t t o e s t a b l i s h t r i a d s , t h e r e s u l t b e i n g t h e a r t i f ic i a li t y o f t h e s y s t e m .Z h u r n a [ r u s s k a g o f i z i k o - k h i m i e h e s k a g o o b s h c he s t va , 1 8 6 9 , 1 9 6 0 - 7 7 .


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The Process of Discovery: Mendeleev and the Periodic Law 193T h i s c o n v e r s io n o c c u r r e d g r a d u a l l y . I n w r i ti n g h i s u n d e r g r a d u a t e

t h e s i s a t t h e P e d a g o g i c a l I n s t i t u t e i n S t. P e t e r s b u r g d u r i n g 1 8 5 5 - 18 5 6 ,M e n d e l e e v e m p l o y e d a t o m i c w e i g h t s b a s e d o n B e r z e l i u s 's t a b l e s ; 19 af e w m o n t h s l a te r , f o r h i s m a s t e r ' s t h e s i s, h e a d o p t e d t h e a t o m i c w e i g h t sp r o p o s e d b y th e F r e n c h c h e m i s t C h ar le s G e r h a r d t, w h o h a l v e d m a n y o fB e r z e l i u s ' s v a l u e s , t h u s c o r r e c t i n g t h o s e f o r s u c h e l e m e n t s a s p o t a s s i u m ,s i l v e r , a n d c h l o r i n e ( w h i c h B e r z e l i u s h a d e r r o n e o u s l y l i s t e d a t t w i c et h e i r a c t u a l w e i g h t ) , b u t w r o n g l y a l te r i n g t h e a l r e a d y c o r r e c t v a l u e s fo rs u c h e l e m e n t s a s c a l c i u m , c o p p e r , a n d z i nc . s ~

U n d e r g r a d u a t e M a s t e r ' st h e s i s t h e s i sK 78 .2 39 .1

Ag 216 .0 108 .0C1 71.0 35.5Ca 40.1 20.0Cu 63 .4 31 .7Zn 65 .0 32 .6

I n n e i t h e r c a se d id M e n d e l e e v di st in g u is h a d e q u a t e l y b e t w e e n a t o m i ca n d e q u i v a l e n t w e i g h t s , n o r d i d h e d o s o i n h i s f i r st p u b l i s h e d a r t i c l ei n 1 85 8 . I n f a c t , i n t h i s a rt i c le h e e x t e n d e d t h e e r r o n e o u s f e a t u r e s o fG e r h a r d t ' s s y s t e m b y h a lv i n g e v e n t h e v a l u e s f o r o x y g e n a n d c a r b o n ,w h i c h r e s u l t e d i n s u c h f o r m u l a e a s H ~ O 2 f o r w a t e r a n d C 1 2H ~ f o r b e n z e n e . ~1H e r e a l i z e d t h a t t h e s e v a l u e s w e r e u n s a t i s f a c t o r y a n d s o o n a b a n d o n e dt h e m , e s p e c i a ll y b e c a u s e o f h i s i n t r o d u c t i o n t o a m o r e l o g ic a l s y s t e m o fa t o m i c w e i g h t s a f t e r 1 86 0. I n t h a t y e a r , w h i l e s t u d y i n g o n a f e l lo w s h i pa t t h e U n i v e r s i t y o f H e i d e l b e r g , h e t o o k t i m e t o a t t e n d t h e i m p o r t a n tK a r l s r u h e C o n g r e s s , a t w h i c h s o m e 1 4 0 p r o m i n e n t c h e m i s t s f r o m t h r o u g h -o u t E u r o p e a t t e m p t e d t o r e a c h a g e n e ra l a g r e e m e n t o n t h e d e f in i ti o n s o fa t o m i c w e i g h t s, e q u i v a l e n t s , a n d r e l a t e d te r m s . I n a le t t e r t o h isf r i e n d a n d f o r m e r t e a c h e r i n S t . P e t e r s b u r g , A . A . V o s k r e s e n s k i i , h er e l a t e d t h a t h e w a s m u c h i m p r e s s e d b y S t a n i s l a o C a n n i z z a r o 's p l e af o r a d o p t i n g a t o m i c w e i g h t s b a s e d c o n s i s t e n t l y o n v a p o u r d e n s i t i e s( a c c o r d i n g t o A v a g a d r o ' s h y p o t h e s i s ) r a t h e r t h a n o n c o m b i n i n g w e i g h t s ,w h i c h s e e m e d t o h a v e c a u s e d so m u c h c o n f u si o n . A s h e e n t h u s i a s t i c a l l yc o m m e n t e d , C a n n i z z a ro e x p o s e d t h e f la w i n G e r h a r d t ' s s y s t e m , w h i c hh a d a t f i rs t a p p e a r e d t o b e s u c h a n i m p r o v e m e n t o n B e r z e l i u s 's v a lu e s .

lgMendeleev, Periodicheskii zakou, 621-624.2o Ibid ., 646-650.21 Mdlanges physiques et chimiques tirds du Bulle tin de l'Acaddmle impdriale des s~iencesde St.-Petersbourg, 1856-1859, 3, 402-428.


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194 Don C. RawsonAccording to Carmizzaro the atomic weights of the elements listed in thetable above would be: ~

K 39Ag 108 ~like Ger hardtC1 35.5 )Ca 40Cu 63 ~ unlike Gerha rdt

Jn 65.5As receptive as was Mendeleev to Cannizzaro's persuasive argument,there is evidence that several more years passed before he thoroughly

conver ted to a consistent use of atomic weights. Among Mendeleev'spapers was found a series of lecture no tes, which include a list of 63 elementsand the ir weights. Although this list is und ate d, one can conclude bycomparing the weights Mendeleev used in it with those that had beenestablished at various points during the 1860s, parti cula rly for recen tlydiscovered elements, th at he wrote it in either 1864 or 1865. In thelist, he used equivalen t weights for some 13 of the elements. Even forcalcium, for which he had used the atomic weight of 40 in his letterfollowing the Karlsru he congress, he now reverted to the equiva lentweight of 20. ~a However, in t he first volume of Principles of Chemistry,written in 1868--several months before his work on the Periodic Law--he included a list of 22 common elements (including calcium), for whichhe used atomic weights throu ghou t. 24 Thus, by the ti me he began towork seriously on the problem of classifying the elements, he was alrea dyusing their atomic weights consi stentl y--an imp erta nt prerequisite to hisdiscovery of the Periodic Law.6. Rec ogn ition of the La w

What, then, was the final process by which Mendeleev arrived a tthe crucial point of disco very --his recognition of periodicity? In22 Mendelee v, Periodic t tesk i i zakon, 663-665.~a B. M. K e(l rov , De n' odnogo ve l ikogo otkr y t l ia , Moscow, 1958, 341. Ke dr ov dat esthe list as 1867 or 1868, but this is probably too late, because Mendeleev included no weightthat had been established or revised after 1865, whereas he did use values tha t had notbeen established until 1864. For example, he listed indi um as 37, the weight its discoverers,Ferdinand Reich and Theodor Richter, reported in Journa l f i i r p rak t i sche Chemic in 1864;h o w e v e r , in an article in this journal in 1865, Clemens Winkler revised the weigh t to 35.9and in 1867 to 37.8. Mendeleev evidently w as familiar wit h these changes, becausehe used the 1867 value for his Periodic Table (actually he doubled this equivalent weight

to i ts atomic weight of 75.6). Similarly, for other eleme nts --y t tr ium , erbium and niobium--Mendeleev did not use weights reported after 1864 or 1865 in his lecture list.24 Mend eleev , Soch inen i ia , vol. xiii, p. 342. In this list Mendeleev did state the weigh tof carbon as 6-- i t s equiva lent wei ght - -bu t thi s apparent l y was an oversight, because onthe following page he calculated the weight of Na2CO a as 106, which req uired carbo n tobe 12, its atomic weight.


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T h e P r o c e s s o f D i s c o v e r y : M e n d e l e e v a n d t h e P e r i o d i c L a w 195his initial article announcing the discovery, he described his methodologybut briefly and vaguely. He wrote tha t he

'selected the bodies with the smallest atomic weights and arrangedthem according to the magnitude of their atomic weights. In so doing,it appeared that there exists a periodicity of properties of the simplebodies, and even that according to valency the elements follow oneanother in the order of an arithmetric sequence of the magnitude oftheir atomic weights.'He ad ded tha t he then proceeded to bring all of the elements into a systembased oft this principle. 25 While indicating that Mendeleev listed thesimpler elements sequentially according to their increasing atomicweights, this description does not at all explain how such an arrangementenabled him to recognize the periodicity of their properties. Never-theless, his idea of arranging the elements in a single sequence became t hegenerall y accepted int erpr eta tion of the process of his discovery. More-over, other fragments of evidence seemed to support and amplify this view.One of his friends, the Czech chemist Bohuslav Brauner, relatedshort ly after Mendeleev's death in 1907 th at Mendeleev once told himhow he had written the symbols and atomic weights of the elementson individual cards an d arranged t hem in various ways, none of themsatisfactory until he placed them one after another according to theirincreasing atomic weights. Having arran ged the following elementsin a row:H=I L i =7 Be=9 B=l l C=12 N=14 0=16 F=19 ,he foun d th at the elements imme diat ely following would form a secondrow direct ly below the first:

Na=23 Mg=24 A1=27 Si=28 P=31 S=32 C1=35.5,each element having properties similar to the element above it, and thatthese p roperties were repeated in successive rows of the e lements arrangedaccording to their increasing atomic weights. 26While the simplicity of this explanation has its appeal, recently pub-lished archival materials, when studied carefully, disclose that thediscovery probably resulted from a different process. Ra the r th an listinga single sequence of elements, Mendeleev compared the atomic weightsof various g r o u p s of elements, gradually accumulating a sufficient numberto enable him to recognize their periodicity.The first of these materials is some notes written on the back of alette r to Mendeleev from A. I. Khodnev , secreta ry of the Free Economic

25Z h u r n a l ru s s ~ s a g o f i z il c o . ] c h im i c h e s ] c a g o o b s h c h e s t v a , 1869, 1~ 60-77.26 C o l l ec t io n o f C z e c h o s l o va k C h e m i c a l C o m m u n i c a t i o n s , 1930, 2, 225. The Russ ianchemist , V. In. Kurba tov , also briefly relates in his memoirs that Mendeleev once told himhow he wrote the propert ies of the elements on cards, arranged them according to thesepropert ies, an d arrived at a table. A. A. Makarenia and I. N. Fi l imonova (eds.), D. I.M e n d e l e e v v v o s p o m i n a n i i a k h s o v r e m e n n i k o v , Moscow, 1969, 107.


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196 Don C. RawsonSociety in St. Petersburg, inquiring about Mendeleev's preparations toinspec t some cheese-processing plants for the Society . ~7 The let ter isdate d 17 February, 1869, and judging from the note of urgency in itsmessage, it was probably delivered by messenger th at day. After jottingdown the symbols for several elements on the back o f the letter, the purposeof which is not a ppare nt, Mendeleev made the following calculations:

23 39 85 133(7) 14 24 65 112(16) 9 15 20 217 12 32 56

(The figures 14 and 9 in the first column were written over 7 and 16,respectively.) Evidently , Mendeleev was comparing the atomic weightsof the following two groups of elements:

Na K Rb Cs[Be?] ~s Mg Zn CdMendeleev's purpose in choosing these two groups of elements waslogical and practical. Havi ng just finished the chap ters on the alkali

metals for his textbook, he was contemplating th at on the basis of chemi-cal properties, as well as valency, the analogues of zinc might well follow thealkali metals, even before the alkaline earths. This arrangem ent accordedwith one of the c urrently accepted concepts tha t the analogues of zincformed an inte rmediate group between the alkali metals and the alkalineearths. Indeed , in his initial article on the Periodic Law, he referred toKreme r's arrangem ent of these three groups: 29

Li Na KMg Zn CdCa Sr Ba~ K e d r o v ( n o t e 23 ) , 4 2 .2s I t i s d i f f ic u l t t o k n o w w h i c h e l e m e n t M e n d e l e e v i n t e n d e d b y t h e f i g u r e ' 7 ' : l i t h i u m ,

w h o s e a t o m i c / e q u i v a l e n t w e i g h t h a d b e e n e s t a b l i s h e d a t t h a t v a l u e , o r b e r y l l iu m , w h o s ee q u i v a l e n t w e i g h t h a d n o t y e t b e e n d e c i d e d a s 4 .7 ( i f i t s o x i d e w a s B e O ) o r 7 . 0 ( if B e 2 O a ).S e e J a h r e s b e r i c h t i i b e r d i e F o r t s c h r i t t e d e r C h e m i e , 1 86 8 , x x x i . O n e m i g h t w e l l c o n j e c t u r et h a t M e n d e l e e v h a s t i l y c h o s e B e 7 , e v e n t h o u g h t h i s m e a n t t h a t B e m u s t b e t r i v a l e n ta n d w o u l d n o t c o r r e s p o n d t o t h e b i v a l e n t M g - Z n - C d . T h a t h e m i g h t h a v e e r r e d in t h i sr e s p e c t i s i n d i c a t e d b y t h e f a c t t h a t i n t h e r u d i m e n t a r y t a b l e s f o l lo w i n g th e s e i n i ti a l ca l-c u l a t i o n s ( se e F i g u r e s 1 a n d 2 ), M e n d e l e e v w a s u n d e c i d e d a b o u t B e , b o t h i n r e g a r d t o i tsw e i g h t a n d v a l e n c y . F u r t h e r m o r e , s i n c e h e e v e n t u a l l y p l a c e d B e w i t h M g - Z n - C d (s ee F i g -g u r e s 3 a n d 4 ) , a lb e i t a s a b i v a l e n t e l e m e n t h a v i n g t h e a t o m i c w e i g h t o f 9 . 4 ( t h a t i s, 2 x 4- 7) ,h e p r o b a b l y h a d t h i s a s s o c ia t i o n i n m i n d i n t h e i n it ia l c a lc u l a t i o n s . I n a n y e v e n t , t h et h e s i g n i f ic a n c e o f t h e f i g u r e ' 7 ' i s n o t t h e k e y t o t h e s i g n i f i c a n c e o f t h e s e c a l c u l a t i o n s , a se x p l a i n e d i n t h e d i s c u s s i o n b e l o w .29 Z h u r n a l r u s s k a g o f i z i k o - k h i m i c h e s k a g o o b s h c h es t v a, 1 8 6 9, 1 , 6 0 - 7 7 . S e e t h e d i s c u s s i o no f K r e m e r ' s i n v e s t i g a t i o n a b o v e .


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The Proces s o f D i scovery : Mende l eev and t he Per iod i c Law 197W i t h t h i s s c h e m e i n m i n d a s he w r o t e o n t h e b a c k o f K h o d n e v ' s l e t te r ,M e n d e l e e v c a l c u l a t e d i n t h e t h i r d r o w o f f ig u r es th e d i f fe r e n c e i n e a c hp a i r o f a to m i c w e i g h ts , p r o b a b l y i n o r d e r t o d e t e r m i n e i f t h e r e e x i s t e da r e g u l a r r a t e o f i n c re a s e . U p o n f in d i ng t h a t t h e d i f f e re n c e i n t h e f i r stc o l u m n - - 1 6 - - w a s l ar ge r t h a n t h a t i n t h e s ec o n d c o l u m n - - 1 5 - - M e n d e l e e vd o u b l e d t h e v a l u e b e i n g s u b t r a c t e d i n t h e f i rs t c o lu m n , c o r r e c t i n g i t f r o mt h e e q u i v a l e n t w e i g h t t o t h e a t o m i c w e i g h t o f b e r y l l i u m a~ a n d g i v in ga n e w d i f f e r e n c e o f 9 . S t i ll t h i s d i d n o t p r o v i d e a l o g ic a l s e q u e n c e o fd i f fe r e n c es , a n d M e n d e l e e v r e v i s e d t h e v a l u e s f o r t h e a n a l o g u e s o f z i nci n t h e f o u r t h r o w o f f ig u r e s b y c h a n g in g 1 4 b a c k t o 7 a n d h a l v i n g t h ev a l u e s f o r t h e o t h e r e le m e n t s f ro m t h e s e c o n d r o w - - a c t u a l l y g i v i n g t h e mt h e i r e q u i v a l e n t w e i g h t s .T h i s a f t e r t h o u g h t p r o v e d n o m o r e i n s t ru c t i v e t h a n M e n d e l e e v ' s f ir stc a l c u la t io n , a n d h e p u r s u e d i t n o f u r t h e r. H o w e v e r , t h e c o m p a r i s o nt h a t h e h a d m a d e w a s e x t r e m e l y i m p o r t a n t , f o r i t w a s t h e b e g i n n in g o ft h e p r o c e s s t h a t l e d t o h is re c o g n it io n o f t h e P e r i o d i c L a w . H e h a dc o m p a r e d t h e a t o m i c w e i g h t s o f t w o d i s si m i la r g r o u p s o f e l e m e n t s , w h e r e a s ,a s o n e re c a ll s , h is p r e c u r s o r s h a d a l m o s t w h o l l y l i m i t e d t h e i r c o m p a r i s o n st o a n a l o g o u s e l e m e n t s , t h e e x c e p t i o n s h a v i n g b e e n K r e m e r s w i t h h i s' c o n j u g a t e d t ri a d s ' a n d D u m a s w i t h hi s i n c r e m e n t a l f o r m u l a e . I n h isa r r a n g e m e n t o f t h e a l k a li m e t a l s , z in c a n a l o g u e s , a n d a l k a li n e e a r t h s , c i t e da b o v e , K r e m e r s m a y n o t c o n s ci o u sl y h a v e b e e n c o m p a r i n g d i ss im i l are l e m e n t s , b u t a s M e n d e l e e v r e p e a t e d t h i s a r r a n g e m e n t , h e a p p a r e n t l yr e a li z e d , a s w e s h a ll s e e, t h e i m p o r t a n c e o f c o m p a r i n g a d d i t i o n a l d i s si m i l a rg r o u p s . T h e n o t e s o n K h o d n e v ' s l e t t e r i n d i c a t e M e n d e l e e v ' s f i rs t b r e a k -t h r o u g h .

T h e s e c o n d m a n u s c r i p t a d d i n g t o o u r k n o w l e d g e o f N [ e n d e le e v 'sr e c o g n i t io n o f t h e P e r i o d i c L a w i s a s h e e t o f p a p e r f o u n d g l u e d i n s id e ab o o k i n M e n d e l e e v ' s p e r s o n a l l i b ra r y , d a t e d 1 7 F e b r u a r y , ] 8 6 9 , o n w h i c hh e h a d w r i t t e n t w o t a b l e s o f e l e m e n t s a n d t h e i r a t o m i c w e i g h t s . 31 B o t ht a b l e s c o n t a in o n l y p a r t o f t h e e l e m e n t s , b u t t h e y s h o w h o w h e w a sp r o c e e d i n g w i t h h is c o m p a r i s o n s o f d i s si m i l a r g r o u p s o f e l e m e n t s . A so n e c a n s e e f r o m t h e c o n f i g u r a ti o n o f t h e f ir s t t a b l e ( F i g u r e 1 ) h e p r o b a b l yw r o t e i t i n s t a g es . J u d g i n g f r o m t h e r e g u l a r i t y o f t h e i r r o w s a n d c o l u m n s ,h e w r o t e t h e F , 0 , N , a n d C g r o u p s f ir st . T h i s w a s n o t a d i r e c t c o n t in u -a t i o n o f h i s p r e v i o u s c o m p a r i s o n o f t h e a l k a li m e t a l s a n d t h e a n a l o g u e s o fz in c , b u t i t w a s a l o gi c al s t e p i f o n e s u r m i s e s t h a t h e i n d e e d h a d d e c i d e dt o p u r s u e t h e c o m p a r i s o n o f d i s si m i l a r g r o u p s . I t a l so r e f le c t s t h e p a t t e r no f M e n d e l e e v ' s t h o u g h t p r o c e s se s : a t t h i s p o i n t h e a p p a r e n t l y w a s b e g i n-n in g t o d e t e c t t h e l in k b e t w e e n t w o s e p a r a t e s e t s o f r e l a t i o n s h i p s - - a

80 See no te 28.31 Ked rov (note 23) , p. 49. In rega.rd to these tables, I tend t o agree with Ked rov 's

analy sis and follow a l ine of reaso ning sim ilar to his.A.S. o


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198 Don C. R~wson

t ~ ~ ~ ( ? ~ - I ~ ~ ~ ~

~ I I r l r l I I I I I I I I I I I I ~ t "

~ ~ l . ~ ~ ~ e ~ ~ ' ~

. II II ""

II II II II 11 II II

~ 0

. % . . ~ " :' .'~ ~ ' ~

~ l J J ! II I! .| ~ -o


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The Process of Discovery: M endeleev and the Periodic La w 199k i n d o f ' b i s o c i a t i o n , .a 2 T h e f ir s t r e l a t io n s h i p w a s t h a t o f a n a l o g o u se l e m e n t s w i t h i n g r o u p s - - t h e a n a l o g u es o f F , 0 , N , a n d C , r e s p e c t i v e l y - -w h o s e a t o m i c w e i g h t s i n c r e a s e d c o n t i n u a l ly , i f n o t b y c o n s t a n t i n c r e m e n t s .T h e s e c o n d r e l a t io n s h i p w a s t h a t o f F , 0 , N , a n d C , t h e m s e l v e s . O nt h e b a s i s o f t h e i r p r o p e r t i e s a s n o n - m e t a l s a n d t h e i r r e g u l a r l y c h a n g in gv a l e n c y f r o m 1 t o 4 , M e n d e l e e v c o u l d d e t e c t t h i s r e l a t io n s h i p r e a d i l y ;a n d h e c o u l d a l s o n o t i c e t h a t t h e i r a t o m i c w e i g h t s i n c r e a s e d c o r r e s p o n -d i n g l y . A l r e a d y h e h a d s u s p e c t e d t h e e f f ic a c y o f c o m p a r i n g d i s s im i l a re l e m e n t s ; n o w , a s h e l i s t e d e n ti r e g r o u p s s i d e b y s i de , h e c o u l d p e r c e i v et h e i r b i - d i r e c ti o n a l r e l a ti o n s h i p s . T h r o u g h t h i s ' b i s o c i a t i o n ' h e r e c o g -n i z e d t h a t h e c o u l d i n t e g r a t e t h e s e s e t s o f r e l a t i o n s h i p s i n t o a s i n g les y s t e m . A s t ri k i n g r e al i z a ti o n , t o b e s u re : a g e n u i n e a c t o f c r e a t i v i t y .Y e t , t h e a p p a r e n t s y n t h e s i s w a s n o t a t t h i s p o i n t c o n c l u s iv e ; a n d~ e n d e l e e v i m m e d i a t e l y s e t a b o u t e x t e n d i n g t h e r e la t io n s h ip s . T o t h isn u c l e u s o f e l e m e n t s , h e a d d e d s e v e r a l o t h e r g r o u p s : t h e a l k a l in e e a r t h so n t o p a n d t h e a l k a l i m e t a l s , a n a l o g u e s o f zi nc , a n d s e v e r a l o t h e r e l e m e n t sa t t h e b o t t o m . A l t o g e t h e r s ix g r o u p s f i tt e d t o g e t h e r a d m i r a b l y - - t w oo t h e r s n o t s o w e l l .

O n t h e b o t t o m h a l f o f t h e s h e e t o f p a p e r , M e n d e l e e v r e a r r a n g e da n d e x p a n d e d t h e t a b l e e v e n f u r t h e r ( F i g u r e 2). S i g n i f ic a n t l y h e p l a c e dt h e a l k a l in e e a r t h s , r a t h e r t h a n t h e a n a l o g u e s o f z in c , n e x t t o t h e a l k a l in em e t a l s . T h i s s o l v e d t h e o r ig i n a l p r a c t i c a l p r o b l e m o f w h i c h e l e m e n t s t oi n c lu d e n e x t in h is t e x t b o o k , b u t f a r m o r e i m p o r t a n t i t c o n s t i tu t e s t h ec o n c l u d i n g s t a g e i n t h e p r o c e s s o f ' b i s o c i a t i o n '. F r o m t h e c o n f ig u r a t io no f t h e t a b l e i t a p p e a r s t h a t a s M e n d e l e e v w r o t e d o w n t h e s i x g r o u p s - -a l k a li n e e a r t h s , a l k a li m e t a l s , F , 0 , N , a n d C - - h e d i d s o n o t horizontallyb y g r o u p s , b u t vertically, e a c h c o l u m n i n s e q u e n c e , i n d i c a t i n g t h a t b yt h i s t i m e h e s u r e l y r e c o g n i z e d t h e i r b i d i r e c ti o n a l r e la t i o n s h i p s. H e h a da c h i e v e d t h e c r u c ia l s y n t h e s i s : i n r e c og n i z in g t h e r e p e t i t i v e r e l a t io n s h i po f p r o p e r t ie s a n d a t o m i c w e i g h ts , M e n d e l e e v h a d a r r iv e d a t the principle ofperiodicity. H a v i n g c o n s t r u c t e d 'th is m u c h o f a t a b l e , h e t h e n p r o c e e d e dt o a d d s e v e r a l o t h e r e l e m e n t s w h i c h f i tt e d re a d i l y . I n a l l, h e i n c l u d e d4 2 e le m e n t s , s o m e t w o - t h i r d s o f t h e t o t a l k n o w n .I t r e m a i n e d t h e n f o r M e n d e l e e v t o e x p a n d h i s t a b l e t o i n c l u d e a llo f t h e e l e m e n t s . Q u i t e p o s s i b l y a t th i s t i m e h e u s e d t h e c a r d s t o w h i c hB r a u n e r re f e r re d . T h e r e m a i n i n g e l e m e n t s w e r e f a r l e s s i n v e s t i g a t e dt h a n t h o s e h e h a d a l r e a d y a rr a n g e d , a n d a l t h o u g h h e u n d o u b t e d l yu n d e r s t o o d b y t h is t i m e t h a t t h e y s h o u l d b e i n c l u d ed a c c o r d i n g t o th e i ra t o m i c w e i g h t s , h e s ti ll n e e d e d t o d e t e r m i n e e x a c t l y w h e r e t h e ys h o u l d f it a l o n g t h e p e r i p h e r y . T h e r e s u l t in g a r r a n g e m e n t a p p e a r si n t h e t h i r d m a n u s c r i p t , M e n d e l e e v ' s fi r st d r a f t o f a t a b l e c o m p r i s in g a l lo f th e k n o w n e l e m e n t s ( F i g u r e 3 ). aa

32 Ar th ur Koest le r ' s te rm in T h e A c t o f C r e a t i o n , London, 1964, 656-660.aa Mendelee v, P e r i o d i c h e s k i l z a k o n , 8 .


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The Process of Discovery: Mendeleev and the Periodic Law 203printer. It served as the fo rm of the tab le Mendeleev included in hisarticle on t he discovery of the Periodic Law. a57. Confirmation

'T he confirmation of a law' , wrote Mendeleev, 'i s possible onlyby deducing consequences from it, which would be impossible andunexpected without it, and by justifying those consequences by experi-mental proof '.~In the case of the Periodic Law, confirmation result ed as othe r chemists

verified various predictions Mendeleev made regarding the existence andcharacteristics of several undiscovered elements, as well as correctionsin the atomic weights of certain elements already known but inadeq uatelystudied. In his table he left several blank spaces, in which he placedquestion marks, along with the approximate atomic weights of elementslleeded to fill these gaps. For example, he suggested two elements,one similar to aluminum, the other to silicon, which would have atomicweights betwee n 65 and 75. Ia 1871, he provided the na mes ' eka-alu-minum ' and ' eka-silieon' for these anticipated elements, and he added' eka -boron ' for an elemen t needed to fill a gap which appe ared in aslightly revised version of the table. 37 In verification of his predictions,all three of his eka-elements had been discovered by 1885, each havingappr oxima tely the properties and atomic weight he h ad specified. ~s

Predi cted atomic weight Discovered atomic weighteka -alu mini um 68 gallium 69.9eka-silicon 72 germaniu m 72.32eka-boron 44 scandium 44.1

Apparently even l~endeleev had not expected such rapid confirmation,for he later admitted:' When in 1871 I wrote an article or~ the application of the PeriodicLaw to the determinatiorl of the properties of as yet undiscovered

a~ Note th at in both draft s of the table seven elemen ts, --E r, Yt, I n, Ce, La, Di, andTh -- do not fit properly. The first three, whose atomic weights are listed betwe en 57 and75, Mendeleev placed between e lements havi ng weights of 40 and 50; the other four, whoseatomic weights range from 92 to 118, fall between elements havi ng weights of 87 and 90.Mendeleev simply left these el ements in the margin, unplaced ; in later years, when correctedatomic weights were established for them, Mendeleev incorporated them into the table.a6 Mendelecv, Periodicheskii zakon, 323, from Osnovy khimii, 8th edition.87 Ibid., 150-153, reprint of Russian draft of an article in Annalen der Chemic undPkarmacie, 1871, Suppl ement VII I, 133-229.a8 Whe n Leeoq de Boi sbaudr an annou nced the discovery of gallium, he reported itsdensi ty as 4.7, considerably lower than Mendeleev's prediction of 5.9. More accuratemea sur eme nt proved Mendeleev correct. Comptes Rendus, 1875, 81, 969-972; 1876, 82,1036-1039.


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204 The Process of Discovery: Mendeleev and the Period Lawelements, I did not think I would live until this consequence of thePeriodic Law was verified; but reality turned out otherwise '

The discoverers of the eke-elements Mendeleev acknowledged as the't ru e corrobor ators' of the Periodic Law. a9In addition to these gaps for undiscovered elements, Mendeleevmade other predictions, most---but not all--of which were fulfilled.To the elements Er= 56, Yt= 60, and In= 75.6, which Mendeleev couldnot satisfactorily incorporate into his original table, he assigned newatomic weights in 1871 (actually higher multiples of their equivalentweights than accepted in 1869) and placed them in positions consistentwith their properties: In--113 between Cd=11 2 and S n =l l8 ; Yt =8 8between Sr = 87 and Zr = 90; and Er-- 178 in the midst of several blankspaces still to be filled. Subsequent atomic weight dete rmina tions provedMendeleev correct in his predictions for indium and ytt riu m, thei r weightsbeing 114.8 and 88.9; erbium, however, was shown to have an atomicweigh~ of 167.27 and belong to the rare earth series, a group whose mem-bers, except for erbium and cerium, were unknown at the time Mendeleevmade his predictions.4~

As scientists learned more about atomic structure, they modifiedMendeleev's fundam ental explana tion of the periodic relationship of theelements, particularly by concluding tha t the periodic arrang ement of theelements is actually not a function of their atomic weight but of theiratomic number. However, in most cases, an increase in atomic numb erentails a corresponding increase in atomic weight, which means thatalthough Mendeleev's explanation had a somewhat incorrect theoreticalbasis, it nevertheless disclosed a periodic relationship of the elementsbased on a genuine law of nature. Thus, from presentiments to recog-nition to confirmation, the discovery of the law was complete.

a9 Mendeleev, Periodicheskii zakon, 323-324, from Osnovy khimii, 8th edition. In his1871 article, Me ndelee v also sugg ested the possibi lity of eka -caesi um [175], dvi-caes ium [220],eke- niob ium [146], ek a-t ant ali um [235], eke- mang anes e [100], an d tr i-ma ngan ese [190].His prediction of eka-manganese approximated technetium [99], discovered in 1939;but his other predictions proved inaccurate, primarily because he could not foresee thateventua lly the lanth anid e and actinide series would become a par t o f the syste m of elements.40 In his system Mendeleev also questioned the current ly ac cepted a tomic weigh t ofgold, since platinum, iridium, osmium, and mercury, all of which he was convinced mustcome before gold, had been assigned higher atomic weights. Additi onal atomic weightdeterminations showed that actually the weights of platinum, iridium, and osmium wereincorr ect--t heir revised values permitted them to remain before gold, as Mendeleev hadplaced them. In la ter tables the positions of gold and mer cury were exchanged, placingthem in different groups than before on the basis of their properties and in the propersequence according to their atomic weights.

process discovery mendeleev

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