The Structure of an AtomCh.4 Notes

Section 1: Early Theories of Matter

•Atom- the smallest particle of an element that retains the properties of the element.

•Size? The world’s population is 6,840,000,000 •The number of atoms in a penny is 29,000,000,000,000,000,000,000

Early Theories of Matter• Ancient Greek philosopher Democritus speculated matter 

was made up of atomos

• The theory was rejected for 2000 years due to being challenged by Aristotle

Discovery of the Electron

• John Dalton­ proposed his atomic theory in 1803 based on his research that was not completely accurate

• Atoms can be subdivided

• Atoms of the same element can have slightly different masses

•Discovering the electron

•Sir William Crookes - used a cathode ray tube to do experiments on the atom based on the relationship between mass and charge. Electrons are negatively charged

Discovering the Electron

•JJ Thomson (1856-1940)- in the 1890’s found a cathode ray could be deflected by magnetic and electric fields. He concluded the ray was a stream of negatively charged particles dislodged from an atom with a mass much less than a hydrogen atom.

Discovering the Atom

http://www.youtube.com/watch?v=GzMh4q­2HjM

Discovering the Atom

•Robert Millikan (1868-1953) – in 1909 he accurately determined the mass of an electron through the Oil Drop Experiment, it had a charge of -1

•The actual mass of an electron is 9.1 x 10-28 g

• Plum Pudding Theory-( aka the chocolate chip cookie model)

• Matter is neutral, so Thomson proposed: spherical atoms have uniform (+) charge with (-) electrons embedded throughout.

Nucleus

Nucleus• Ernest Rutherford, in 1911

• To test the theory, he aimed a thin beam of (+) charged Alpha particles at a thin sheet of gold.

• He expected the alpha particles to pass through with little deflection

• Instead, most passed through, but few bounced back and deflected at big angles.

• He concluded atom is mostly empty space with large (+) charged nucleus in center

http://www.youtube.com/watch?v=wzALbzTdnc8

Atomic Summary

*Nucleus -protons + neutrons

-contains all the mass-positive charge

*Electrons-located in the cloud-fast moving-very small

Properties of Subatomic Particles 

Particle

Electron

Proton

Neutron

Symbol Location ChargeRelative Mass

Actual Mass

e-

p+

n0

Electron Cloud

Nucleus

­1

Nucleus

+1

none 1

1

1/1840

1.675 x 10-24

1.673 x 10-24

9.11 x 10-28

How atoms differ:

•Atomic number is how an atom is identified, it is equal to the number of protons in an atom, & determines the location of the atom in the periodic table.

•The periodic table is arranged left to right & top to bottom by increasing atomic number.

1

1.008

H

atomic #

Chemical Symbol

Average atomic mass

Number of protons in nucleus of an element

This number identifies the element

Periodic table is arranged in sequence of increasing atomic numbers

Atoms are neutral: (+) charges must equal  (­) charges

Atomic Number

We know: atomic # = #protons = # electrons

•How many protons and electrons are there in:

•Be: p= 4 e= 4

•F: p= 9 e= 9

  

Ions

• Charged particles: Number of electrons has changed­ proton # stays the same

• Be2+ has lost 2 electrons: 

• F1­ has gained one electron:

p= 4 e= 2

p=9    e=10

•To identify an isotope a number is added to the end of the name, it is called a mass number.

•Mass Number = # of protons + # of neutrons

•Ex: Carbon­14, Neon­22, Potassium­41

Isotopes

•Isotopes

• All atoms of same element have some number of protons

• Most have different numbers of neutrons

Mass Number

• An isotope is identified by its mass number

• Mass number= number of protons + number of neutrons

• Number of neutrons = mass number – atomic number

Atomic Mass

• Atomic mass units (amu) = ½ the mass of a Carbon­12 atom

• Atomic mass given in Periodic Table is  not given in whole numbers

• Atomic mass is the weighted average of the atomic masses of all naturally occurring isotopes. This weighted average takes into account the mass and abundance of each isotope

Atomic Mass• Average atomic mass= mass * abundance for each 

isotope: then add

Copper­63 Copper­65

Number of protons 29 29

Number of neutrons 34 36

Atomic mass 62.930 amu 64.928 amu

Abundance 69.17% 30.83%

• Mass Contribution = mass x abundance

• Cu­63= (62.930)(.6917)= 43.53

• Cu­65= ( 64.928)(.3083) =20.02