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Periodic Trends: Atomic Radii
Elemental Properties and Patterns
The Periodic Law
Dimitri Mendeleev (1869/1871) was the first scientist to publish an organized periodic table of the known elements.
He was taking a chemistry course in Russia and tried to find a way to organize the periodic table.
The Periodic Law
Mendeleev even went out on a limb and predicted the properties of 2 at the time undiscovered elements.
He was very accurate in his predictions, which led the world to accept his ideas about periodicity and a logical periodic table.
The Periodic Law
Mendeleev understood the ‘Periodic Law’ which states:
When arranged by increasing atomic number, the chemical elements display a regular and repeating pattern of chemical and physical properties.
The Periodic Law
Atoms with similar chemical properties and behavior appear in groups or families (vertical columns named by Roman numerals with A or B) on the periodic table.
They are similar because they all have the same number of valence (outer shell) electrons, which governs their chemical behavior.
Periods– horizontal rows on periodic table
Periodic Trends
There are several important atomic characteristics that show predictable trends that you should know.
Atomic properties— Deal with only single atoms
The full, positive nuclear charge is “SHIELDED” from
outer shell electrons by all the other electrons between them
Electrons closer to the nucleus shield other electrons further away. Full attractive force is not felt by outer electrons
Electrons have different energies and therefore, located at different regions surrounding the nucleus Based on electrostatic field + positively charged nucleus
Electron Shielding
The amount of nuclear charge influencing or
attracting an electron
= actual nuclear charge—screening influence of other electrons
Effective Nuclear Charge (Zeff)
An atom’s size affects chemical and physical
properties of an atom
Atom’s size is related to the radius of the atom Radius of atom = ½ distance between nuclei of 2
adjacent atoms of the same element Radius is the distance from the center of the nucleus to the “edge”
of the electron cloud. half the distance between the nuclei of 2 bonded atoms.
Atomic Radii
Atomic Radius General Trend
Decreases
Increases
Atomic Radius
The effect is that the more positive nucleus has a greater pull on the electron cloud. The nucleus is more positive and the electron cloud is
more negative. The increased attraction pulls the cloud in,
making atoms smaller as we move from left to right across a period—MOVING ACROSS TABLE
More and more electrons are added and placed at higher and higher energy levels so atomic size increases—MOVING DOWN TABLE
Half the distance between nuclei of adjacent
atoms in a metal
Radius of metallic atoms
> 75% of elements are metals
1. Metallic Radii
2. Covalent Radius
Half the distance between the nuclei of 2 adjacent, bonded atoms in a chemical compound.
Atoms MUST be identical Radius of nonmetallic atoms (Ex. Cl2. Br2)
Smaller radii than metallic—covalent bonds hold the atoms close
Ex. Br2
2.86
Å1.43 Å
1.43 Å
Ions
Atoms that have gained or lost electrons (anion, cation)
Electrons gained/lost from valence shell (outer electron shell)
Defined by the distance between the nuclei of 2 ions Studies with crystal structures Radii related to original atomic radii
Ionic Radii
1) Anions
LARGER size than original neutral atom Generally nonmetals
Additional electrons cause the atom’s size to increase
Electrons repel as increase in number, causes size to increase
Ionic Radii (cont.)
2) Cations
SMALLER size than original neutral atom Generally metals
Removal of electrons causes the atom’s size to decrease more attractive force from nucleus, nucleus can
pull remaining electrons towards it.
Ionic Radii (cont.)
Element
Covalent Radius (nm)
Ion Ionic Radius (nm)
F 0.064 F- 0.136
Cl 0.099 Cl- 0.181
Li 0.123 Li+ 0.068
Na 0.157 Na+ 0.095
Compare atomic sizes of Na and Cl
Neutral Ion Na > Cl Na+ < Cl-
Example 1
Applies to either atoms or ions
Atoms or ions with the SAME number of electrons So electron configuration is the same as well
Does not state the protons stay the same As the proton number increases but the electrons
stay the same, the positive nucleus holds electrons more tightly
Isoelectric
Atom/Ion Radius (nm) Electron Configuration
O2- 0.140 1s22s22p6
F- 0.136 1s22s22p6
Ne 0.112 1s22s22p6
Na+ 0.095 1s22s22p6
Mg+2 0.065 1s22s22p6
Which of the following ions has the LARGEST
radius?
A) Na+B) Mg+2
C) S2-
D) Cl-
E) Se2-
Example 2:
Which of the following ions has the SMALLEST
radius? Why?
A) K+
B) Li+
C) Be+2
D) O2-
E) F-
Example 3:
Which of the following isoelectric ions is the
LARGEST? Why?
A) Mn+7
B) P3-
C) S2-
D) Sc3+
E) Ti4+
Example 4:
Atomic Theory III Worksheet #1-17
Homework
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