1. Lecture 20: Periodic Trends
Reading: Zumdahl
Outline
Periodic Trends
Ionization Energy, Electron Affinity, and Radii
A Case Example

2. Periodic Trends
The valence electron structure of atoms can be used to explain various properties of atoms.
In general, properties correlate down a group of elements.
A warning: such discussions are by nature very generalized…exceptions do occur.

3. Periodic Trends: Ionization
If we put in enough energy, we can remove an electron from an atom.
The electron is completely “removed” from the atom (potential energy = 0).

4. Periodic Trends: Ionization
Generally done using photons, with energy measured in eV (1 eV = 1.6 x J).
The greater the propensity for an atom to “hold on” to its electrons, the higher the ionization potential will be.
Koopmans’ Theorem: The ionization energy of an electron is equal to the energy of the orbital from where the electron came.

5. Periodic Trends: Ionization
One can perform multiple ionizations:
Al(g) Al+(g) + e- I1 = 580 kJ/mol
first
Al+(g) Al2+(g) + e- I2 = 1815 kJ/mol
second
Al2+(g) Al3+(g) + e- I3 = 2740 kJ/mol
third
Al3+(g) Al4+(g) + e- I4 = 11,600 kJ/mol
fourth

6. Which reaction represents the ionization of F?
A. 1s22s22p5
1s22s22p6
B. 1s22s22p5
1s22s22p43s1
C. 1s22s22p5
1s22s22p4
D. 1s22s22p5
1s22s12p6

7. Periodic Trends: Ionization
First Ionization Potentials:
Column 8
Column 1

8. Periodic Trends: Ionization
First Ionization Potentials:
• Increases as one goes from
left to right.
• Reason: increased Z+
• Decrease as one goes down
a group.
• Reason: increased distance
from nucleus

9. Periodic Trends: Ionization
Removal of valence versus core electrons
Na(g) Na+(g) + e- I1 = 495 kJ/mol
[Ne]3s1
[Ne]
(removing “valence” electron)
Na+(g) Na2+(g) + e- I2 = 4560 kJ/mol
[Ne]
1s22s22p5
(removing “core” electron)
Takes significantly more energy to remove a core electron….tendancy for core configurations to be energetically stable.

10. Which atom would you expect to have the lowest ionization energy?
1s22s22p3
B. 1s22s22p63s23p5
C. 1s22s22p63s23p64s2
D. 1s22s22p63s23p64s23d104p65s1

11. Periodic Trends: Electron Affinity
Electron Affinity: the energy change associated with the addition of an electron to a gaseous atom.

12. Periodic Trends: Electron Affinity
We will stick with our thermodynamic definition, with energy released being a negative quantity.
Wow!

13. Periodic Trends: Electron Affinity
Elements that have high electron affinity:
• Group 7 (the halogens) and Group 6 (O and S specifically).

14. Periodic Trends: Electron Affinity
Some elements will not form ions: N?
• Orbital configurations can explain both observations.

15. Periodic Trends: Electron Affinity
Why is EA so great for the halogens?
F(g) + e F-(g) EA = kJ/mol
1s22s22p5
1s22s22p6
[Ne]
Why is EA so poor for nitrogen?
N(g) + e N-(g) EA > 0 (unstable)
1s22s22p3
1s22s22p4
(e- must go into occupied orbital)

16. Periodic Trends: Electron Affinity
How do these arguments do for O?
O(g) + e O-(g) EA = -140 kJ/mol
1s22s22p4
1s22s22p5
Bigger Z+ overcomes
e- repulsion.
What about the second EA for O?
O-(g) + e O2-(g) EA > 0 (unstable)
1s22s22p5
1s22s22p6
[Ne] configuration, but electron
repulsion is just too great.

17. Which diagram indicates the evolution in electron affinity from high affinity to low affinity?

18. Atomic Radii • Atomic Radii are defined as the covalent radii,
and are obtained by taking 1/2 the distance of a bond:
r = atomic radius

19. Atomic Radii • Decrease to right due due increase in Z+
• Increase down column
due to population of
orbitals of greater n.

20. Looking Ahead
• We can partition the periodic table into general types of elements.
Metals: tend to give up e-
non-Metals: tend to gain e-
Metalloids: can do either