Lecture 19: Periodic Trends
•  Reading: Zumdahl 12.14-12.15
•  Recommended Problems:12.81,12.83,12.87
•  Exam on 2 Dec. –  See Study Guide (3 pages). It will be covered at Discussion
Sections and at the Th (1 Dec.) review session.
–  Exam 2 Covers: 10.11,10.12, 11.1-11.4, 12.1-12.10; Lectures
12-18
–  Lectures 19-23 covering 12.15 and 13.1-13.13 will be covered on
the final exam •  Outline
–  Periodic Trends
•  Ionization Energy, Electron Affinity, and Radii
–  A Case Example
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.
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).
Periodic Trends: Ionization
•  Generally done using photons, with energy
measured in eV (1 eV = 1.6 x 10-19 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.
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
Periodic Trends: Ionization
•  First Ionization Potentials:
Column 8
Column 1
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
Periodic Trends: Ionization
•  Removal of valence versus core electrons
Na(g) Na+(g) + e-
I1 = 495 kJ/mol
(removing “valence” electron)
[Ne]3s1
[Ne]
Na+(g)
Na2+(g) + e- I2 = 4560 kJ/mol
[Ne]
1s22s22p5 (removing “core” electron)
•  Takes significantly more energy to remove a core
electron…. core configurations are energetically
stable.
First Ionization E Trends
Increases from left to
right across a period.
 Reason: increasing Z+ (the number of
protons in the nucleus) which attracts
the valence electron
Decreases down a group.  Reason: increasing distance between
electron and nucleus
Periodic Trends: Electron Affinity
•  Electron Affinity: the energy change associated
with the addition of an electron to a gaseous atom.
Periodic Trends: Electron Affinity
•  We will stick with our thermodynamic definition,
with energy released being a negative quantity.
Wow!
Periodic Trends: Electron Affinity
•  Elements that have high electron affinity:
• Group 7 (the halogens) and Group 6 (O and S specifically).
Periodic Trends: Electron Affinity
•  Some elements will not form ions:
N?
• Orbital configurations can explain both observations.
Periodic Trends: Electron Affinity
•  Why is EA so great for the halogens?
F(g) + e1s22s22p5
F-(g)
1s22s22p6
EA = -327.8 kJ/mol
[Ne]
•  Why is EA so poor for nitrogen?
N(g) + e1s22s22p3
N-(g)
EA > 0 (unstable)
1s22s22p4
(e- must go into occupied orbital)
Periodic Trends: Electron Affinity
•  How do these arguments do for O?
O(g) + e1s22s22p4
O-(g)
EA = -140 kJ/mol
1s22s22p5
Bigger Z+ overcomes
e- repulsion.
•  What about the second EA for O?
O-(g) + e1s22s22p5
O2-(g)
EA > 0 (unstable)
1s22s22p6
[Ne] configuration, but electron repulsion is just too great.
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
Atomic Radii
• Decrease to right due
to increase in Z+
• Increase down column
due to population of orbitals of greater n.
Ionization and Atomic Radii
Which atom would you expect to have the lowest
ionization energy?
A.  1s22s22p3
B. 1s22s22p63s23p5
C. 1s22s22p63s23p64s2
D. 1s22s22p63s23p64s23d104p65s1
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
1A
8A
3A 4A 5A 6A 7A
2A
Non-metals
Metals
•  Metals … good conductors of heat, electricity; malleable
solids
•  Non-metals … poor conductors; not malleable
•  Metalloids … both metallic and nonmetallic properties