Lecture 20: Periodic Trends

• Reading: Zumdahl 12.14-12.15

• 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.

+Z

Z-

+Z

(Z-1)-

e-

Energy

• 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

Which reaction represents the ionization of F?

A. 1s22s22p5

B. 1s22s22p5

1s22s22p6

1s22s22p43s1

C. 1s22s22p5 1s22s22p4

D. 1s22s22p5 1s22s12p6

Periodic Trends: Ionization

• First Ionization Potentials:

Column 1

Column 8

Periodic Trends: Ionization

• First Ionization Potentials:

• Increases as one goes from left to right.

• Decrease as one goes down a group.

• Reason: increased Z+

• Reason: increased distance from nucleus

Periodic Trends: Ionization

• Removal of valence versus core electrons

Na(g) Na+(g) + e- I1 = 495 kJ/mol

Na+(g) Na2+(g) + e- I2 = 4560 kJ/mol

[Ne]3s1 [Ne]

[Ne] 1s22s22p5

(removing “valence” electron)

(removing “core” electron)

• Takes significantly more energy to remove a core electron….tendancy for core configurations to be energetically stable.

Which atom would you expect to have the lowest ionization energy?

A. 1s22s22p3

B. 1s22s22p63s23p5

C. 1s22s22p63s23p64s2

D. 1s22s22p63s23p64s23d104p65s1

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

+Z

Z-

+Z

(Z+1)-

e-

Energy

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:

• Orbital configurations can explain both observations.

N?

Periodic Trends: Electron Affinity

• Why is EA so great for the halogens?

F(g) + e- F-(g) EA = -327.8 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)

Periodic Trends: Electron Affinity

• How do these arguments do for O?

O(g) + e- O-(g) EA = -140 kJ/mol

1s22s22p4 1s22s22p5

• 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.

Bigger Z+ overcomes e- repulsion.

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

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 due increase in Z+

• Increase down column due to population of orbitals of greater n.

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