1. Lecture 19: Periodic Trends Reading: Zumdahl 12.14-12.16 Outline –Periodic Trends Ionization Energy, Electron Affinity, and Radii –A Case Example

2. The Aufbau Principal (cont.) Lithium (Z = 3) 1s2s2p 1s2s2p Berillium (Z = 4) Boron (Z = 5) 1s2s2p 1s 2 2s 1 1s 2 2s 2 1s 2 2s 2 2p 1

3. The Aufbau Principal (cont.) Carbon (Z = 6) 1s2s2p 1s2s2p Nitrogen (Z = 7) Hund’s Rule: Lowest energy configuration is the one in which the maximum number of unpaired electrons are distributed amongst a set of degenerate orbitals. 1s 2 2s 2 2p 2 1s 2 2s 2 2p 3

4. The Aufbau Principal (cont.) Oxygen (Z = 8) 1s2s2p 1s2s2p Fluorine (Z = 9) 1s 2 2s 2 2p 4 1s 2 2s 2 2p 5 1s2s2p Neon (Z = 10) 1s 2 2s 2 2p 6 full

5. The Aufbau Principal (cont.) This orbital filling scheme gives rise to the modern periodic table.

6. The Aufbau Principal (cont.) After Lanthanum ([Xe]6s 2 5d 1 ), we start filling 4f.

7. The Aufbau Principal (cont.) After Actinium ([Rn]7s 2 6d 1 ), we start filling 5f.

8. The Aufbau Principal (cont.) Heading on column given total number of valence electrons.

9. 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.

10. 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).

11. 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.

12. Periodic Trends: Ionization One can perform multiple ionizations: Al(g)Al + (g) + e - I 1 = 580 kJ/mol first Al + (g)Al 2+ (g) + e - I 2 = 1815 kJ/mol second Al 2+ (g)Al 3+ (g) + e - I 3 = 2740 kJ/mol third Al 3+ (g)Al 4+ (g) + e - I 4 = 11,600 kJ/mol fourth

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

14. 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

15. Periodic Trends: Ionization Removal of valence versus core electrons Na(g)Na + (g) + e - I 1 = 495 kJ/mol Na + (g)Na 2+ (g) + e - I 2 = 4560 kJ/mol [Ne]3s 1 [Ne] 1s 2 2s 2 2p 5 (removing “valence” electron) (removing “core” electron) Takes significantly more energy to remove a core electron….tendency for core configurations to be energetically stable.

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

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

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

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

20. Periodic Trends: Electron Affinity Why is EA so great for the halogens? F(g) + e - F - (g)EA = -327.8 kJ/mol 1s 2 2s 2 2p 5 1s 2 2s 2 2p 6 [Ne] Why is EA so poor for nitrogen? N(g) + e - N - (g)EA > 0 (unstable) 1s 2 2s 2 2p 3 1s 2 2s 2 2p 4 (e - must go into occupied orbital)

21. Periodic Trends: Electron Affinity How do these arguments do for O? O(g) + e - O - (g)EA = -140 kJ/mol 1s 2 2s 2 2p 4 1s 2 2s 2 2p 5 What about the second EA for O? O - (g) + e - O 2- (g)EA > 0 (unstable) 1s 2 2s 2 2p 5 1s 2 2s 2 2p 6 [Ne] configuration, but electron repulsion is just too great. Bigger Z + overcomes e - repulsion.

22. 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

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

24. 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