Exceptions to trends

How to explain exceptions


The basic idea is that some orbital positions are intrinsically more or less stable because of the details of the configuration.

More stable:


Any electron that completes a set of orbitals, for example the S
2, p6 or d10. Because this is the last electron in the space of these orbitals, the "harmonics" are complete. Removing that electron would be harder than expected just based on the trends. An example would be the ionization energy of Zn. The first electron removed would be from either the S2 or d10. So, Zn's first ionization energy is higher than predicted.

Less stable:


Any electron that is the first lone electron in a set of orbitals, such as the p
1. As a result, aluminum has a lower ionization energy than expected.
Any electron that is the first spin-paired electron in a set of orbitals, such as the p
4 or or d6. As a result, Oxygen has a lower ionization energy than expected (the p4 electron is easier to remove). Similarly Nitrogen will not accept an electron to become N-. Can you explain why?
A more extreme version of this is what happens with Chromiums ground-state. Chromium has 24 electrons. 6 of them are in the valence level (18 are in the configuration of argon).
Then you fill in the 4s
2 and start on the "3d" orbitals. A funny thing happens when you get to 24 electrons. You would think that the last 6 after "argon" would end up 4s2, 3d4. But, there is the tendency to have maximum parallel spin proposed by Hund's rule. So, one of the 4s electrons gets "promoted" to give you 4s1 3d5. Thus, you get 6 electrons all in parallel spin. This contributes to some of chromium's odd properties.