Exceptions to Trends

[Maria1E]

What are the exceptions to the periodic trends for ionization energy, electron affinity, and atomic radii? Thank you!

[Chem_Mod]

The most important exceptions for ionization energy are Be/B and N/O pairs. Both Berillium and Nitrogen have symmetrically filled orbitals (2s and 2p respectively) and thus higher energy is required to remove an electron. Boron and oxygen, however, have one additional electron that they really want to lose to achieve symmetrical configuration. Therefore, their ionization energy will be lower. Therefore, Be has a higher ion. energy than Boron and Nitrogen has higher than oxygen.

For electron affinity, two exceptions are important. Firstly, Carbon has higher electron affinity than nitrogen, as it really wants to gain this electron to achieve symmetry. Noble gases, however, already have a complete shell and therefore have extremely low electron affinity.

[Suhail Zaveri]

There are also exceptions to electron affinity between C and N, as Carbon as the higher electron affinity, even though it is expected for nitrogen to have a higher electron affinity, this is due to the orbitals being half filled or not half filled.