What are the trends and exceptions to the trends in electron affinity

Electron affinity is the energy released in adding an electron to a neutral atom in the gas phase.

It is a measure of the readiness of an atom to gain an electron. This property is very peculiar to non-metals. The higher the value, the greater the tendency to accept electrons.

Across a period electron affinity increases due to the increasing nuclear charge not being compensated for.

Down a group, electron affinity decreases due to the low nuclear charge and the large atomic radii.

The exception to this rule is the stability of half-filled sublevels. For example, nitrogen has a configuration of 2,5 with sublevel notation of 1s²2s²2p³.

The p-sublevel has a degeneracy of three and the three electrons goes in singly. This makes the configuration stable.

We expect such an atom to have a higher electron affinity but its configuration is stable and carbon would have a higher affinity than it across the same period.

Half filled sublevels are exception to the trend of electron affinity.