Question

Why is the electron gain enthalpy of phosphorus, magnesium almost zero?

Answer 1

In the case of nitrogen, it has a half-full 2p subshell. While this is not as stable as a full or empty valence shell, it is still more stable than it would be if it gained or lost one or two electrons.

For beryllium and magnesium, they both have full 2s subshells, meaning that the valence subshell is full. This is much more stable than if they had one or two more electrons, which would leave the new valence subshell with an awkward number of electrons.