Question

Account for thr low value of electron affinity of nitrogen and phosphorus

Answer 1

When we look at electron gain enthalpies (EGEs), we are looking at the energy change (at standard pressure) for the process of adding a mole of electrons to a mole of gaseous atoms of a specified element, forming a mole of negative ions:

X(g) + e- → X-(g)

This process is also commonly referred to as first electron affinity, but it depends on whose definition you’re using: Oxford actually define electron affinity as the opposite to this process.

The factors we must consider that contribute to this energy change are:

The additional electrostatic attraction between the electron and the positively charged nucleus that lowers the energy of the product. We’ll call this the attractive termThe additional electrostatic repulsion that arises as a result of adding the electron raises the energy of the product. We’ll call this the repulsive term

The attractive term generally outweighs the repulsive term, and so in most cases these processes are exothermic.*

*However, there are a few cases where the repulsive term outweighs the attractive term: nitrogen being one of them… back to this later

Right, so the attractive term makes the process more exothermic, whereas the repulsive term makes the process less exothermic.

More electronegative atoms have a larger attractive term: due to a combination of the electron experiencing a greater effective nuclear charge and being closer on average to the nucleus in more electronegative atoms.

Down a group electronegativity decreases, so on these grounds we would expect EGEs to become less exothermic as we move down the group.