Why does phosphorus (P) have a less favorable electron affinity (Eea) than its neighbor on either side, Si and?

This stems from phosphorus's electron configuration. It has three 3p electrons, so one electron can go into each p orbital (px,py, and pz) unpaired. However, by adding another electron, you begin to pair previously unpaired electrons. While not too hard to do, this is still accompanied by a small energy barrier.