why electron affinity of nitrogen and phosphorus is very low ?
The configurations of electrons with completely full shells and polunapolnennymi shells have increased stability in comparison with other electronic configurations. Therefore, nitrogen (1s22s22p3), which already has half the full 2p shell, will not want to shift the stability of the half-filled shell, producing an electron. Therefore, it has a lower affinity for an electron than was expected, only electronegative. For Phosphorus, the level 3p is exactly half filled (note that filled full and fully filled orbits have a more stable configuration), so for the adoption of another electron with phosphorus (ie, electron affinity), it will place a stable electronic configuration and, consequently, electron affinity to P is less than the N.