The electron affinities of the elements from aluminum to chlorine are -44, -120, -74, -200.4, and -384.7 kJ/mol respectively. Rationalize the trend in these values.

Electron affinity is the energy absorbed (endothermic) or emitted (exothermic) by the addition of one electron to form the -1 ion. There is a progression from Al to Cl except for P. The electron configurations follows:
Al 1s2 2s2 2p6 3s2 3p1 = 13
Si ----------------3p2 = 14
P -----------------3p3 = 15
S -----------------3p4 = 16
Cl ----------------3p5 = 17
We note that P, with a 3p3 outside orbital, is half filled. Orbitals that are half filled or completely filled are more stable, thus, P is more stable (less energy given off by adding an electron) than Si and S on either side of it.