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.

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  1. This explains why the electron affinity of P is lower than the other elements. The other elements follow the trend of increasing electron affinity as the atomic number increases. This is because as the atomic number increases, the outermost electron is farther away from the nucleus, and thus, more energy is required to add an electron.

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