Problem 13-105

Electron affinity

The term "rationalize" in fact means: "Explain the trend of the numbers in this series"

explanation

Electron affinity is the addition of an extra electron to an atom (added to the lowest available orbital)

This trend (−44, −120, −74, −200, −385) kJ/mol) follows the filling order of the 3p orbitals.
adding one e⁻ to Al (−44 kJ/mol) will place two electrons in the 3p orbitals (or subshell).
adding one e⁻ to Si (−120 kJ/mol) will place three electrons in the 3p subshell. Three electrons paired in the same subshell with the same spin (+½) will ensure subshell stability (same as half or fully filled "d" orbitals).

Therefore, adding an extra electron to phosphorus (−74 kJ/mol) is less exothermic than the previous addition (−120 kJ/mol) since the next electron added −½, will be in repulsion with the three "synchronized +½" electrons already present.

Finally, the trend continue with sulfur and chloride that increase drastically since their electronic configuration is closer than the corresponding noble gas Ar.