Problem 7-135

Electron Affinity: The term "rationalize" actually means: "Explain the trend of the numbers in this series"

Solution and explanation

Electron affinity is the addition of an extra electron to an atom: added to the next lowest available orbital.
This trend from Al to Cl (−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 (similar to half or fully filled "d" orbitals).

Therefore, the addition of one more electron to phosphorus, −74 kJ/mol, is less "exothermic" or favorable than the previous electron addition since this new electron is spin = −½, therefore, will be in repulsion and spin opposition to the three already "synced +½" electrons present.

Finally, the trend is completed with sulfur and chlorine which drastically increase their exothermic values ​​since their electronic configuration is closer than that of the corresponding noble gas Ar.