Final answer:
The greater energy release when adding an electron to Si compared to P is due to Si's more favorable electron configuration and lower electron-electron repulsion in its valence shell, resulting in a higher electron affinity for Si.
Step-by-step explanation:
The addition of an electron to a gas-phase Si atom results in the release of more energy than addition of an electron to a gas-phase P atom due to differences in electron affinity (EA). The electron affinity is a measure of the energy change when an electron is added to a gaseous atom to form an anion, with the energy change often reported in kJ/mol. Generally, electron affinities increase in magnitude across the periodic table from left to right and decrease from top to bottom within a group.
For Si and P, their positions on the periodic table suggest that Si, being situated below P in the same group, should have a lower electron affinity. However, variations can occur due to electronic configuration and subshell placement. Silicon has a vacant p orbital in its valence shell, which allows it to better accommodate an additional electron with less electron-electron repulsion compared to phosphorus, whose valence p orbitals are half-filled and experience more repulsion when adding an extra electron.
Thus, silicon releases more energy upon gaining an electron due to its more favorable subshell arrangement for accepting an extra electron, leading to a more negative electron affinity.