Periodic Trends in Electron Gain Enthalpy

Electron gain enthalpy ($\Delta_{eg}H$) is the enthalpy change that occurs when one mole of isolated gaseous atoms of an element each gains one electron to form one mole of gaseous anions:

Most atoms release energy when they gain an electron (the added electron is attracted by the nucleus), so $\Delta_{eg}H$ is usually negative; the more negative the value, the greater the atom's tendency to accept an electron. A second electron gain (forming, e.g., $O^{2-}$ from $O^{-}$) is always energetically unfavourable ($\Delta_{eg}H$ positive) because the incoming electron is repelled by an already negatively charged ion.

General trend. $\Delta_{eg}H$ becomes more negative on moving left to right across a period (elements more readily accept an electron as effective nuclear charge — and hence attraction for an incoming electron — increases), and generally less negative on descending a group (the added electron enters a progressively larger, more shielded shell farther from the nucleus, so it is attracted less strongly). The halogens (Group 17), one electron short of a full octet, have the most negative electron gain enthalpies in their respective periods.

Three exceptions students must know:

• The noble gases (Group 18) have positive $\Delta_{eg}H$ — the added electron would have to start an entirely new, higher-energy shell (since the atom's own valence shell is already completely filled), which is energetically unfavourable rather than favourable. This is the single biggest deviation from the general trend, breaking the pattern sharply at the end of every period.
• Nitrogen and the rest of Group 15 have $\Delta_{eg}H$ values much less negative than their immediate neighbours (in nitrogen's case, essentially zero or slightly positive). Nitrogen's ground-state configuration $1s^2\,2s^2\,2p^3$ is the same extra-stable, half-filled $2p^3$ arrangement seen in the ionization-enthalpy exceptions; forcing a fourth electron into this subshell disrupts that stability and requires the electron to pair up against extra repulsion, so the process is far less favourable than the smooth trend would predict. …