Electron Gain Enthalpy

Adding an electron to a neutral atom X forms the anion $X^-$; the enthalpy change accompanying this addition to an isolated gaseous atom in its ground state is called the electron gain enthalpy, $\Delta_{eg}H$, expressed in kJ mol⁻¹: $X(g) + e^- \rightarrow X^-(g)$. Unlike ionization enthalpy, $\Delta_{eg}H$ can be either positive (if adding the electron is energetically unfavourable, i.e. endothermic) or negative (if it is favourable, i.e. exothermic), depending on the element. Group 17 elements (the halogens) show very high NEGATIVE electron gain enthalpies, because adding just one electron completes a stable, noble-gas-like octet configuration. Group 18 elements (the noble gases), by contrast, show high POSITIVE values, since the added electron has no choice but to enter an entirely new, higher-energy shell — an arrangement made very unstable by the extremely low effective nuclear charge and heavy core shielding present in that outer region. The alkali metals show only weakly negative electron gain enthalpies. In general, the most strongly negative electron gain enthalpies belong to elements toward the upper-right of the periodic table, excluding group 18. The overall trend, while somewhat less regular than the ionization-enthalpy trend, is: electron gain enthalpy becomes progressively MORE negative moving across a period (up to the s …