Electron Affinity

Definition. Electron affinity (electron gain enthalpy) is the energy released -- or, for the noble gases, required -- when an electron is added to the valence shell of an isolated, neutral, gaseous atom in its ground state, forming its anion:

expressed in kJ mol-1.

Trend across a period: not as systematic as ionisation energy. Moving from the alkali metals toward the halogens across a period, electron affinity generally increases (more energy is released), driven by the same rising nuclear charge and shrinking atomic size that drives the ionisation-energy trend. But elements such as beryllium ($1s^2\,2s^2$) and nitrogen ($1s^2\,2s^2\,2p^3$) are exceptions with almost zero electron affinity, because adding an extra electron would disturb their already-stable filled (Be) or half-filled (N) configuration (Figure 3.4).

Noble gases, with a stable $ns^2\,np^6$ configuration, actually require energy to accept a further electron (positive electron affinity), since there is no favourable subshell left for it. Halogens, with configuration $ns^2\,np^5$, sit at the opposite extreme: adding just one electron completes their octet to the very stable noble-gas configuration $ns^2\,np^6$, so halogens readily accept an electron and show the highest (most negative) electron affinity in every period. …