electron affinity
/ uh-FIN-ih-tee /
If ionization energy asks how reluctantly an atom gives up an electron, electron affinity asks the opposite: how eagerly does a gaseous atom accept an extra electron? It is the energy change when one electron is added to a neutral gaseous atom to make a negative ion. For many atoms this releases energy (the new electron settles into a welcoming spot), so by the common convention a large negative number means a strong appetite for electrons.
The mechanism is the flip side of ionization. An atom with a high effective nuclear charge and a partly empty valence shell can pull in an extra electron and lower its energy — the halogens are the champions, because adding one electron completes their outer shell. Across a period electron affinity generally becomes more negative (more energy released) toward the right, peaking near the halogens, then collapses at the noble gases, which have full shells and actually resist a new electron. The pattern is bumpier than ionization energy: nitrogen and the group with half-filled p subshells accept electrons reluctantly, and adding an electron to a small, crowded atom like fluorine is slightly less favourable than for chlorine because of electron-electron repulsion in the tight 2p shell.
Electron affinity matters because it is half the story of why ionic compounds form: a metal gives up electrons (governed by ionization energy) and a nonmetal grabs them (governed by electron affinity), and the overall energy balance is settled in a Born-Haber cycle. Two honest cautions: the sign convention is genuinely confusing (some books report electron affinity as a positive 'energy released', others as a negative enthalpy change), so always check; and second electron affinities (adding a second electron to an already-negative ion) are always endothermic, because you are forcing an electron onto something that already repels it.
Chlorine has one of the most favourable electron affinities (about -349 kJ/mol released), which is why Cl readily becomes Cl- — yet fluorine's is slightly less favourable (about -328 kJ/mol) because its small 2p shell is cramped.
Cl beats F — small atoms crowd a new electron.
Watch the sign convention: an exothermic electron gain is often quoted as a negative enthalpy but sometimes as a positive 'affinity'. The noble gases and the alkaline earths have near-zero or unfavourable values — full or full-s shells do not want more electrons.