Electron Affinity
The energy change when a neutral, gas-phase atom gains one extra electron — how much an atom "wants" an additional electron.
See electron affinity colored on the interactive table →Across a period (left → right)
Generally rises left to right across a period (with real exceptions).
Down a group (top → bottom)
Generally falls down a group (also with exceptions).
Why it happens
The same shrinking-radius, growing-nuclear-pull logic applies: a smaller atom with a stronger effective nuclear charge releases more energy when it captures an extra electron, because that electron is pulled in closer. Halogens, one electron short of a full shell, have some of the highest electron affinities of any elements for this reason.
Electron affinity is the least uniform of these trends. Group 2 (alkaline earth) and group 18 (noble gas) elements already have full or stable subshells, so adding an electron is actually unfavorable for them — their electron affinities break the general pattern noticeably.