electron affinity
Picture offering a spare electron to different atoms, like handing out a sample at a market. Some atoms snatch it greedily and give off a little 'thank-you' burst of energy; others take it only reluctantly, and a few will not take it at all without being forced. Electron affinity measures just how welcoming an atom is to one extra electron.
Electron affinity is the energy change when a neutral atom in the gas phase gains one electron to become a negative ion. For most atoms this gain releases energy, so the value is conventionally given as a positive number meaning energy given off; a larger electron affinity means the atom holds the new electron more tightly and 'wants' it more. The halogens, one electron short of a full shell, have the largest affinities.
It complements ionisation energy: where ionisation energy measures the cost of losing an electron, electron affinity measures the payoff of gaining one, and together they shape an element's bonding behaviour. The caveats are real: sign conventions differ between textbooks, and not every atom releases energy — noble gases and full or half-full subshells often resist a new electron, giving small or even unfavourable values.
Chlorine has one of the largest electron affinities of all: adding an electron to a chlorine atom releases about 349 kJ/mol, because the new electron completes its outer shell. That eagerness is the flip side of why chlorine forms stable chloride ions in table salt.
How welcoming an atom is to one extra electron — halogens are the keenest.
Do not confuse electron affinity with electronegativity: affinity is a measured energy for a lone gas-phase atom, while electronegativity is a relative scale for an atom's electron-pulling power inside a chemical bond.