EA = enthalpy change when a gaseous atom in ground state gains an electron: X(g) + $e^-$ → $X^-$(g). Conventionally, negative EA means energy is released (exothermic). General trend: EA becomes more negative across a period (atoms want electrons more), becomes less negative down a group. Exceptions: (1) EA(F) less negative than EA(Cl): F is so small that the incoming electron experiences strong repulsion from the already dense electron cloud. Cl (larger) accommodates the extra electron better. EA: F (-328 kJ/mol) vs Cl (-349 kJ/mol). (2) EA(N) ≈ 0: Nitrogen's half-filled 2$p^3$ is stable — adding an electron to a half-filled shell is unfavourable. (3) EA of noble gases ≈ 0: Fully filled shell resists addition. EA of Group 2 ($ns^2$) and Group 15 ($np^3$) elements is very low or near zero for same reasons.