Define electron affinity and explain its trend ... - SS2 Chemistry Atomic Structure and Periodicity Question

Electron affinity refers to the energy change that occurs when an atom or ion in the gaseous state gains an electron to form a negatively charged ion.

Across a period in the periodic table, electron affinity does not have a consistent trend. However, there is a general increase in electron affinity as you move from left to right across a period. This is because, as the atomic radius decreases and effective nuclear charge increases, the attraction between the nucleus and incoming electrons strengthens. Therefore, atoms across a period have a higher tendency to gain electrons and exhibit higher electron affinity.

Down a group in the periodic table, electron affinity generally decreases. This is mainly due to the increasing atomic radius and the shielding effect. The larger atomic radius and shielding reduce the effective nuclear charge, weakening the attraction between the nucleus and incoming electrons. As a result, atoms down a group have a lower tendency to gain electrons and exhibit lower electron affinity.