Final answer:
Electron affinity measures the energy change when an electron is added to a gaseous atom, and the trend is more negative from left to right across a period and less negative from top to bottom down a group. Forming a halide ion releases more energy due to the electron entering lower energy orbitals.
Step-by-step explanation:
Electron affinity (EA) is the energy change that occurs when an electron is added to a gaseous atom, resulting in the formation of an anion (negative ion). The process can be represented as A(g) + e- → A-(g) with ΔH being the equivalent of the electron affinity, usually expressed in kJ/mol. The trend across the periodic table shows that, generally, electron affinities become more negative as one moves from left to right, indicating that the process is more exothermic and thus more energy is being released. On the other hand, as one moves down a group, electron affinities typically become less negative, which means that less energy is released when an electron is added.
For example, the formation of a halide ion is more exothermic than other elements because electrons are added into lower energy orbitals, closer to the nucleus. Period and group trends for electron affinities are not as regular as for ionization energy, with various exceptions due to the complexity of their accurate measurement. In spite of the exceptions, the general trends remain useful for predicting chemical reactivity and stability of the elements.