Hint: Electron affinity depends on atomic size, nuclear charge and the symmetry of the electronic configuration.
Complete step by step answer: Various factors that affect electron affinity are atomic size, nuclear charge and the symmetry of the electronic configuration.
Atomic size: With increase in the atomic size, the distance between the nucleus and the incoming electron also increases. The attraction between the nucleus and the incoming electron decreases. Due to this, the electron affinity will have smaller value. For example, potassium has higher atomic size than sodium. The electron affinity of potassium is lower than that of sodium. Thus, in the group of alkali metals, on moving from top to bottom, with increase in the atomic size, the electron affinity values decreases. Similar is the trend observed for halogens.
Nuclear charge: As the nuclear charge increases, the force of attraction between the nucleus and incoming electron increases. Due to this, the electron affinity increases.
The symmetry of electronic configuration: Atoms having half filled or completely filled subshells in the valence shell have stable electronic configuration. They have little tendency to accept an additional electron. Hence, they have very low values of electron affinity. Based on the above factors, the following trends are observed in the electron affinity. 1) In a period, on moving from left to right, the electron affinity values increases. 2) In a group, on moving from top to bottom, the electron affinity values decrease due to increase in atomic size.
Note: Do not mix the concepts of electron affinity and electronegativity. Electron affinity is similar to electron gain enthalpy. When a neutral gaseous electron gains an electron, the energy change is called electron affinity. In covalent compounds, electronegativity gives the measure of tendency of the atom to attract a shared pair of electrons towards it.