Predict: Electron affinity (EA) refers to the energy released when an electron is added to an atom. This release of energy is always expressed as a negative value. The greater the magnitude of the negative value, the greater the attraction for electrons. (An EA of –100 kJ/mol would indicate a stronger attraction for electrons than an EA of –50 kJ/mol.) How do you think the size of an atom will affect its ability to attract additional electrons?

Explanation:

Electron affinity is a measure of the tendency of an atom to gain an electron. The more positive the electron affinity, the greater the tendency for the atom to gain an electron (this means that the negative ion formed after accepting the electron is very stable).

The overall trend is an increase in the tendency to accept electrons (electron affinity values become more positive) from left to right across a period, and from the bottom to the top across a group. The halogens (group 7A) have the highest electron affinity values.  That means that the halide ions are very stable.

As we can see from the atomic radii, the halogens are rather small atoms. Smaller atoms will attract additional electrons more easily since the incoming electrons (added on the outermost shell) will be closer to the nucleus, therefore the attraction will be stronger.