The observed periodic trends in electron affinity are that electron affinity will generally become more negative, moving from left to right across a period, and that there is no real corresponding trend in electron affinity moving down a group in the periodic table. It is important to note that, in this case, an increased electron affinity is actually the value of the electron affinity becoming more negative.
The electron affinity of an atom is is the amount of energy that is needed to add one electron to that atom. It can be thought of as the likelihood of a neutral atom to gaining an electron, making it a negatively charged ion. Electron affinity is used only when atoms are in a gaseous state because if they were in a solid or liquid state, their energy levels would change when they come into contact with other atoms.
Typically, metals like to lose their valence electrons, while nonmetals like to gain electrons. This means that the electron affinity of metals is significantly lower than that of nonmetals. Although there are exceptions as you go from left to right across a period in the periodic table, electron affinities generally tend to increase. As you move down a group, there are very small changes in electron affinity.