Elements are arranged in groups by similar atomic structure on the periodic table. this allows for an element's properties to be predicted based on general periodic trends. one of these trends, electron affinity, increases up a group and to the right along a period and can be defined as the

The change in energy when a neutral atom in the gaseous state adds an electron to form a negative ion.

Explanation:

That is, it is the energy involved in the reaction

X(g) + e⁻ ⟶ X⁻(g)

For most elements, the electron affinity is negative.

However, there are two major exceptions  — the values are positive for the elements of Groups 2 and 18 (note the troughs in the graph below).