What happens to electron affinity as you move across a period beginning with Group 1? Why do these values change as they do?

The periodic table is one of the most informative and comprehensive tools for understanding chemical behavior. It is organized in such a way that elements with similar properties fall into the same column, known as a group, and elements with increasing atomic number are placed from left to right in rows, known as periods.

When discussing electron affinity, we focus on the trends observable as we move across a period. Beginning with Group 1, which houses the reactive alkali metals, electron affinity values increase as you move towards Group 17, the halogens, which are highly eager to gain an extra electron to complete their outer shell. This trend reflects how the chemical properties of elements change in a predictable way across a period.

Atomic structure plays a crucial role in understanding why electron affinity trends are observable across a period. At the heart of an atom lies the nucleus, containing positively charged protons and uncharged neutrons. Orbiting this nucleus are electrons, which are arranged in electron shells based on their energy levels.

As you move horizontally from Group 1 to Group 18, the number of protons in the nucleus increases, enhancing the positive charge at the center of the atom. Electrons added to the same energy level experience an increased pull from the nucleus due to the additional protons. This resulting enhanced attraction of the atom for incoming electrons exemplifies why electron affinity generally increases across a period.

Electron shells are like concentric circles around the nucleus where electrons reside. The first shell is closest to the nucleus and has the lowest energy level, and as you move outward, the energy of shells increases. Each shell can hold a certain maximum number of electrons, and a shell must be filled before electrons can start filling the next one.

Elements with a nearly complete outermost shell have a high electron affinity because adding one more electron would complete the shell, resulting in a more stable and energetically favorable state. This is why, despite the overall trend of increasing electron affinity across a period, Group 18 elements have a low electron affinity; their shells are already full, and they have no tendency to accept additional electrons that would upset their stable configuration.