When a neutral atom gains or loses an electron, what is this process called?

When a neutral atom gains or loses an electron, the process is referred to as ionization. Ionization occurs because the atom transitions from a neutral state, where the number of protons equals the number of electrons, to an ionized state where there is an imbalance between the number of protons and electrons. If an electron is lost, the atom becomes a positively charged cation, while gaining an electron results in a negatively charged anion. This change in charge significantly alters the chemical properties and reactivity of the atom.

The concept of ionization is foundational in understanding how atoms interact in chemical reactions and how they respond to various forms of energy, including electromagnetic radiation. Unlike other options, such as excitation, which refers to an electron moving to a higher energy level without changing the overall charge of the atom, or electrification and electron transfer, which might involve broader concepts of charge and electricity rather than specifically focusing on the addition or removal of electrons from an atom's outer shell, ionization directly addresses the altering of an atom's charge through electron gain or loss.