How is electron affinity defined?

Electron affinity is defined as the energy change that occurs when a neutral gaseous atom gains an electron. This process can either release energy or require energy, depending on the atom and the circumstances. When an atom captures an electron, it typically moves from a higher energy state to a lower energy state, resulting in the release of energy, which is indicative of a favorable process.

Understanding this concept is crucial, as electron affinity provides insight into an atom's reactivity and the tendency to form negative ions. The greater the electron affinity, the more likely the atom is to gain an electron and form an anion. This concept is particularly important in predicting the behavior of elements in chemical reactions, especially for nonmetals, which generally have higher electron affinities compared to metals.

The other choices relate to different concepts in atomic structure and behavior. The removal of an electron pertains to ionization energy, and stability in noble gas configurations requires a different context focused on electron configurations rather than on the addition of electrons. Thus, the correct definition is specifically tied to the process of gaining an electron, encapsulating the core principle of electron affinity.