What term describes the amount of energy released when an electron joins an atom to form a negative ion?

The term that describes the amount of energy released when an electron joins an atom to form a negative ion is electron affinity. This concept specifically relates to the energy change associated with the addition of an electron to a neutral atom in the gas phase. When an electron is added, and the atom becomes negatively charged, energy is typically released as the electron experiences an attractive force from the positively charged nucleus.

Electron affinity values can vary across the periodic table; generally, elements that have a strong tendency to gain electrons (like halogens) exhibit higher electron affinity, indicating that they release more energy upon gaining an electron. Understanding electron affinity is crucial in the context of how atoms interact with electrons and form anions, thus affecting many chemical properties and reactivities of elements.

Ionization energy, in contrast, refers to the energy required to remove an electron from an atom, while nuclear binding energy pertains to the energy that holds the nucleons together in the nucleus. Electronegativity describes an atom's ability to attract shared electrons in a chemical bond and doesn't directly relate to the process of gaining an electron to form a negative ion.