The energy change that accompanies the addition of an electron to an isolated gaseous atom is

Step 1: Analyze the question The question asks for the term that describes the energy change when an electron is added to an isolated gaseous atom.

Step 2: Explain the concepts

• Bond energy: the energy required to break a covalent bond between two atoms. It involves bonds, not adding an electron to a single atom.
• Electronegativity: the tendency of an atom to attract shared electrons in a bond. It is a relative tendency with no units of energy, and it applies to atoms in bonds, not isolated atoms.
• Electron affinity: the energy change when one mole of isolated gaseous atoms each gain an electron to form gaseous negative ions. X(g) + e⁻ → X⁻(g)
• Ionization energy: the energy required to remove an electron from an isolated gaseous atom. X(g) → X⁺(g) + e⁻

Step 3: Match the definition The key phrases are "addition of an electron" and "isolated gaseous atom". Together these match the definition of electron affinity. Ionization energy is the opposite process, since it involves removal of an electron.

Step 4: Answer The correct option is C. electron affinity.

Exam tip: Remember "Affinity = Adding" an electron, while "Ionization = Injuring (removing)" an electron. For the first electron affinity of most elements, energy is released (exothermic), which is why halogens form negative ions so easily.