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

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

Step 2: Explain the concepts

• Atomization: the energy needed to convert one mole of a substance into gaseous atoms.
• Electronegativity: the ability of an atom in a bond to attract the shared pair of electrons. It is a relative tendency, not an energy change for an isolated atom.
• Electron affinity: the energy change when one mole of electrons is added to one mole of isolated gaseous atoms 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 words are "addition of an electron" and "isolated gaseous atom". These match the definition of electron affinity exactly. Ionization is the opposite process (removal of an electron), and electronegativity applies to atoms in bonded situations.

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

Tip: Think of "Affinity = Attraction for an extra electron" (addition), and "Ionization = Isolating an electron by removal". Addition means affinity, removal means ionization.