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

Step 1: Analyze the question We need the term for the energy change when a gaseous atom gains an electron.

Step 2: Explain the concepts

• Atomization: energy required to break a substance into gaseous atoms (e.g., the enthalpy of atomization).
• Electron affinity: energy change when one mole of gaseous atoms each gains an electron to form gaseous negative ions. X(g) + e⁻ → X⁻(g)
• Electronegativity: a measure of an atom's tendency to attract shared electrons in a covalent bond. It is not an energy change for a gaseous atom, and it has no units on the Pauling scale.
• Ionization energy: energy required to remove an electron from a gaseous atom. X(g) → X⁺(g) + e⁻

Step 3: Match the definition The question describes the addition of an electron to a gaseous atom, which is exactly the definition of electron affinity. Ionization is the opposite process (removal of an electron), and electronegativity concerns bonding rather than an isolated gaseous atom.

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

Tip: Remember "Affinity = Add, Ionization = Imove (remove)". Electron Affinity → Adding an electron; Ionization → taking an electron out. Also note that for most atoms, such as the halogens, the first electron affinity is exothermic (energy is released), which is why chlorine readily forms Cl⁻.