What is the standard enthalpy change of atomisation defined as?

• A. The enthalpy change when one mole of gaseous atoms is formed from its elements.
• B. The enthalpy change when a solution forms.
• C. The enthalpy change when a gas dissolves.
• D. The enthalpy change when one mole of molecules is formed.

Answer: (A)

Standard enthalpy change of atomisation is the energy required to turn the elements in their standard states into gaseous atoms. In other words, it is the enthalpy change when one mole of gaseous atoms is formed from its elements in their standard states. For a diatomic element like chlorine, this means breaking Cl2(g) into 2 Cl(g), so the value is half the bond dissociation energy of Cl2 per mole of atoms. This differs from dissolving a gas or forming a solution, and from forming molecules from elements, which is the standard enthalpy of formation.