Atomisation energy is defined as which of the following?

• A. The enthalpy change that takes place when one mole of gaseous atoms forms from the element in its standard state
• B. The energy change when a molecule is vaporized
• C. The energy released when a bond is formed
• D. The energy change when a solid is melted

Answer: (A)

Atomisation energy is the energy required to convert one mole of a substance from its standard state into gaseous atoms. This enthalpy change reflects breaking all the bonds holding the atoms together in the element or compound, producing isolated gas-phase atoms. Because bonds are being broken, the process is endothermic and the value is positive. For a diatomic molecule, it equals the energy needed to break the bond and yield two gaseous atoms. By contrast, vaporization energy relates to turning a liquid into gas, bond formation energy is the energy released when bonds form, and melting energy is the energy to turn a solid into a liquid.