The enthalpy change of atomisation refers to forming what from what?

• A. One mole of gaseous atoms from its elements
• B. One mole of molecules from atoms
• C. One mole of ions from atoms
• D. One mole of electrons from atoms

Answer: (A)

Atomisation energy is the energy required to convert the element from its standard state into gaseous atoms. In other words, it’s the enthalpy change when one mole of gaseous atoms is produced from the element in its most stable form at 1 atm and 298 K. For elements that exist as diatomic molecules in their standard state, such as oxygen, you break the bond to yield two atoms, and the process is endothermic. This definition matches the option describing forming gaseous atoms from the elements. The other options describe forming molecules from atoms, forming ions, or producing electrons, which are not what atomisation measures.