Define the "standard enthalpy change of atomisation".

This is a standard definition question that is relatively common in A level papers.

The standar molar enthalpy change of atomisation is defined as the enthalpy change when 1 mol of an element in its standard state is atomised to produce 1 mol of gaseous atoms.

By "standard state" we mean the state the element is naturally found, for example for Chlorine it is Cl2(g) and for Sodium it is Na(s).