What do you mean by the standard enthalpy of atomisation of chlorine at `25^(@)C=+121kJ*mol^(-1)`?

Explanation

This means that at `25^(@)C` and 1 atm pressure, 121 kJ of heat is required to produce 1 mol of gaseous Cl-atom from `Cl_(2)(g)`. Thus the change in enthalpy for the process,
`(1)/(2)Cl_(2)(g) to Cl(g),DeltaH_("atom")^(0)=+121kJ`.