The enthalpy of atomisation (also atomization in American spelling) is the enthalpy change that accompanies the total separation of all atoms in a chemical substance (either a chemical element or a chemical compound). This is often represented by the symbol ΔatHo or ΔHao. The associated standard enthalpy is known as the Standard enthalpy of atomization, ΔatHo/(kJ mol−1), at 298.15 K (or 25 degrees celsius) and 101.3 kPa.
The enthalpy change of atomization of gaseous H2O is, for example, the sum of the HO–H and H–O bond dissociation enthalpies. The enthalpy of atomization of an elemental solid is exactly the same as the enthalpy of sublimation for any elemental solid that becomes a monatomic gas upon evaporation.
There is a difference between a solid converting to a gas, and a diatomic gaseous element converting to gaseous atoms. The standard enthalpy change is based purely on the production of one mole of gaseous atoms.
Standard enthalpy of atomization is the enthalpy change when 1 mol of atoms in the gas phase is formed from its element in its defined physical state under standard conditions (298.15K, 1 atm). All bonds in the compound are broken in atomization and none are formed, so enthalpies of atomization are always positive.
This is incorrect. The enthalpy of atomisation is the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state and under standard conditions.