Enthalpy of atomization

The enthalpy of atomization (also standard enthalpy of atomisation - US 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 ΔH_at^O. The associated standard enthalpy is known as the Standard enthalpy of atomization, ΔH_at^O/(kJ·mol^-1), at 298 K and 1 atmosphere of pressure, or 100 kPa (kilopascals.)

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The enthalpy 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 is purely based on the production of one mole of gaseous atoms.