Enthalpy of Atomisation, Ha

•The heat change when 1 mole of gaseous atoms is formed from its element

     •DHa is always positive because it involves only breaking of bonds

•e.g:

  Na(s) ¾® Na(g)  DHa = +109 kJ mol-1

  ½Cl2(g) ¾® Cl(g)  DHa = +123 kJ mol-1

The enthalpy of atomization (also atomisation in British 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 Δ_atH^o or ΔH_a^o. The associated standard enthalpy is known as the Standard enthalpy of atomization, Δ_atH^o/(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).