Standard enthalpy change of atomisation

The standard enthalpy change of atomisation, ΔHat o, is the change in enthalpy when one mole of atoms in the gaseous state is formed from the most stable form of its element under standard conditions.

The atomisation process is always endothermic. Some examples are:

C(graphite)\rightarrow C(g)\; \; \; \; \; \; \; \Delta H_{at}^{\: o}=717\: kJmol^{-1}

\frac{1}{2}O_2(g)\rightarrow O(g)\; \; \; \; \; \; \; \Delta H_{at}^{\: o}=249\: kJmol^{-1}

The standard enthalpy of atomisation of an atom is the same as the standard enthalpy of formation of that atom and the standard enthalpy of the reaction to form that atom. The standard enthalpy of atomisation of a species that exist as a homonuclear diatomic molecule under standard conditions, e.g. O2, is also the bond enthalpy of that species. So,

\Delta H_{at}^{\: o}[O]=\Delta H_{be}[O=O]=249\: kJmol^{-1}


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