What defines the specific enthalpy of atomization?
ChemGuide says that 'The standard atomisation enthalpy is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state.'

For example,
\begin{array}{rc}
\ce{1/2 Cl2 (g) -&gt; Cl (g)} &amp; \Delta H_{a}^{0} = +122~\mathrm{kJ/mol} \\
\ce{1/2 Br2 (l) -&gt; Br (g)} &amp; \Delta H_{a}^{0} = +112~\mathrm{kJ/mol} \\
\ce{Na (s) -&gt; Na (g)} &amp; \Delta H_{a}^{0} = +107~\mathrm{kJ/mol}
\end{array}