= Electron attachment enthalpy
{title2=$\Delta H_{\rm attach}^{\circ}(T)$}
An <electron attachment enthalpy> is the signed molar <enthalpy> change for a stated attachment reaction,
$$
X(g)+e^-(g)\longrightarrow X^-(g).
$$
For favourable attachment such as <chlorine>'s first attachment, the <electron attachment enthalpy> is negative. Its leading isolated-ground-state <energy> contribution is $-N_{\rm A}A_{\rm ea}$, where $N_{\rm A}$ is the <Avogadro constant>. At finite <temperature>, consistent thermal populations, translational contributions and the $pV$ term in <enthalpy> must also be included, together with the same gaseous-<Electron> reference used in the corresponding <ionization energy> term. Consequently a measured spectroscopic <electron affinity> is not automatically an exact $298.15\,\mathrm K$ attachment <enthalpy> with a minus sign. A <Born-Haber cycle> must use compatible <enthalpies> throughout.
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