= Electron affinity
{title2=$A_{\rm ea}$}
{wiki}
<Electron affinity> measures how strongly an isolated neutral species binds an additional <electron>. In the spectroscopic convention used here, it is the positive <energy> required to detach that <electron> from a bound <anion>. With the free <electron> at rest assigned zero <energy>,
$$
A_{\rm ea}=E(X)+E(e^-)-E(X^-),\qquad
\Delta E_{\rm attach}=-A_{\rm ea}.
$$
The <quantum states> of $X$ and $X^-$ must be specified, usually their <ground states>. Positive $A_{\rm ea}$ means that attachment lowers the <energy>; a signed attachment <energy> is then negative. This convention agrees with the IUPAC definition and with the atomic tables used for <Mulliken electronegativity>.<IUPAC Compendium of Chemical Terminology (Gold Book) by International Union of Pure and Applied Chemistry/electron affinity>{ref}
For <chlorine>, a published <photodetachment threshold> is $A_{\rm ea}/(hc)=29138.59(22)\,\mathrm{cm^{-1}}$, approximately $3.612725\,\mathrm{eV}$ per <chemical atom>. Here $h$ is the <Planck constant>, $c$ the <speed of light>, and the inverse-length unit expresses an <energy> through a spectroscopic <wavenumber>. The parentheses give the reported uncertainty in the last digits, not a range of possible sign conventions.<Isotope shift in the electron affinity of chlorine by U. Berzinsh and others>[§2.3, printed pp. 6–7: reported threshold 29138.59(22) inverse centimetres]{ref}
\Image[/electron-affinity-attachment-detachment-levels.png]
{title=<Electron affinity> of <chlorine>: positive detachment <energy> and negative attachment <energy> connect the same isolated-species levels; a finite-temperature <electron attachment enthalpy> needs additional corrections}
{id=image-electron-affinity-attachment-detachment-levels}
An <electron affinity> is neither an <electronegativity> nor a <lattice energy>. The first describes a specified isolated-species <energy> difference, <electronegativity> compares attraction for <electron density> in a <chemical bond>, and <lattice energy> refers to an entire <ionic crystal> and separated gaseous <ions>. A stable <anion> can therefore help explain <ionic bonding> without describing the full <energy> balance for making a <salt>.
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