= Radical
{disambiguate=chemistry}
{wiki}
= Free radical
{synonym}
A <radical (chemistry)> has one or more unpaired <Electrons>. In a simple <atomic orbital> or <molecular orbital> occupancy picture, an unpaired <Electron> occupies a spatial orbital without an opposite-<spin> partner in that orbital. A dot such as $\mathrm H^{\bullet}$ marks this electronic property; it does not mean a missing <Electron> or positive <electric charge>. <Free radicals> can be neutral or charged. Terminology also has a boundary: metal <ions> exhibiting <paramagnetism> are not normally called <free radicals> just because they have unpaired <Electrons>.<Glossary of Terms Used in Physical Organic Chemistry (IUPAC Recommendations 1994): R entries by International Union of Pure and Applied Chemistry>[Glossary of Terms Used in Physical Organic Chemistry, IUPAC Recommendations 1994: radical (or free radical), opening definition and metal-ion qualification]{ref}
Compare three species containing the same single <proton>. The <hydrogen atom> $\mathrm H^{\bullet}$ has one <Electron> in its $1s$ <atomic orbital>, and is a neutral <radical (chemistry)>. The <hydrogen> <cation> $\mathrm H^+$ has no <Electrons>, so no unpaired <Electron>. The <hydrogen anion> $\mathrm H^-$ has two <Electrons> paired in its $1s$ <atomic orbital>. It is charged but is not a <radical (chemistry)> in its <ground state>. With $N_p$ <protons>, $N_e$ <Electrons>, and positive <elementary charge> $e$, the independent <electric charge> count is
$$
q=(N_p-N_e)e;\qquad
\mathrm H^+:q=+e,\quad \mathrm H^{\bullet}:q=0,\quad \mathrm H^-:q=-e.
$$
The <electron pair> and <electric charge> questions require different information. An odd $N_e$ ensures that not all <Electrons> can be paired; an even $N_e$ alone does not ensure pairing, as the <molecular orbital> description of <molecular oxygen> illustrates.
\Image[/radical-charge-and-orbital-occupancy.png]
{title=<Electric charge> and unpaired <Electrons> are different properties}
{id=image-radical-charge-and-orbital-occupancy}
{description=Left: actual <ground state> $1s$ occupancies of $\mathrm H^+$, $\mathrm H^{\bullet}$, and $\mathrm H^-$. Right: stipulated two-orbital models with fixed $N_p=2$ illustrate neutral, positive, and negative <free radicals>; those boxes are not assignments for particular <chemical substances>. Arrows label <spin> occupancy, not particle trajectories.}
The distinction matters whenever a <chemical reaction> mechanism uses dots and charge signs together. <Homolysis> gives one bonding <Electron> to each fragment, while <heterolysis> assigns both to one fragment. A <radical chain reaction> can pass an unpaired <Electron> from one species to another without generating net <electric charge>. Detection then exploits the unpaired <Electron>'s <spin> and <magnetic moment>, rather than merely measuring whether the sample carries an <electric charge>.
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