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Radicals are highly reactive species, that have one or more unpaired electrons. Whether a species has unpaired electron(s) can be experimentally determined through electron pair resonance spectroscopy. Radicals are paramagnetic. This tag should be applied to questions that involve free radicals as a species or radicals in reactions and their mechanisms.

Free radicals, or commonly abbreviated as radicals, are atoms, molecules or ions that bear unpaired electrons in their valence shell, usually in their highest occupied molecular orbital (HOMO). They are therefore usually highly reactive species, that are often unstable. They react with each other or the system to form more stable diamagnetic products, or they fragment into smaller molecular entities.

Example Benzoyl peroxide $\ce{Ph-CO-O-O-OC-Ph}$:

The peroxyl bond can be split thermally or photoelectrically, resulting in a chain reaction to form carbon dioxide and the phenyl radical. $$\ce{Ph-CO-O-O-OC-Ph ->[h\nu/\Delta T] 2 Ph-COO. ->[][-2\ce{CO2}] 2Ph.}$$ It is most commonly used in radical polymerisation as a starting agent.

There are many examples of small persistent or stable molecules based on carbon nitrogen and oxygen. For example, $\ce{NO}$ and $\ce{NO2}$ have been vigorously investigated, both, experimentally and theoretically.

Radicals can also be stabilised with sterically demanding moieties, at least long enough to investigate these compounds. One of the earliest examples dates back to the 19th century, the so-called Gomberg radical, triphenylmethyl radical, $\ce{Ph3C.}$. This can be considered kinetically stable since recombination with itself is prevented.

Usually, a combination of electrical effects, partial spin delocalisation and/or steric hindrance is used to make radicals more persistent.

Radicals can be synthesised from homolytic bond cleavage, and one-electron reductions or oxidations.

A prominent example for molecules with more than one unpaired electrons is the dioxygen molecule, $\ce{O2}$, which has a triplet ground state.

Whether a species has unpaired electron(s) can be experimentally determined through electron pair resonance spectroscopy, as Radicals are paramagnetic. Analysis of the Lewis structure and molecular orbital theory can also predict unpaired electrons. It is also possible to predict the stability of radicals with modern quantum chemical investigations.