1.3.9—Free radical as a species with one or more

A free radical is a species containing one or more unpaired electrons. The defining evidence is the electron arrangement, not whether the species is neutral or charged.

In an electron-box diagram, an unpaired electron is shown by a single arrow in an orbital; in a configuration, inspect the outer sub-shell occupancy. A dot can also represent the unpaired electron in a radical symbol.

A chlorine radical has a 3p⁵ outer arrangement: two 3p orbitals contain paired electrons and one 3p orbital contains one unpaired electron. The single electron is the evidence for radical status.

Do not equate radical with ion: a neutral species can be a radical, and charge alone does not establish an unpaired electron. The SME also links radical formation to homolytic fission, where a covalent-bond electron pair separates evenly; do not extend this card beyond that supported boundary.