The term radical can have two distinct meanings in chemistry. The first is that a radical is an atom or molecule with free unpaired electrons. This causes them to be highly reactive as they try to bond these electrons with other atoms. In this sense, a radical is no different from a free radical. In the other sense, a radical is a collection of atoms that act as a common entity in reaction. For example, the ammonium ion, NH₄⁺, is a radical in that it can react as a single entity:

NH₄OH + NaHSO₄ → Na(NH₄)SO₄ + H₂O

Radicals in this second sense are often enclosed in parentheses in the expressions of chemical formulas (in this case the radical is the trivalent phosphate anion, PO₄^3-):

Fe₃(PO₄)₂

Radicals, depending on whether they are largely ionic or covalent in character, may also be referred to as a polyatomic ion or a functional group. A radical, such as ferrocyanide, with a central metal atom and dominated by coordination chemistry with ligands, would be called a complex ion.

It needs to be understood that radicals in the first sense may not be radicals in the second sense, and vice versa (though there are radicals that have both properties, for example the superoxide anion is both a functional group and has an unpaired electron). An example of the first kind of radical is the hydroxyl radical, an example of the second is the ammonium ion. Simple examination of the Lewis structure of ammonium will reveal a completed octet around the central nitrogen, hence it has no unpaired electrons.

Historically, the term radical in the second sense was in common use by the second decade of the 20th century, long before the concept of spin angular momentum was discovered by Dirac.

Diagnostics

Radical diagnostic techniques include:

• Electron spin resonance (EPR)
• Chemical labelling by quenching with free radicals, e.g. with NO or DPPH , followed by spectroscopic methods like X-ray photoelectron spectroscopy (XPS) or absorption spectroscopy, respectively.