Pen"tad (?), n. [Gr. &?;, &?;, a body of five, fr. &?; five.] (Chem.) Any element, atom, or radical, having a valence of five, or which can be combined with, substituted for, or compared with, five atoms of hydrogen or other monad; as, nitrogen is a pentad in the ammonium compounds.

Pen"tad, a. (Chem.) Having the valence of a pentad.