PENTAD

Pen″tad (?), n. [[Gr. πεντάσ -άδοσ, a body of five, fr. πέντε 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.