Pentad
Pentad , noun
[Greek penta`s penta`dos, a body of five, from pe`nte five.]
(Chemistry) 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.
2.
Any grouping of five things.
Pentad , adjective
(Chemistry) Having the valence of a pentad.