Hexad

Hex″ad (?), n. [L. hexas, hexadis, the number six, Gr. �, �, fr. ἕξ six.] (chem.) An atom whose valence is six, and which can be theoretically combined with, substituted for, or replaced by, six monad atoms or radicals; as, sulphur is a hexad in sulphuric acid. Also used as an adjective.