pen·tad /ˈpɛnˌtæd/
  五個一組;五年間;五價元素
  Pen·tad n.  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.
  ◄ ►
  pentad
       n : the cardinal number that is the sum of four and one [syn: five,
            5, V, cinque, quint, quintet, fivesome, quintuplet,
            fin, Phoebe, Little Phoebe]