octad
  八個一組,八價元素
  Oc·tad n.
  1. Chem. An atom or radical which has a valence of eight, or is octavalent.
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  octad
       n : the cardinal number that is the sum of seven and one [syn: eight,
            8, VIII, eighter, eighter from Decatur, ogdoad,
            octonary, octet]