(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.
Nitrogen is a pentad in the ammonium compounds.
Origin of pentad
Greek pentaspentad-group of fivefrompentefivepenkwe in Indo-European roots
American Heritage Dictionary of the English Language, 5th Edition
Pentad Sentence Examples
Those elements which are equivalent in combining or displacing power to a single atom of hydrogen are said to be univalent or monad elements; whilst those which are equivalent to two atoms of hydrogen are termed bivalent or dyad elements; and those equivalent to three, four, five or six atoms of hydrogen triad, tetrad, pentad or hexad elements.
Thus, it must be supposed that in nitric oxide, NO, an odd number of affinities are disengaged, since a single atom of dyad oxygen is united with a single atom of nitrogen, which in all its compounds with other elements acts either as a triad or pentad.
The oxide NO 2 must be regarded as another instance of a compound in which an odd number of affinities of one of the contained elements are disengaged, since it contains two atoms of dyad oxygen united with a single atom of triad or pentad nitrogen.
The month may be divided in two ways: a fractional part may be taken (decad or pentad), as in East Africa or Ancient Egypt (moon-week), or the week may be settled without regard to the length of the month (market-week, &c.).