Chem. [f. Gr. ἄρτι-ος even + -AD 1 a.] A chemical element or radical with even degrees of quantivalency or atomicity, e.g., a dyad, tetrad or hexad, in opposition to perissads, which include monads, triads and pentads.
1870. Watts, Dict. Chem., VI. 238. The elements may be divided in this respect into two classes, one of odd, the other of even equivalence, the former distinguished as perissads, the latter as artiads.
