> as lemmas

1900 E. F. Smith tr. V. von Richter Org. Chem. (ed. 3) II. 435 The basal element of these rings is carbon, and accordingly the members not produced by C-atoms are designated as hetero-atoms.

1949 G. B. Bachman Org. Chem. xxvii. 336 O-, S-, and NH-containing rings undergo substitution practically exclusively at the carbons holding the hetero atom.

1966 McGraw-Hill Encycl. Sci. & Technol. (rev. ed.) VI. 427/2 The number of heteroatoms in any one ring is commonly one or two, less commonly three or more.

1967 A. R. Katritzky & J. M. Lagowski Princ. Heterocyclic Chem. v. 144 (heading) Four or five heteroatoms. Tetrazoles are formed by the action of nitrous acid on amidrazones, and pentazoles from the reaction of diazonium cations with azide anions.

1968 New Scientist 31 Oct. 268/3 Heterocyclic compounds containing nitrogen, sulphur and oxygen as the heteroatom.