Valence

If we assume that the molecule is held together by chemical bonds, without knowing more, we could write numerous structures such as H−H−H−H−H−C−C−Br,H−C−Br−H−H−C−H−H, and so on. However, if we also know of the existence of stable H2, but not H3; of stable Br2, but not of Br3; and of stable CH3Br, CH2Br2, CHBr3, and CBr4, but not of CH4Br, CHBrCHBr, CBrCBr, and so on, a pattern of what is called valence emerges. It will be seen that the above formulas all are consistent if hydrogen atoms and bromine atoms form just one bond (are univalent) while carbon atoms form four bonds (are tetravalent). This may seem almost naively simple today, but a considerable period of doubt and uncertainty preceded the acceptance of the idea of definite valences for the elements that emerged about 1852.