This year marks the sesquicentennial of Edward Frankland's paper, On a New Series of Organic Bodies containing Metals, [Phil. Trans. Royal Soc., 142, 417-444 (1852)] in which he reported the preparation of organometallic compounds of Zn, Hg, As, Sb, and Sn. "...the compounds of nitrogen, phosphorus, antimony, and arsenic especially exhibit the tendency of these elements to form compounds containing 3 or 5 equivs. of other elements, and it is in these proportions that their affinities are best satisfied ... Without offering any hypothesis regarding the cause of this symmetrical grouping of atoms may be, the combining power of the attracting element, if I may allow the term, is always satisfied by the same number of atoms." Today we use the term valence rather than "affinity" or "combining power".
Edward Frankland (1825-1899)