Kossel and Lewis approach to chemical bonding

W. Kossel and G. N. Lewis, working independently, gave the first satisfactory electron-based account of chemical valence in 1916, building their Electronic Theory of Valence on the observed chemical inertness of the noble gases. Lewis pictured an atom as a positively charged 'kernel' (its nucleus plus all the inner, non-valence electrons) surrounded by an outer shell able to hold at most eight electrons — a stable octet. From this came the octet rule: during bond formation, an atom loses, gains, or shares electrons until its own outermost (valence) shell contains eight electrons, i.e. until it reaches the electronic configuration of the nearest noble gas ($ns^2np^6$). The octet rule usefully predicts the normal valence of most elements and helps explain how atoms combine into molecules, but it is not universal — hydrogen and lithium, in particular, are perfectly stable with only TWO valence el …