James Richard Fromm
When the stoichiometry of molecules was first understood in the nineteenth century, and came to be used to discuss their structure, the concept of valence or combining capacity was employed. A valence was generally indicated by a short line joining the atoms in a molecule. For example, the structure of water was written H-O-H which indicated two one-valence hydrogen atoms attached to a two-valence oxygen atom. The number of valences was generally characteristic of the type of atom; thus hydrogen had one valence, oxygen two, and chlorine one. Nitrogen had either three or four valences depending upon the particular compound in which it was found.
A more quantitative picture of bonding was introduced by the American physical chemist G. N. Lewis. Lewis suggested that the valences should be replaced by the valence (outer) electrons. The valence electrons are the electrons used by an atom in chemical bond formation. Valence electrons include the electrons in the outermost shell of the atom, which is often called the valence shell. When electrons in inner shells of the atom are also used in bonding, as occasionally happens, then these electrons are considered valence electrons also even though they are not in the valence shell.
The Lewis structure of an atom is written as the element symbol, which represents the nucleus of the atom and its surrounding inner electrons which are not used in chemical bonding, and a surrounding group of dots which represent the valence electrons. Since two electrons, or more correctly one pair of electrons, are used in the formation of a single covalent bond, the surrounding group of electrons are usually written in pairs.
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