1852 – England
‘The capacity of a given element to combine with other elements to form compounds is determined by the number of chemical bonds that element can form with other elements’
This ‘combining power’ is now termed valency or valence.
Valency is the number of electrons an atom of an element must lose or gain, either completely or by sharing, in order to form a compound. This leaves the atom with the stable electronic configuration of a noble gas (that is a completely full outer shell). For example, in H2O, hydrogen has a valency of +1 (H+) and Oxygen -2 (O-2). Two hydrogen atoms lose one electron each; one oxygen atom gains these two electrons.
Every atom has a fixed number of bonds that it can form, and to be stable all of these must be employed. If a hydrogen atom bonds to another hydrogen atom, then the bonds on each atom will be fully used in forming H2, a molecule of hydrogen. The same can occur between two atoms of oxygen. Alternatively, the two bonds on oxygen could be occupied by the bonds on two hydrogen atoms, forming water, H2O.
Frankland understood that only molecules in which atoms had all of their bonds occupied were stable. Most elements have a fixed valency, although some have more than one. The numerical values of valences represent the charge on the ion.
The concept of valence was further developed by FRIEDRICH AUGUST KEKULE who decided that the valence of carbon must be four which allowed carbon to form into chains of atoms or link into closed, six-atom rings. In the simplest such molecule, three of each carbon’s bonds are used to keep the ring together and the remaining bond on each carbon binds to a hydrogen atom. The resulting molecule of benzene contains six atoms of carbon and six atoms of hydrogen.
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