For ions, the valence equals the electrical charge. In molecules, the various atoms are assigned chargelike values so the sum of the oxidation numbers equals the charge on the molecule. For example, in the H 2O molecule, each H has an oxidation number of +1, and the O is –2.
In Table, the common oxidation numbers in the last column are interpreted as the result of either losing the valence electrons (leaving a positive ion) or gaining enough electrons to fill that valence subshell. Table 2 compares three ions and a neutral atom.
The charges on the chlorine, potassium, and calcium ions result from a strong tendency of valence electrons to adopt the stable configuration of the inert gases, with completely filled electronic shells. Notice that the three ions have electronic configurations identical to that of inert argon. These ions and the atom of argon are known as isoelectronic.