Redox Reactions Involving Nonmetals Only

The situation is a bit more complex when nonmetals atoms are involved. As all nonmetals have similarly high electronegativity values, it is unreasonable to assume that there will be a transfer of electrons between them in an oxidation-reduction reaction. In these instances the valence electrons involved can no longer be thought of as being "lost or gained" between the atoms, but instead, are only partially transferred, moving closer to that atom which has the higher electronegativity (and away from the atom of lower electronegativity). This "shift" of electrons results in an unequal distribution of charge, as the more electronegative atom becomes more "negative" and the atom of lower electronegativity becomes more "positive".

The accurate determination of the distribution of charge resulting from these "electron shifts" is very difficult, but guidelines have been devised to simplify the process. In general, these guidelines assign the more electronegative atom a negative oxidation state, and the atom with the lower electronegativity, a positive oxidation state. One should be aware that these guidelines are at best, arbitrary approximations, and in some instances may have to be supplemented by additional methods.

Guidelines - Oxidation States of Nonmetals

  1. When two, nonmetals react with each other, the more electronegative element is assigned the negative oxidation state.

    1. Fluorine, the most electronegative element, is always assigned an oxidation state of "-1" when combined with any other element.

    2. Hydrogen, whenever it is combined in a molecule, is assigned an oxidation state of "+1".

    3. When hydrogen combines with metals in forming compounds called, metal hydrides, it is assigned an oxidation state of (-1)

  2. Oxygen, in most compounds, is usually assigned an oxidation state of "-2".

    1. However, when it is found in peroxides (" O - O bonds ") it is assigned a value of "-1"; or when combined with fluorine, it is assigned a value of "+1".

  3. The sum of the oxidation states of every element in a substance or species (it may be an ion or a molecule) must always equal the electrical charge indicated for that substance or species.

    1. any monatomic ion has an oxidation state equal to its charge

    2. the sum of the oxidation states of all atoms in a compound must equal zero.

    3. the sum of the oxidation states of all atoms in a polyatomic ion must equal the charge of the ion.

See exercise 5