May 31st, 2007
The term oxidation state is a convenient way of describing the effective condition of an atom in a compound in terms of the charge that it would have if it were ionic. It is not intended to suggest that atoms have a specific charge, but it is a useful way of describing the state or condition that an atom of an element is in within a compound; useful for the purposes of redox chemistry.
Whether an element is assigned a specific oxidation state, for example, +1 or -1 oxidation state depends on what it is bonded to. THIS IS IMPORTANT
If an atom of a specific element is bonded to an atom that is MORE electronegative, then it is assigned a positive oxidation state, and vice versa.
In reality this assignment is for descriptive terms only – it doesn’t really imply anything about the atoms or the bonds apart from the fact that one atom will tend to attract electrons more than the other.
Take the following nitrogen example:
In ammonia, NH3, nitrogen is more electronegative than hydrogen and so is assigned a negative oxidation state of 3- (assuming that each hydrogen is +1)
When bonded to oxygen however, as is N2O, the nitrogen is assigned a positive oxidation state of +1 (as oxygen is -2 (usually except for in peroxides) when attached to something less electronegative)
The extreme case that highlights this is F2O.
What is the oxidation state of oxygen here? well since oxygen is LESS electronegative that fluorine, and fluorine always has an oxidation state of -1, it must be +2.
In summary, the most important factor to establish is which is the more electronegative of the two attached atoms. This atom takes the negative oxidation state, which can then be calculate from the valencies of the atoms in question.
May 31st, 2007