These oxidation numbers (ON) of H and O in H₂O arise because O is more electronegative than H, so both the electrons in each OH bond are assigned to O. If you are unsure of this process, look back at Chapter 6. This is such a simple chemical reaction that it can be balanced by inspection - by making sure that the same number of atoms of H and O are on each side of the equation. The balanced equation is



We also could have balanced the equation by seeing to it that the net change in oxidation number of all substances was zero. If the oxidation number of one oxygen atom decreases by two, then two hydrogen atoms each must increase by one. In physical terms, if one oxygen atom pulls two electrons toward itself, then two hydrogen atoms are required to donate one electron each. In terms of changes in oxidation number,

2H O
Changes in ON: 2(+1) + (-2) =0

This was a trivial example, but the following example is not quite so trivial. If oxygen is in short supply, some bacteria can respire using nitrates as sources of oxidizing power instead of O₂. Rather than reducing oxygen to water, these bacteria reduce nitrates to NO₂ , NO, or N₂ .