In order for us to find oxidation numbers, we need to learn some oxidation rules first.
1. Oxygen is -2, unless in peroxides.
2. Group 1 metals = +1
3. Group 2 metals = +2
4. If the molecule is neutral, all of the oxidation numbers have to add up to zero.
5. If the molecule is charged, all of the oxidation numbers have to add up to the charge of the molecule.
When it comes to calculation the oxidation number of N in KNO3, we can make a small equation. From our rules we know that O= -2 and we can find K on the periodic table, in the first group, thus giving it a +1 charge. Now let's put it all together. K=+1 N=x O=-2. Let's take into account the number of atoms of each element we have and make an equation since we know everything has to add up to zero since the molecules is neutral.
+1 +x+3(-2) =0 (notice we multiplied 3 by -2, because in the formula we have 3 atoms of oxygen with -2 charge each)
Therefore, the oxidation number of N in KNO3 is +5
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