# Why is NO2 formed when N valency is 3

Sorry if the question is silly but this is troubling me more than enough. Here's what I know about chemical formula: Reverse the valencies of the combining elements. But in this case, $\ce{NO2}$ is the formula for nitrogen dioxide while N valency is 3. Please explain it to me if I am wrong somewhere.

• What do you think the formula should be? – bon Dec 30 '15 at 13:55
• N2O3 - this should be the formula – Imaginary Pumpkin Dec 30 '15 at 13:56
• Your rule of swapping the valencies only works for a very specific set of compounds. In general there are a huge number of compounds where this is not the case and indeed the rule is merely a coincidence and has no real theoretical basis. – bon Dec 30 '15 at 14:02
• Because it is useful for quickly getting the formulae of many compounds that you are likely to meet at that level. – bon Dec 30 '15 at 14:10
• @SujithSizon $\ce{N2O3}$ is a separate compound. It is formed from the reaction of $\ce{NO}$ and $\ce{NO2}$ but it is not a mixture of them. – bon Dec 30 '15 at 14:25

In particular, it works for binary 'ionic' compounds such as $\ce{NaCl}$, $\ce{CuCl2}$, $\ce{Al2O3}$ etc. because the valencies of the atoms correspond to the charges on the ions and the net charge has to be zero.
However, it obviously doesn't work for a huge range of other compounds where there is more complicated bonding. For example there are a whole range of covalent nitrogen oxides of varying degrees of stability which do not fit the rule ($\ce{N2O}$, $\ce{NO}$, $\ce{NO2}$, $\ce{N2O3}$, $\ce{N2O4}$ etc).