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.
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$\begingroup$ What do you think the formula should be? $\endgroup$ – bon Dec 30 '15 at 13:55
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1$\begingroup$ N2O3 - this should be the formula $\endgroup$ – Imaginary Pumpkin Dec 30 '15 at 13:56
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3$\begingroup$ 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. $\endgroup$ – bon Dec 30 '15 at 14:02
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1$\begingroup$ Because it is useful for quickly getting the formulae of many compounds that you are likely to meet at that level. $\endgroup$ – bon Dec 30 '15 at 14:10
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1$\begingroup$ @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. $\endgroup$ – bon Dec 30 '15 at 14:25
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The rule of swapping the valencies of the atoms to get their ratios only works for a very specific set of compounds and is more of a convenient trick than a rule based on any sort of hard theory.
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).
Ultimately the only way to determine the formula of any arbitrary substance is through experiment, typically with a mass spectrometer.
If you are interested in finding out more about the different types of substances that are possible I suggest that you start reading about different types of bonding, in particular ionic versus covalent, to begin to understand the different types of compounds that exist.
