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.




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.


 9

Well. Here is an explanation which is valid at the secondary level. At the university level, it is more correct to speak of bonding and anti-bonding molecular orbitals.


Let's go ! $\ce{NO2}$ is a unusual molecule that has one lonely electron on the $\ce{N}$. This should not happen. But look ! It also contains $4$ $\ce{N-O}$ bonds, with a negative partial charge on the oxygen side. As a consequence, there are $4$ partial positive charges on the central $\ce{N }$ atom. This is much ! Now, experience shows that $\ce{NO2}$ exists only at high temperatures. When cooling it at low temperatures ($T < 0°C$), it gets dimerized and becomes $\ce{N2O4}$. Unfortunately this molecule $\ce{N2O4}$ has a drawback. It contains a bond $\ce{N-N}$ between two atoms holding both $4$ partial positive charges. Similar electric charges are repelling one another. So the bond $\ce{N-N}$ is rather weak in $\ce{N2O4}$, and does not resist at absolute temperatures higher than $\pu{270 K}$. At room temperature for example, $\ce{N2O4}$ is dissociated into $\ce{2 NO2}$, at least partially (as it is an equilibrium).


As a consequence, $\ce{NO2}$ can be considered as a half-molecule ! Exactly like $\ce{H2}$ or $\ce{Cl2}$ which produce $\ce{H}$ or $\ce{Cl}$ atoms when heated to rather high temperatures.


 0