# 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.

• N2O3 - this should be the formula Dec 30, 2015 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, 2015 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, 2015 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, 2015 at 14:25
• @SujithSizon $\ce{NO2}$ exists in equilbrium with $\ce{N2O4}$. The position of equilibrium varies with temperature, pressure etc.
– bon
Dec 30, 2015 at 14:34

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.

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.