some gases such as NO2 and NF2 don't obey boyle's law at any pressure.why?

In the case of NO2, if you count up the valence electrons, you will realize that it has an odd number of electrons. The presence of an unpaired electron makes it particularly reactive, and in fact, it always exists in an equilibrium with the gas N2O4. When you decrease the volume or increase the pressure of the gas, NO2 is more likely to interact with itself and shift the equilibrium in favor of N2O4. Doing the reverse has the opposite effect, and N2O4 tends to fall apart into two molecules of NO2.

NO2 essentially violates the notion that ideal gas molecules do not interact with each other at every temperature and pressure. It's not so much that NO2 isn't ideal, because no gas is ideal. It just doesn't approximate an ideal gas at normal temperatures and pressures like many other gases do. It is too reactive.

To my knowledge, NF2 does not exist naturally, but if it did, it would probably have the same problem of reacting with itself. It would have an odd number of electrons and would probably tend to dimerize.

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