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Ideal gases are assumed to be point particles (have no volume) and not interact with each other apart from elastic collisions

The ideal gas law is a reasonable approximation for low pressures, high volumes, and high temperatures

[math]a[/math] accounts for electrostatic interaction between the gas molecules.  Highly polar molecules (e.g. ammonia) have high values of [math]a[/math].  [math]b[/math] accounts for the nonzero volume of molecules.  Large molecules (e.g. propane) have high values of [math]b[/math].