is a mixture of gases. Nitrogen and oxygen are both diatomic gases.
This means that they consist of molecules of two atoms. Therefore some
energy is taken up in the vibration of the bond between the two atoms.
There are also molecules with
three atoms such as carbon dioxide and water.
In these more energy would be
taken up in the vibration of bonds.
Atmospheric pressure is 100 kPa,
which is quite a high pressure, and the temperatures at which we are
comfortable, about 290 K are quite high as far as gasses are concerned.
However, to a first
approximation, we can model ideal gas behaviour with air.