::You can look it up in any contemporary elementary book on thermodynamics. I'm sure you can find something on the internet as well.  
 ::The equipartition theorem says that at temperature T the energy of any quadratic degree of freedom is ½kT. So Uthermal = N f ½ k T, where N, is the number of molecules in the system and f is the degrees of freedom the molecule has. So, since a monatomic ideal gas has only three degrees of freedom (translational motion only) the thermal energy . A diatomic or triatomic molecule has more degrees of freedom (because of "internal flexing") however you have to go into quantum physics to understand how to count degrees of freedom for molecules properly. At room temperature N2 and O2 has 5 degrees of freedom for example. —