where Um(0) is the molar internal energy at T = 0, when all translational motion
has ceased and the sole contribution to the internal energy arises from the internal
structure of the atoms. This equation shows that the internal energy of a perfect
gas increases linearly with temperature. At 25°C, RT = 3.7 kJ mol−1, so translational
motion contributes about 4 kJ mol−1 to the molar internal energy of a gaseous sample
of atoms or molecules.