The average thermal energy for a monoatomic gas is (where, k_B is Boltzmann constant and T is absolute temperature.) 

The average thermal energy of a system with degree of freedom f is equal to its average energy, which is given as

                                        $= \frac{f}{2}{k}_B·T$

For monoatomic gas, f = 3

$\therefore$ Average thermal energy $=\frac{3}{2}{k}_{B}T$