= Solution
One mole of original atoms contains $(1-\zeta)$ moles of neutral atoms, $\zeta$ moles of <protons> and $\zeta$ moles of <electrons>. All are monatomic translational species, so their combined <thermal energy> is $\frac32RT(1+\zeta)$. Taking the neutral ground state as zero <internal energy>, <ionization> stores $\zeta N_AE_I$. Thus
$$
\boxed{U=\frac32RT(1+\zeta)+\zeta N_AE_I=\frac32RT(1+\zeta)+\zeta\frac{E_I}{k_BT}RT.}
$$
Radiation, excited states and nonideal interactions have been excluded consistently with the elementary <Saha ionization equation>.
Back to article page
