Past exam of the short options paper shared by Parts A and B of the physics course of the University of Oxford / 2019 / S16 / 1 / b / 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 $\frac{3}{2}RT(1+\zeta )$. Taking the neutral ground state as zero internal energy, ionization stores $\zeta N_A{E}_I$. ThusRadiation, excited states and nonideal interactions have been excluded consistently with the elementary Saha ionization equation.