Ionization Energy Calculation

Question

How do you calculate the third ionization energy of a Be atom in its ground state?

Answer 1

Not an exact solution, but one can approximate the IE as follows:

En = (2.18x10^-18)Z² (1/n²)

Be = 1s² 2s² so Z=4 and n=1 (the energy level of the electron being removed)

En = (2.18x10^-18)(4)²(1/1²) = 3.5x10^-17 J = energy for the electron in 1 atom

3.5x10^-17 J/atom x 6.02x10²³ atoms/mole = 2.1x10⁷ J/mole = 2.1x10⁴ kJ/mole ≈ 21,000 kJ/mole