Consider the following statements about first ionization energies:

I. Because the effective nuclear charge for Mg is greater than that for Be, the first ionization energy of Mg is greater than that of Be.
II. The first ionization energy of O is less than that of N because in O we must pair electrons in the 2p orbitals.
III. The first ionization energy of Ar is less than that of Ne because a 3p electron in Ar is farther from the nucleus than a 2p electron in Ne.

Which of the statements I, II, and III is or are true?

Firstly, the magnitude of nuclear charge affects the first ionization energy of an element. Hence,as effective nuclear charge increases, the attraction between the nucleus and the outermost electron increases and ionization energy consequently increases. Thus Mg has a higher first ionization energy than Be.

Secondly, oxygen has an electron configuration of 1s2 2s2 2p4 while nitrogen has an electron configuration of 1s2 2s2 2p3. Now, recall that extra energy is often associated with half filled orbitals hence nitrogen has a higher first ionization energy than oxygen. Furthermore, the addition of electron to an already half filled 2p orbital in oxygen (pairing) leads to inter electronic repulsion and drastic fall in first ionization energy. Therefore, as we move from nitrogen to oxygen in the periodic table, greater inter-electron repulsion between two electrons in the same p-orbital counter balances the increase in effective nuclear charge hence nitrogen has a greater first ionization energy than oxygen.

Lastly, the 3p orbital is far away from the nucleus hence we expect it to feel less of nuclear attraction than a 2p orbital. Hence the first ionization energy of Ar is less than that of Ne.