Consider the following statements about first ionization energies: 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. The first ionization energy of O is less than that of N because in O we must pair electrons in the 2p orbitals. 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?

Stament I : Because the efective nuclear charge for Mg is greater than that for Be, the first ionization energy of Mg is greater than that of Be. FALSE

The first ionization energy decreases as we go down a group because of the shielding efect electrons have on the farthest electron that is going to be ionized. That is the ionized electron is shielded from its nucleus because we are filling higher energy levels.

Statement II :The first ionization enery of O is less than that of N because in O we must pair electrons in the 2p orbital. FALSE

Again we have the shielding effect of electrons on the first electron to ionize. This effect does not have anything to do with the pairing of electrons in the 2p orbital. since as we go across a row in the periodic table we are increasing the number of electrons. The first electron to ionize thus will see less of its nucleus and therefore O will have a lower first ionization energy.

Statement 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. TRUE

This is another way to say the shielding effect is responsible for the first ionization energy being in Ar is less than that of Ne. The electron is shielded from a nucleus with a higher atomic number and it is farther away ( 3p vs 2p orbital)