In: Chemistry
(A) a 3s electron is shielded by 1s, 2s and 2p orbitals. Whereas a 3p electrons is shielded by 1s,2s,2p and 3s orbitals.
As 3p electron is shielded from the nucleus by one extra orbital (3s), the effective nuclear charge faced by this electron is less than a 3s electron.
(B) Al and Ga are in same group but the atomic size of Ga is more than Al. So, the distance between the valence shell and nucleus is more in Ga than Al. As the atomic size increases, the effective nuclear charge on valence electron decreases. So, the effective nuclear charge for the valence shell of Ga is less than Al.
(C) Size of N is less than the size of O. For the reason, the effective nuclear charge for the valence shell of O is less than N.