(a) In beryllium [z = 4, 1s2, 2s2], the electron removed during the ionization is an s-electron while the electron removed during ionization of boron (Z = 5, 1s2,2s2,2p1) is a p-electron.
The penetration of a 2s electron to the nucleus is more than that of a 2p-electron , so the 2p electron of boron is more shielded from the nucleus by the inner core of electrons than the 2s electrons of beryllium. Hence, it is easier to remove the 2p- electrons from boron compared to the removal of 2s electron from beryllium. Hence, first ionization enthalpy of boron (Z =5) is slightly less than that of beryllium (Z= 4).
(b) Electron gain enthalpy of fluorine is less negative than that of chlorine because when an electron is added to F, the added electron goes to the smaller n = 2 quantum level and suffers significant repulsion from the other electrons present in this level. While for the n=3 quantum level (Cl), the added electron occupies a larger region of space and the electron-electron repulsion is much less.
(c) Size of an anion is always larger than that of parent atom because the addition of one or more electrons would result in increased repulsion among the electrons and a decrease in effective nuclear charge.
