Question

Give the reasons of the following:
(a) Fluorine has less negative electron gain enthalpy than chlorine.
(b) Noble gases tend to be less reactive.
(c) First ionisation enthalpy of ‘Mg’ is more than that of ‘Na’ but second ionisation enthalpy of ‘Mg’ is less than that of ‘Na’.

Answer 1

a) Due to the small size of the fluorine atom there exists inter ionic repulsion between the electrons in the 2p orbitals, so the incoming electrons in the fluorine atom does not show much attraction. So, fluorine has less negative electron gain enthalpy than chlorine

b) The noble gases already have most stable electronic configuration. They do not tend to lose or gain electron to attain stable configuration. Hence, they are less reactive or inert in nature

c) The electronic configuration magnesium is $1s^{2}2s^{2}2p^{6}3s^{2}$ whereas the electronic configuration of sodium is  $1s^{2} 2s^{2} 2p^{6} 3s^{1}$.

During first ionization the electron removed from the outer shell is from the 3s orbital in both the cases. Here the charge of Magnesium is (+12) whereas it is (+11) for sodium. So, the first ionization energy of magnesium is higher than that of sodium. Then for removal electron during second ionization, the electron is removed from 3s orbital for magnesium whereas in case of sodium it attains the inert gas configuration. So, more energy amount is required to remove an electron from sodium during second ionization. Hence first ionization enthalpy of magnesium is more than sodium whereas the second ionization enthalpy of magnesium is less than that of sodium .