Question

In which of the following arrangements the order is $NOT$ according to the property indicated against it?

(a)	$A{l}^{3+}<M{g}^{2+}<N{a}^{+}<F^{-}$	-	increasing ionic size
(b)	$B<C<N<O$	-	increasing first ionisation enthalpy
(c)	$I<Br<F<Cl$	-	increasing electron gain enthalpy (with negative sign)
(d)	$Li<Na<K<Rb$	-	increasing metallic radius

• A. a
• B. b
• C. c
• D. d

Answer 1

Answer: (B)

As we move from left to right across a period, ionisation enthalpy increases with increasing atomic number. So the order of increasing ionisation enthalpy should be$B<C<N<O.$
But $N(1s^{2}2s^{2}2p^3)$has a stable half filled electronic configuration.
So, ionization enthalpy of nitrogen is greater than oxygen. So, the correct order of increasing first ionization enthalpy is $B<C<O<N.$