1. Tardigrade
  2. Question
  3. Chemistry
  4. In which of the following arrangements the order is NOT according to the property indicated against it? (a) Al3+ < Mg2+ < Na+ < 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

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Q. In which of the following arrangements the order is $ NOT$ according to the property indicated against it?
(a) $ Al^{3+} < \,Mg^{2+} < \,Na^+ < \,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

Classification of Elements and Periodicity in Properties

Solution:

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 $B < \, C < \,O < \,N.$