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Question

Which of the following is/are wrong order(s)?
  1. B+<B<B   Size
  2. O<O<O+   Zeff
  3. Na<Al<Mg<Si   Ionisation potential
  4. I<Br<Cl<F   Electron gain enthalpy


Solution

The correct option is D I<Br<Cl<F   Electron gain enthalpy
Consider the factors on which these properties depend:
(A) Cation is smaller while anion is bigger than its parent atom. correct order is B+<B<B
(B) Correct order of electron gain enthalpy is Cl>F>Br>I
(C) Cation is smaller as it is formed by the loss of electron(s). The anion is formed by the gain of electron(s). The size of anion increases with increase in charge on anion i.e.. as the Z/e ratio decreases the size increases.
(D) Across the period the size decreases and nuclear size increases. So, ionisation energy increases. However, the first ionisation energy of Mg is greater than Al because of high penetration power of 2s2 electrons of Mg as compared to that of 2p1 electron of Al.
hence, Na<Al<Mg<Si is the order of Ionisation potential.

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