Question

Example trends of groups 12, 13 (a) atomic radii (b) ionization enthalpy (c) ionic radii

Answer 1

• Group 12: (a) Atomic radii increase from Zn to Cd with a contraction for Hg; (b) Ionization enthalpy decreases down the group; (c) Ionic radii of Zn²⁺, Cd²⁺, Hg²⁺ increase from Zn²⁺ to Cd²⁺ with Hg²⁺ showing contraction effects.

• Group 13: (a) Atomic radii increase from B to Tl (Ga showing slight deviation due to d-electron effects); (b) Ionization enthalpy decreases down the group; (c) Ionic radii of the +3 ions increase from Al³⁺ to Tl³⁺ (with possible anomalies for Tl due to inert pair effect).

Answer 2

Solution Explanation:

For Group 12 (Zn, Cd, Hg):

1. Atomic Radii:

   • Increases from Zn to Cd as extra electron shells are added.
   • However, Hg shows a slight contraction compared to the expected trend due to the lanthanide contraction and relativistic effects.

2. Ionization Enthalpy:

   • Decreases down the group because atoms have larger radii and increased shielding, making it easier to remove an electron.

3. Ionic Radii:

   • The ionic radii of their +2 ions (Zn²⁺, Cd²⁺, Hg²⁺) increase from Zn²⁺ to Cd²⁺; Hg²⁺, however, may show a smaller radius than expected owing to contraction effects.

For Group 13 (B, Al, Ga, In, Tl):

1. Atomic Radii:

   • Increase from B to Tl as additional shells are added.
   • Note: Ga shows a slight deviation due to poor shielding by the (n-1)d electrons.

2. Ionization Enthalpy:

   • Decreases down the group since increasing size and electron shielding reduce the nucleus’ hold on the valence electrons.

3. Ionic Radii:

   • For the common +3 ions (Al³⁺, Ga³⁺, In³⁺, Tl³⁺), the radii generally increase from Al³⁺ to Tl³⁺, although for Tl, the inert pair effect can sometimes result in anomalous behavior.