Question

Give reasons for the following: 
(i) Zn, Cd and Hg are not considered transition elements. 
(ii) Transition metals form complex compounds. 
(iii) Sc(3+) is colourless, but Ti(3+) is coloured.

• A.
• B.
• C.
• D.

Hint:

To have a colored ion via $ d-d $ transitions, the metal must have a partially filled $ d $-subshell ($ d1 $ to $ d9 $). Ions with $ d0 $ (like c3+, \textTi4+ $) or $ d10 $ (like $ \textZn2+, \textCu+ $) configurations are generally colourless.

Answer 1

The Correct Option is A

Concept:
Transition elements are defined as elements which have incompletely filled d-orbitals in their ground state or in any of their oxidation states. The presence of unpaired electrons in d-orbitals governs their color and complex-forming ability. 
Step 1: Electronic configuration of Group 12 elements.
Zn, Cd, and Hg have the general electronic configuration (n-1)d10 ns2. Since their d-orbitals are completely filled in both their atomic state and their common ionic state (M2+), they do not fit the definition of transition elements. 

Step 2: Complex formation.
Transition metals form complexes because:

• They have small ionic sizes and high nuclear charges, which attract ligands.
• They possess vacant d-orbitals of appropriate energy to accept lone pairs of electrons from ligands.

Step 3: Color in Cr3+ vs Ti3+.
The color of transition metal ions is typically due to d-d transitions. When ligands approach the central metal ion, the degenerate d-orbitals split into different energy levels (typically t2g and eg in octahedral fields).

Cr3+: The electronic configuration is [Ar] 3d3. Electrons occupy lower d-orbitals and can absorb visible light, giving color.
Ti3+: The electronic configuration is [Ar] 3d1. The single electron can be excited to a higher d-orbital, absorbing visible light (green and yellow), resulting in the transmitted light appearing purple or violet.