Explain the following observations : (i) Transition elements generally form coloured compounds (ii) Zinc is not regarded as a transition element. "

(i) Transition elements generally form colored compounds:

The color exhibited by transition metal compounds arises from the d-d transition, which involves the movement of electrons between the d orbitals of the metal ions. Transition metals have partially filled d orbitals, which allow for the absorption of visible light. When light strikes a transition metal complex, it can promote an electron from a lower-energy d orbital to a higher-energy d orbital, resulting in the absorption of certain wavelengths of light and the reflection or transmission of others. The color observed depends on the energy difference between the d orbitals involved in the transition.

The intensity and nature of the color can be influenced by various factors such as the oxidation state of the metal ion, the ligands surrounding the metal ion, and the coordination geometry of the complex. Ligands with different electron-donating abilities can lead to different splitting patterns of the d orbitals, resulting in different absorption spectra and hence different colors.

(ii) Zinc is not regarded as a transition element:

Zinc is often not considered a transition element because it lacks partially filled d orbitals in its common oxidation states. In its most common oxidation state, +2, the 3d orbitals are completely filled, which means there are no available d electrons for d-d transitions to occur. Therefore, zinc typically forms colorless compounds.

Transition metals, by definition, have incompletely filled d orbitals in at least one oxidation state, which allows them to exhibit characteristic transition metal properties such as forming colored compounds and acting as catalysts. Since zinc does not fulfill this criterion, it is often excluded from the list of transition elements despite being located in the d-block of the periodic table.