The coloured nature of transition metal ions are associated with their partially filled

Analysis

The question asks for the specific electron orbital responsible for the characteristic colours observed in transition metal ions.

Concept Explanation

Transition metals are defined by the presence of partially filled d-orbitals. In an isolated transition metal atom, the five d-orbitals have the same energy (they are degenerate). However, when ligands (atoms, ions, or molecules) approach the metal ion to form a complex, these d-orbitals split into different energy levels.

When white light hits these ions, electrons in the lower-energy d-orbitals absorb specific wavelengths of light to "jump" (excite) to the higher-energy d-orbitals. This process is known as d-d transition. The light that is not absorbed is transmitted or reflected, which is what we perceive as the colour of the ion.

Step-by-Step Solution

1. Identify the electronic configuration: Transition metals (elements in the d-block of the periodic table) have their valence electrons in the d-subshell.
2. Consider the condition for colour: For an ion to be coloured, it must have at least one unpaired electron in its d-orbital to allow for the d-d transition.
3. Evaluate the options: • s-orbitals and p-orbitals: These are typically fully filled in stable ions or do not participate in the energy-gap transitions that produce visible light colours in transition metals. • f-orbitals: These are associated with Lanthanides and Actinides (inner transition metals), not the standard transition metals referred to in this context. • d-orbitals: These are the specific orbitals that split in energy when transition metals form complexes, allowing for the absorption of visible light.

Conclusion

The coloured nature of transition metal ions is directly linked to the presence of partially filled d-orbitals, which facilitate d-d electronic transitions.

Exam Tip

Remember the mnemonic "D for Dazzling": Transition metals are Dazzling (coloured) because of their d-orbitals. If an ion has a completely empty d-orbital (like $Sc^{3+}$) or a completely full d-orbital (like $Zn^{2+}$), it will be colourless!