Which of the following set represents only coloured ions? (1) \(\mathrm{Fe}^{3+}\) (2) \(\mathrm{Sc}^{3+}\) (3) \(\mathrm{Cu}^{2+}\) (4) \(\mathrm{Cr}^{3+}\) (a) 1,3 (b) \(2,3,4\) (c) \(1,2,4\) (d) 1,3 and 4

Understanding the behavior of transition metal ions is key to exploring the captivating world of inorganic chemistry. These ions are formed when transition metals, found in the central block of the periodic table, lose electrons to achieve a positive charge. The particular significance of these ions lies in their capability to exhibit various oxidation states, contributing to their diverse chemical reactivity and their ability to form complex structures with rich colors.

The color phenomena associated with transition metal compounds are not just visually appealing but also serve as a fingerprint for identifying the presence of specific metal ions. This is widely applied in both qualitative and quantitative chemical analysis.

When we delve into the reasons behind the intense colors of transition metal ions, we encounter the concept of d-d transitions. These refer to the movement of an electron from one d-orbital to another of a higher energy level within the same atom. As the electrons jump in energy levels, they absorb certain wavelengths of light. The particular wavelengths that are absorbed determine which colors will be missing from the spectrum and, conversely, which colors the compound will appear as. For instance, if a compound absorbs the green portion of the light spectrum, it will appear red to the human eye due to the complementarity of colors.

It is crucial to note that not all transition metal ions exhibit d-d transitions. Only those ions with partially filled d-orbitals exhibit this phenomenon, which is why the presence of unpaired electrons is a key factor in determining an ion's color.

The electron configuration of a transition metal ion provides the blueprint for understanding its chemical and physical properties. Electrons within an atom occupy specific energy levels, and for transition metals, the d-orbitals are of particular interest. These orbitals can hold up to 10 electrons, and their fill pattern influences magnetic properties and the propensity for d-d transitions.

For example, a Sc3+ ion jumps to a d0 configuration, meaning all its d-orbitals are empty, which translates to no unpaired electrons and typically results in a colourless appearance. In contrast, ions like Fe3+, Cu2+, and Cr3+ have d-orbitals with unpaired electrons, responsible for their colored appearance. Whether an ion is colorless or colored can be predicted by the electron configuration, making it a fundamental aspect for chemists to understand transition metal ions' behavior and their resulting complex compounds.