The Cr3+(aq) cation is violet, but Y3+(aq) is colorless. Explain why this difference in color occurs.

Verified step by step guidance

1. Step 1: Understand the concept of color in transition metal complexes. The color of a transition metal complex is due to the d-d electron transitions within the d-orbitals, which are split in energy in the presence of a ligand field.
2. Step 2: Recognize that Cr3+ is a transition metal ion. Chromium in the +3 oxidation state has a partially filled d-orbital (specifically, it has 3 electrons in the 3d orbitals), which allows for d-d transitions when light is absorbed, resulting in the violet color.
3. Step 3: Note that Y3+ is not a transition metal ion. Yttrium in the +3 oxidation state has a completely empty d-orbital (it has no electrons in the 4d orbitals), so there are no d-d transitions possible, leading to it being colorless.
4. Step 4: Consider the role of ligand field theory. In Cr3+, the presence of ligands causes the d-orbitals to split into different energy levels, allowing for the absorption of specific wavelengths of light, which is perceived as color.
5. Step 5: Conclude that the difference in color between Cr3+ and Y3+ is due to the presence of d-electrons in Cr3+ that can undergo electronic transitions, whereas Y3+ lacks d-electrons, preventing such transitions and resulting in a colorless appearance.

Key Concepts

d-Block Elements and Color

Transition metals, such as chromium (Cr) and yttrium (Y), have partially filled d-orbitals. The color of a transition metal ion in solution is primarily due to the presence of d-electrons, which can absorb specific wavelengths of light. When these d-electrons transition between energy levels, they absorb certain colors, resulting in the observed color of the solution.

Crystal Field Theory

Crystal Field Theory explains how the arrangement of ligands around a metal ion affects the energy levels of its d-orbitals. In the case of Cr3+, the presence of ligands causes a splitting of the d-orbitals, allowing for electronic transitions that absorb visible light, leading to the violet color. In contrast, Y3+ has a full d-subshell, resulting in no such transitions and thus no color.

Ligand Field Strength

The strength of the ligands surrounding a metal ion influences the extent of d-orbital splitting. Strong field ligands cause greater splitting, which can lead to more significant color differences. In the case of Cr3+, the ligands create a suitable environment for electronic transitions, while Y3+ does not exhibit such transitions due to its electronic configuration, resulting in a colorless solution.