Problem 67 - General Chemistry

Question

The hexaaquascandium(III) ion, \(\left[\mathrm{Sc}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\), is colorless. Explain why this might be expected.

Step-by-Step Solution

Answer

The Sc(III) ion is colorless because it has no d-electrons for d-d transitions.

Step 1: Understanding electronic configuration

Scandium is the first element in the d-block of the periodic table, with the electronic configuration \([ ext{Ar} ] 3d^1 4s^2\). As \( ext{Sc}^{3+} \) ion is formed, it loses three electrons, resulting in \( [ ext{Ar} ] \) configuration, with no d-electrons.

Step 2: Evaluating d-electron involvement in color

The color in transition metal complexes generally arises from transitions between d-orbitals (d-d transitions) that are split in energy by the ligands surrounding the metal ion. These transitions absorb certain wavelengths of light, and the complementary color is observed.

Step 3: Linking colorlessness to electron configuration

Since \( ext{Sc}^{3+} \) has no d-electrons, there are no possible d-d transitions. Thus, there is no absorption of visible light, resulting in the complex being colorless.

Key Concepts

Scandium(III) Ion

The Scandium(III) ion is an essential topic in coordination chemistry. Scandium, symbolized by Sc, is an element located in the d-block of the periodic table. Its Scandium(III) ion, noted as \( \text{Sc}^{3+} \), is especially interesting because of its electronic configuration. When scandium forms its 3+ ion, it loses three electrons: one from the 3d orbital and two from the 4s orbital, resulting in an electron configuration resembling that of argon - \([\text{Ar}]\). This means that the Scandium(III) ion is a d^0 system, lacking any electrons in its d-orbitals. This absence is crucial since it affects the ion's properties, including its color, which we explore further in connection with transition metals.

d-block Elements

d-block elements, also known as transition metals, are located in the central block of the periodic table. They include elements from groups 3 to 12. These metals are characterized by their ability to form various oxidation states and by the presence of d-orbitals in their electronic structure. The properties of d-block elements, such as catalytic activity, magnetic properties, and distinctive colors, are primarily attributed to the behavior of their d-electrons. Even though scandium is part of the d-block, its ion, \( \text{Sc}^{3+} \), stands out because it lacks d-electrons, influencing its unique characteristics like being colorless in solutions.

Electronic Configuration

Understanding electronic configuration is key to explaining the properties of any element, including Scandium(III). Typically, an atom's electron configuration reveals the arrangement of electrons across various orbitals. In scandium's case, the electronic configuration of the atom is \([ \text{Ar} ] 3d^1 4s^2\). However, when scandium forms the 3+ ion, it loses its 3d and 4s electrons, leaving it with the electron configuration \([ \text{Ar} ]\). This means that \( \text{Sc}^{3+} \) does not have any d-electrons available for electron transitions, which is pivotal in explaining the ion's lack of color.

Color in Transition Metals

The presence of color in transition metals is often due to electronic transitions. These transitions occur between split d-orbitals, which absorb specific wavelengths of visible light. When light passes through or reflects from a compound, certain wavelengths are absorbed, and the non-absorbed wavelengths combine to produce a perceived color. In coordination compounds, ligands affect the energy levels of d-orbitals through ligand field theory, inducing these transitions. However, as with the \( \text{Sc}^{3+} \) ion, when no d-electrons are present, as there are no available d-d transitions. This absence of electron movement in the d-orbitals leads to no absorption of visible light frequencies, rendering the compound colorless.