Question

The \(\mathrm{Ni}^{2+}(a q)\) cation is green, but \(\mathrm{Zn}^{2+}(a q)\) is colorless. Explain.

Short answer

Ni2+ has unpaired d electrons causing color; Zn2+ has filled d orbitals, so it's colorless.

Step-by-step solution

Understanding Ion Colors

The colors observed in transition metal ions like \( \text{Ni}^{2+} \) are due to electron transitions between d orbitals. The difference in energy between these orbitals corresponds to the energy of visible light.

The Role of d-Orbitals

\( \text{Ni}^{2+} \) has unpaired electrons in its d orbitals (specifically, d\(^8\)) which allow for electron transitions that absorb certain wavelengths of light, resulting in the observation of a green color.

Comparison with \( \mathrm{Zn}^{2+} \)

\( \text{Zn}^{2+} \), on the other hand, has a completely filled d subshell (d\(^{10}\)) so there are no available d-d transitions. This lack of electron transitions means that \( \text{Zn}^{2+} \) does not absorb visible light and thus appears colorless.

Key concepts

d-orbitals

In transition metals, d-orbitals play a crucial role in determining the color of ions in solution. The d-orbitals are a type of atomic orbital that can hold up to ten electrons. They come in five different shapes, each with a specific orientation in space. This arrangement is key to understanding how these elements interact with light.

Transition metals are unique because their d orbitals are partially filled, which allows for electron movement within these orbitals. When light strikes a transition metal ion, electrons may absorb specific wavelengths and transition between different d orbitals. These movements depend on the energy difference between the d orbitals.

The absorbed light energy corresponds to particular wavelengths in the visible spectrum, which results in the vibrant colors associated with different metal ions. This is why, for instance, a piece of jewelry containing nickel might appear green under certain lighting conditions.

electron transitions

The fascinating colors seen in transition metal ions are caused by electron transitions. When we talk about electron transitions, we mean the movement of electrons from one energy level to another within an atom. In transition metals, these movements happen mainly between d orbitals.

When an electron absorbs energy, for instance from visible light, it can jump from a lower energy d orbital to a higher energy d orbital. This is known as an electron transition. The specific energy required for this transition is equal to the energy carried by the absorbed light, which usually corresponds to certain visible wavelengths. These wavelengths are then missing from the reflected or transmitted light, producing the observable color.

For example, in the case of the nickel ion (\(\text{Ni}^{2+}\)), the electron absorbs specific wavelengths that result in green light being reflected, making nickel solutions appear green.

Ni2+ and Zn2+ comparison

Comparing \( \text{Ni}^{2+} \) and \(\text{Zn}^{2+}\) ions helps illustrate the principle of electron transitions and their role in determining color. \(\text{Ni}^{2+}\) has a di{8} configuration, which means there are unpaired electrons in its d orbitals. This allows for electron transitions that absorb specific light wavelengths, resulting in the characteristic green color of nickel solutions.

In contrast, \( \text{Zn}^{2+} \) has a completely filled di{10} subshell. Since there are no vacant spots for electrons to jump to in the same energy level, there are no d-d transitions occurring, and thus, no absorption of visible light. Consequently, \(\text{Zn}^{2+}\) solutions are colorless because all visible light passes through without being absorbed.

This comparison showcases why some transition metal ions have vibrant colors while others, like zinc, do not. The presence or absence of available electron transitions directly influences the color you see.