Question

Suggest a reason why copper(II) compounds are often colored but copper(I) compounds are colorless. Which oxidation number results in paramagnetic compounds?

Short answer

Copper(II) compounds are often colored because they have a d9 electronic configuration, which allows for d-d transitions that can absorb visible light, whereas copper(I) compounds are colorless due to their full d10 configuration with no available d-d transitions. Copper(II) compounds, with one unpaired electron, are paramagnetic.

Step-by-step solution

- Understanding Copper Oxidation States

Copper can have multiple oxidation states, the most common of which are copper(I) with an oxidation state of +1, and copper(II) with an oxidation state of +2. The coloration of copper compounds is generally due to the electronic transitions between d-orbitals that absorb light. Since copper(I) has a full d10 configuration, there are no electronic d-d transitions to absorb visible light, making these compounds colorless. Copper(II), on the other hand, has a d9 configuration, allowing for such electronic transitions and resulting in colored compounds.

- Understanding Paramagnetism

Paramagnetism is a form of magnetism whereby certain materials are attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. This effect is due to the presence of unpaired electrons in the atomic or molecular structure of the material. In the case of copper(II) compounds, the d9 configuration has one unpaired electron, which makes copper(II) compounds paramagnetic. Copper(I) compounds have a d10 configuration with all electrons paired, making these compounds diamagnetic, not paramagnetic.

Key concepts

Copper Oxidation States

Understanding the oxidation states of copper is crucial when studying its compounds and their varied properties. The most commonly encountered copper oxidation states are copper(I), denoted as Cu(I) or cuprous, with an oxidation state of +1, and copper(II), referred to as Cu(II) or cupric, with an oxidation state of +2.

The phenomenon behind the colorless nature of copper(I) compounds lies in their electronic configuration. Copper(I) has a full d-orbital with 10 electrons, symbolized as d¹⁰. This stable arrangement means that all electrons are paired and occupy the entire d-orbital, leaving no room for electronic transitions that can absorb visible light — a key factor in rendering color to compounds. Hence, copper(I) compounds are typically colorless.

On the other hand, copper(II) compounds possess a d⁹ configuration, indicating an unoccupied space within the d-orbital. This vacancy allows for the absorption of certain wavelengths of visible light as electrons jump between d-orbital levels. The absorbed light corresponds to specific color wavelengths, which are not reflected back to the eye, giving copper(II) compounds their characteristic color.

d-Orbital Electronic Transitions

Delving into d-orbital electronic transitions provides insight into why certain copper compounds display vivid colors. For transition metals like copper, the d-orbitals offer a stage for electrons to absorb energy and transition to higher levels, a process intimately connected with light absorption and color manifestation.

In the case of copper(II) with its d⁹ electron configuration, one electron can move to different energy levels within the d-orbitals when light is shined upon the compound. This electron absorbs light's energy, leaping from a lower to a higher d-orbital energy level, and this energy corresponds to a specific wavelength of visible light. The compound thus appears colored as it reflects the spectrum of visible light minus the absorbed wavelength.

This phenomenon is absent in copper(I) compounds, as their d¹⁰ configuration implies a full and stable d-orbital with no room for such transitions. The lack of these electron jumps means no light is selectively absorbed and as such, copper(I) compounds appear colorless to the human eye.

Paramagnetism and Diamagnetism

The magnetic properties of copper compounds can be understood through the concepts of paramagnetism and diamagnetism, which describe how substances interact with magnetic fields. These phenomena are directly linked to the electronic structure of the compounds.

Materials exhibiting paramagnetism contain at least one unpaired electron and are magnetically mutable. In paramagnetic compounds, like copper(II) complexes, the d⁹ configuration features one unpaired electron that can align with an external magnetic field, causing the material to exhibit a magnetic response and become weakly attracted to the magnetic source. This unpaired electron is fundamental to the property of paramagnetism.

Conversely, diamagnetic compounds have all their electrons paired and evenly spread over orbitals, which results in a slight repulsion from a magnetic field. Since copper(I) has a complete d¹⁰ electron configuration, where all electrons are paired, it does not have magnetic unpaired electrons to contribute to paramagnetism. Instead, it is diamagnetic and will show a repelling interaction when exposed to a magnetic field. This clear distinction in magnetism is an additional factor that can be used to differentiate between copper(I) and copper(II) compounds in a laboratory setting.