Question

Aluminum has only one oxidation state, while chromium has several. Which of the following is the best explanation for this difference? A. Electrons in the \(d\) orbitals of \(\mathrm{Cr}\) may or may not be used to form bonds. B. Electrons in the \(p\) orbitals of \(\mathrm{Cr}\) may or may not be used to form bonds. C. Electrons in the \(d\) orbitals of Al may or may not be used to form bonds. D. Electrons in the \(p\) orbitals of \(\mathrm{Al}\) may or may not be used to form bonds.

Short answer

A. Electrons in the d orbitals of Cr may or may not be used to form bonds.

Step-by-step solution

Understand the Electronic Configuration

First, note the electronic configurations for both aluminum (Al) and chromium (Cr). Aluminum has the configuration [Ne]3s²3p¹, and chromium has [Ar]3d⁵4s¹.

Identify the Orbitals Involved in Bonding for Each Element

Aluminum only involves electrons in the 3s and 3p orbitals for bonding as it does not have d electrons in its valence shell. Chromium, on the other hand, has electrons in the 3d and 4s orbitals that can participate in bonding.

Analyze the Ability to Vary Oxidation States

Since chromium can use its 3d electrons in addition to its 4s electron, it has multiple possible oxidation states. Aluminum, lacking d orbital electrons in its valence shell, generally exhibits a single oxidation state (+3) as it can only lose the 3s²3p¹ electrons.

Determine the Correct Explanation

Among the given options, only electrons in the d orbitals of chromium (Cr) are versatile and can participate in bonding, not the p orbitals. Therefore, the correct explanation for the difference in oxidation states between aluminum and chromium is: A. Electrons in the d orbitals of Cr may or may not be used to form bonds.

Key concepts

Electronic Configuration

Electronic configuration refers to the distribution of electrons in an atom's orbitals. Each element has a unique configuration that helps determine its chemical properties. For instance, the electronic configuration for aluminum (Al) is [Ne]3s²3p¹ and for chromium (Cr) is [Ar]3d⁵4s¹. The configuration shows that aluminum has electrons in the 3s and 3p orbitals, whereas chromium has electrons in the 3d and 4s orbitals. These configurations play a significant role in the chemical behavior and bonding patterns of the elements.
Understanding electronic configurations is crucial because it explains why some elements have multiple oxidation states while others do not. By examining their configurations, we can see that chromium's 3d and 4s orbitals allow for more flexibility in bonding compared to aluminum, which only uses its 3s and 3p orbitals.

d Orbitals

d orbitals are a set of five orbitals found in the d subshell, which can hold up to 10 electrons. These orbitals start appearing in the third energy level (n=3) and are significant in the chemistry of transition metals, such as chromium (Cr).
Unlike main group elements that typically use s and p orbitals for bonding, transition metals can use their d orbitals. For example, chromium, with its [Ar]3d⁵4s¹ configuration, can leverage 3d orbitals to form various complex ions and would exhibit multiple oxidation states like +2, +3, and +6.
This flexibility in using d orbitals for bonding is key to understanding why elements like chromium have more varied chemistry compared to elements such as aluminum, which doesn't have occupied d orbitals in its valence shell.

Bonding in Metals

Bonding in metals is characterized by the presence of free electrons, which can move easily throughout the metal lattice. This 'sea of electrons' concept explains many metallic properties such as conductivity and malleability.
For transition metals like chromium, the involvement of d orbitals in bonding provides additional variation. These orbitals can participate in various types of bonds, including metallic and covalent bonds. The ability of d electrons to form bonds contributes to the metal's varied oxidation states, seen in elements like chromium, which can easily switch between multiple oxidation states.
On the other hand, aluminum relies mainly on its 3s and 3p electrons for bonding, limiting its oxidation state to +3. The absence of available d orbitals in the valence shell confines aluminum's bonding capabilities.

Transition Metals

Transition metals are elements found in the d-block of the periodic table. They are known for their ability to form various oxidation states and complex compounds due to the involvement of their d orbitals. The electronic configurations of these metals typically include unfilled d orbitals, which can actively participate in chemical bonding.
Chromium (Cr) is a classic example of a transition metal with the electronic configuration [Ar]3d⁵4s¹. This setup allows chromium to use both its 3d and 4s electrons for bonding, resulting in multiple oxidation states such as +2, +3, and +6. The versatility of transition metals in forming bonds is a direct consequence of their d orbital configurations.
In contrast, main group elements like aluminum (Al) usually do not have accessible d orbitals, leading them to exhibit fewer oxidation states. For example, aluminum typically shows an oxidation state of +3, as its bonding involves only the 3s and 3p orbitals.