Problem 43 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of d electrons in \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) [atomic no. of \(\mathrm{Cr}=24]\) is (a) 2 (b) 3 (c) 4 (d) 5

Step-by-Step Solution

Answer

(b) 3

Step 1: Determine the Chromium Electron Configuration

The atomic number of Chromium (Cr) is 24, which means it has 24 electrons in its neutral state. The electron configuration of neutral chromium is \[ ext{Cr}: [ ext{Ar}] 3d^5 4s^1\]

Step 2: Account for Ionization

The complex \[[\mathrm{Cr}(\mathrm{H}_2\mathrm{O})_6]^{3+}\] involves a chromium ion with a 3+ charge. This means three electrons are removed from its neutral state electron configuration. When electrons are removed, they come from the outermost shell first (4s before 3d), leading to: \[ ext{Cr}^{3+}: [ ext{Ar}] 3d^3\]

Step 3: Count the Number of d Electrons

After forming the \text{Cr}^{3+} ion, we see from the electron configuration \[3d^3\] that there are 3 electrons remaining in the d subshell.

Key Concepts

Chromium Electron Configuration

Chromium is a fascinating element, particularly when it comes to its electron configuration. The neutral atom of Chromium has an atomic number of 24, indicating it possesses 24 electrons. These electrons are arranged in specific orbitals. Typically, you might expect it to fill the 3d subshell completely before moving to the 4s subshell, but Chromium is an exception to the norm.
For Chromium, the electron configuration is \[\text{Cr}: [\text{Ar}] 3d^5 4s^1\] This configuration is interesting because having a half-filled 3d subshell provides additional stability due to electron exchange energy. As a result, Chromium adopts this specific configuration instead of the expected \(3d^4 4s^2\).
Understanding this unique configuration is crucial, especially when Chromium forms ions, such as the 3+ ion in coordination compounds. Removing electrons from Chromium during ion formation affects the stability and chemical properties of the resulting ion.

Coordination Chemistry

Coordination chemistry is the study of compounds formed between metal atoms or ions and other molecules or ions. These compounds are called coordination complexes or coordination compounds. In these complexes, the metal atom acts as the center and binds with surrounding molecules or ions called ligands.
One classic example of coordination chemistry is the complex \[[\mathrm{Cr}(\mathrm{H}_2\mathrm{O})_6]^{3+}\]. In this complex, the central chromium ion is surrounded by six water molecules, which act as ligands.
The ligands attach to the chromium ion through coordinate covalent bonds, sharing their electron pairs with the metal atom. This sharing allows stable complex formation. Coordination numbers and geometries, such as the octahedral shape seen in this Chromium complex, are innate features of coordination chemistry that guide the formation and stability of such compounds.

Transition Metal Complexes

Transition metal complexes, like the chromium complex \[[\mathrm{Cr}(\mathrm{H}_2\mathrm{O})_6]^{3+}\], are fascinating due to their unique chemical and physical properties. Transition metals, positioned in the central area of the periodic table, are characterized by their ability to form multiple oxidation states and complex combinations with various ligands.
In the case of chromium \[\text{Cr}^{3+}\], it forms a 3+ ion by losing three electrons. These electrons originate from the 4s and then the 3d orbitals, reflecting chromium's position as a transition metal. Transition metals are known for their partially filled d orbitals, enabling a broad scope of coordination chemistry.
These complexes often exhibit colorful presentations. This is due to electron transitions between d orbitals, which absorb specific wavelengths of light and reflect others. Such characteristics make transition metal complexes significant in industries ranging from pigments to catalysis, highlighting their diverse applications.