Problem 9 - A Complete Resource Book in Chemistry for JEE Main

Question

Chromium is represented by the electronic configuration (a) \([\mathrm{Ne}] 3 \mathrm{~s}^{2} 3 \mathrm{p}^{6} 3 \mathrm{~d}^{1} 4 \mathrm{~s}^{2}\) (b) \([\mathrm{Ne}] 3 \mathrm{~s}^{2} 3 \mathrm{p}^{6} 3 \mathrm{~d}^{2} 4 \mathrm{~s}^{1}\) (c) \([\mathrm{Ne}] 3 \mathrm{~s}^{2} 3 \mathrm{p}^{6} 3 \mathrm{~d}^{5} 4 \mathrm{~s}^{1}\) (d) \([\mathrm{Ne}] 3 \mathrm{~s}^{2} 3 \mathrm{p}^{6} 4 \mathrm{~s}^{2} 3 \mathrm{~d}^{4}\)

Step-by-Step Solution

Answer

The correct electronic configuration for Chromium is option (c): \\[ \text{[Ne]} 3 \text{ s}^2 3 \text{ p}^6 3 \text{ d}^5 4 \text{ s}^1 \\].

Step 1: Understand the Problem

The task is to determine the electronic configuration of Chromium from the given options. Chromium is a chemical element with an atomic number of 24.

Step 2: Recall Chromium's Electronic Configuration

Chromium has an atomic number of 24, which means it has 24 electrons. The electronic configuration is unique due to electron stability considerations, and it is known to be \[ ext{[Ar]} ext{ 3d}^5 ext{ 4s}^1 \]. This setup follows the principle that a half-filled \[3d\]\ sub-shell lends extra stability to the atom.

Step 3: Compare with Given Options

Now, compare the known electronic configuration \[ ext{[Ar]} ext{ 3d}^5 ext{ 4s}^1 \] with each of the given options. We recognize that \[ ext{[Ne]} ext{ 3s}^2 ext{ 3p}^6 \] in the options represents \[ ext{[Ar]} \]. This makes option (c) \[ ext{[Ne]} 3 ext{ s}^2 3 ext{ p}^6 3 ext{ d}^5 4 ext{ s}^1 \] identical to Chromium's configuration.

Key Concepts

Electron Stability

Atoms strive for a state of minimal energy, making them more stable. A unique aspect of electron stability in atoms manifests in Chromium's electronic configuration. Normally, electrons fill the 4s orbital before filling the 3d orbital, due to energy levels. However, Chromium deviates from this standard in pursuit of stability.

Instead of having an electronic configuration of \[ \text{[Ar]} \text{ 3d}^4 \text{ 4s}^2 \], Chromium achieves stability by adopting the configuration \[ \text{[Ar]} \text{ 3d}^5 \text{ 4s}^1 \]. This balance between the 3d and 4s orbitals allows half-filling of the 3d subshell, which is particularly stable.

Half-filled subshells, like the 3d in Chromium, are less reactive and evade pairing repulsion, lowering energy and increasing atom stability. Consequently, Chromium's electron configuration exemplifies how deviations from electron filling orders may yield a more stable atomic structure.

Atomic Number

The atomic number is an essential property that determines an element's identity and its position on the periodic table. Chromium's atomic number is 24, signifying it holds 24 protons in its nucleus and equally, 24 electrons in its neutral state.

Understanding the atomic number is critical to predicting an element's electronic configuration and chemical behavior. The atomic number influences aspects like atomic mass and isotopes, contributing to our comprehension of elemental characteristics.

For Chromium, the atomic number dictates the arrangement of electrons across the available energy levels and subshells, crucial for determining its unique electron configuration that enhances stability.

Transition Metals

Transition metals, including Chromium, reside in the d-block of the periodic table. They are characterized by their larger range of available oxidation states, variable electron configurations, and ability to form complex ions.

What sets transition metals apart is the occupation of d-orbitals. Chromium, for example, with the electron configuration \[ \text{[Ar]} \text{ 3d}^5 \text{ 4s}^1 \], offers a prime illustration of complex electron arrangements.

Being a part of transition metals, Chromium exhibits features like:

• High density and melting points
• Variable oxidation states facilitating compound formation
• The presence of unpaired d-electrons leading to paramagnetism

Transition metals' unique properties ensure their pivotal roles in technological advancements, catalysis, and biological processes.