Question

Which of the following is the correct electron configuration for chromium in the ground state? A. \([\mathrm{Ar}] 3 \mathrm{~d}^4 5 \mathrm{~s}^2\) B. \([\mathrm{Kr}] 4 \mathrm{~d}^5 5 \mathrm{~s}^1\) C. \([\operatorname{Ar}] 4 \mathrm{~s}^1 4 \mathrm{p}^5\) D. \([\operatorname{Ar}] 3 \mathrm{~d}^5 4 \mathrm{~s}^1\)

Short answer

D: \([\text{Ar}] 3d^5 4s^1\)

Step-by-step solution

Understand Electron Configurations

An electron configuration describes the distribution of electrons among the various orbitals in an atom. The electrons fill the orbitals in a specific order based on the energy levels and subshells.

Determine the Order of Filling

Electrons fill according to the Aufbau principle, which states that lower-energy orbitals fill before higher-energy orbitals. For elements, the order is typically: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, etc.

Note Chromium's Electron Configuration

Chromium has an atomic number of 24, so it has 24 electrons. However, chromium is an exception to the expected order due to electron stability preferences.

Recognize Special Cases

Chromium is a special case where the electron configuration is not as predicted. Instead of filling 3d with 4 electrons and 4s with 2 electrons, one electron from the 4s orbital moves to the 3d orbital to provide a more stable, half-filled 3d subshell.

Identify the Correct Configuration

The unique electron configuration for chromium results in \([\text{Ar}] 3d^5 4s^1\). This provides a more stable configuration due to the half-filled d subshell.

Confirm the Correct Answer

Among the given options, option D: \([\text{Ar}] 3d^5 4s^1\) is the correct electron configuration for chromium in the ground state.

Key concepts

Electron Configuration

The electron configuration of an atom describes how electrons are distributed in different orbitals around the nucleus. This configuration helps to understand the atom's chemical behavior.
Electrons occupy orbitals starting from the lowest energy level, moving up to higher levels. For example, they fill the 1s orbital before moving to the 2s orbital.
Using notations like [Ar] 3d^5 4s^1, we can represent the electron configuration succinctly. Here, [Ar] represents the electron configuration of argon, and 3d^5 4s^1 shows the arrangement of additional electrons.

Aufbau Principle

The Aufbau principle is essential for determining the correct electron configuration. It states that electrons fill lower-energy orbitals before occupying higher-energy ones. This principle ensures that atoms achieve the most stable, lowest energy configuration possible.
For most elements, the filling order is straightforward: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, and so forth.
However, exceptions like chromium occur due to electron stability preferences, where electrons rearrange for a more stable configuration.

Atomic Number

The atomic number of an element is the number of protons in its nucleus, which also equals the number of electrons in a neutral atom. For chromium, it has 24 protons, so its atomic number is 24.
This means chromium has 24 electrons to distribute among its orbitals. Understanding the atomic number is key to determining an element's electron configuration accurately.
Special cases like chromium's unique electron configuration show that the atomic number guides the initial electron distribution, but stability factors can lead to rearrangements.