Question

What is correct electron configuration for elemental chromium? A. [Ar] 3p 6 B. [Ar] 3d 54s 1 C. [Ar] 3d 6 D. [Ar] 3d 44s 2

Short answer

Option B: [Ar] 3d⁵ 4s¹

Step-by-step solution

Identify the Atomic Number

Find the atomic number of chromium. Chromium has an atomic number of 24, meaning it has 24 electrons.

Write the Basic Electron Configuration

The electron configuration is written by filling subshells in the order of their increasing energy levels. Follow the order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, etc.

Apply the Aufbau Principle

Following the aufbau principle (building-up principle), we add electrons to the subshells in order of increasing energy level. For chromium, the configuration would typically be: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁴

Consider Electron Configuration Exception

Chromium is an exception to the expected filling order due to electron repulsion and stability considerations. One of the 4s electrons is promoted to 3d, resulting in a half-filled 3d subshell and a half-filled 4s subshell. This makes the configuration more stable.

Write the Correct Configuration

Considering the exception, the correct electron configuration for chromium is: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹

Match with the Given Options

Compare the correct electron configuration with the provided options. The correct configuration: [Ar] 3d⁵ 4s¹ corresponds to Option B.

Key concepts

Atomic Number

The atomic number of an element is a fundamental concept in chemistry. It represents the number of protons in the nucleus of an atom of that element. For chromium, the atomic number is 24. This means every chromium atom has 24 protons.

The atomic number also tells us the number of electrons in a neutral atom. Hence, a neutral chromium atom contains 24 electrons. These electrons are arranged in various energy levels and subshells around the nucleus. Knowing the atomic number is the first step in determining the electron configuration of an element.

Electron Configuration

Electron configuration describes the distribution of electrons in an atom's orbitals. For chromium, which has 24 electrons, we must place these electrons in increasing energy levels using the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on.

To determine the electron configuration, we start by placing electrons in the lowest energy subshell before moving to higher ones. Initially, the configuration for chromium would appear as:

• 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁴

However, this is not the final configuration due to certain exceptions that we'll discuss later. Accurate electron configurations help in understanding chemical properties and reactivity.

Aufbau Principle

The aufbau principle, also known as the 'building-up' principle, is a rule used to determine an atom's electronic configuration. According to this principle, electrons fill atomic orbitals in order of increasing energy levels.

This means that electrons will occupy the lowest available energy subshell first before moving to higher ones. The typical order of filling is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on. Following the aufbau principle, the initial configuration for chromium should be:

• 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁴

But as we consider exceptions to this rule, chromium does not follow this standard filling order.

Electron Configuration Exceptions

Certain elements, like chromium, display exceptions to the typical electron configuration rules. These exceptions occur because some electron arrangements are more stable. In the case of chromium, a more stable configuration is achieved by promoting one electron from the 4s orbital to the 3d orbital.

As a result, chromium has the electron configuration:

• 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹

This setup occurs because a half-filled d-subshell (3d⁵) and a half-filled s-subshell (4s¹) provide additional stability due to reduced electron-electron repulsions and symmetrical distribution of electrons. Understanding these exceptions is crucial, as they impact an element's chemical behavior and its interactions with other elements.