The Electron Configuration: Exceptions (Simplified): Videos & Practice Problems

Electron orbital stability is enhanced when d subshells are half-filled or fully filled, following Hund's rule. Notable exceptions in electron configurations occur in elements like chromium (Cr) and copper (Cu), where electrons are promoted from the 4s to the 3d orbitals to achieve stability. For chromium, the configuration shifts from Ar4s²d⁴ to Ar4s¹d⁵. Similarly, copper transitions from Ar4s²d⁹ to Ar4s¹d¹⁰.

The Electron Configuration: Exceptions (Simplified) Concept 1

Understanding the stability of electron orbitals, particularly the d subshell, is crucial in chemistry. The d orbitals achieve maximum stability when they are either half filled or fully filled. This stability arises from the symmetry of the electron distribution within these orbitals.

According to Hund's rule, when electrons occupy degenerate orbitals (orbitals of the same energy), they will first fill each orbital singly with parallel spins before pairing up. This means that in a half-filled d subshell, each of the five d orbitals contains one electron with the same spin orientation, represented as ↑. For example, a half-filled d subshell would look like this: ↑ ↑ ↑ ↑ ↑.

In contrast, a fully filled d subshell has all five orbitals filled with two electrons each, with the spins paired. This configuration is depicted as: ↑↓ ↑↓ ↑↓ ↑↓ ↑↓. Both configurations—half filled and fully filled—contribute to the overall stability of the atom, making them significant in understanding electron behavior and chemical properties.

The Electron Configuration: Exceptions (Simplified) Concept 2

Exceptions to electron configurations begin with chromium (Cr), which has an atomic number of 24. As the atomic number increases, particularly starting from chromium, certain elements exhibit deviations from the expected electron configurations. These exceptions primarily occur in six key elements: chromium, manganese, iron, cobalt, nickel, and copper.

To remember the sequence of these exceptions, note that after chromium, you skip the next four elements in the periodic table: manganese (Mn), iron (Fe), cobalt (Co), and nickel (Ni), before reaching copper (Cu). This pattern highlights the significance of these elements in understanding electron configurations.

In general, electron configurations are determined by the Aufbau principle, which states that electrons fill atomic orbitals in order of increasing energy levels. However, in certain cases, such as with chromium and copper, electrons may be promoted to higher energy orbitals to achieve a more stable electron arrangement, often resulting in half-filled or fully filled subshells. This phenomenon is crucial for explaining the unique chemical properties and stability of these elements.

Understanding these exceptions is essential for mastering the concepts of electron configurations and their implications in chemistry.

The Electron Configuration: Exceptions (Simplified) Concept 3

Chromium is a notable example in the study of electron configurations due to its unique behavior. Initially, one might expect the electron configuration of chromium to be represented as Ar 4s₂ 3d₄, where argon (Ar) serves as the noble gas core. However, chromium exhibits an exception to the expected pattern. The 3d subshell, which contains 4 electrons, has a tendency to achieve a half-filled or fully filled state for stability.

To facilitate this, one of the electrons from the 4s subshell is promoted to the 3d subshell. This results in the electron configuration being adjusted to Ar 4s₁ 3d₅. By promoting an electron from the 4s subshell, the 3d subshell now contains 5 electrons, achieving a half-filled configuration. This phenomenon highlights the principle that s and d subshells strive for stability through half-filled or fully filled states, which is a key concept in understanding transition metals and their electron configurations.

In summary, the correct electron configuration for chromium is Ar 4s₁ 3d₅, illustrating the importance of electron promotion in achieving a more stable arrangement of electrons in the d orbitals.

The Electron Configuration: Exceptions (Simplified) Concept 4

Copper is an interesting element when examining its electron configuration. Initially, one might expect its configuration to be Ar 4s²3d⁹ based on the periodic table. However, a closer look reveals that the 3d subshell is just one electron short of being completely filled. The stability of electron configurations is enhanced when d and p subshells are either half-filled or fully filled.

To achieve a more stable configuration, an electron from the 4s orbital can be promoted to the 3d subshell. This process transforms the electron configuration of copper from Ar 4s²3d⁹ to Ar 4s¹3d¹⁰. By donating one electron from the 4s orbital to the 3d orbital, copper achieves a fully filled 3d subshell, which is more stable. This phenomenon is not unique to copper; several other elements exhibit similar behavior, where one electron is transferred from the s orbital to the d orbital to create either half-filled or fully filled d orbitals. Understanding this driving force behind electron configuration exceptions is crucial for grasping the stability of transition metals.

The Electron Configuration: Exceptions (Simplified) Example 1

To determine the condensed electron configuration for a silver atom, which has an atomic number of 47, we start by recognizing that it contains 47 electrons in its neutral state. The initial electron configuration can be derived from the periodic table, leading us to the configuration of krypton, followed by the filling of the 5s and 4d orbitals. This gives us the preliminary configuration of [Kr] 5s² 4d⁹.

However, silver is known for its exception in electron configuration. To achieve a more stable arrangement, one electron from the 5s orbital is promoted to the 4d orbital. This results in the final electron configuration of [Kr] 5s¹ 4d¹⁰. This adjustment allows the d orbitals to be completely filled, which is energetically favorable.

It's important to note that this behavior is characteristic of several transition metals, where one electron is often transferred from the s orbital to the d orbital to achieve either half-filled or fully filled d subshells, enhancing stability. Silver, along with other elements exhibiting similar behavior, exemplifies this principle in electron configuration.

Illustrate the exception to the electron configuration of molybdenum.

Molybdenum (Mo), with an atomic number of 42, has a unique electron configuration that deviates from the expected pattern. Typically, one would anticipate its electron configuration to follow the order of filling orbitals, leading to the configuration of Krypton (Kr) 5s² 4d⁴. However, molybdenum is an exception, similar to chromium, due to the stability associated with half-filled d orbitals.

To achieve a more stable electron configuration, one electron from the 5s orbital is promoted to the 4d orbital. This results in the electron configuration of molybdenum being Krypton 5s¹ 4d⁵. The promotion of the electron allows for a half-filled d subshell, which is energetically favorable.

In terms of the electron orbital diagram, the configuration can be illustrated as follows: the 5s subshell contains one electron, while the 4d subshell contains five electrons. According to Hund's rule, the five electrons in the 4d subshell will occupy separate orbitals before pairing up, resulting in the arrangement of one electron in the 5s orbital and five unpaired electrons in the 4d orbitals.

This unique configuration highlights the importance of electron stability and the exceptions that can arise in the periodic table, particularly among transition metals.

Which of the following is the correct electron configuration of gold? 

• A.
• B.
• C.
• D.
• E.

Answer: (D)

A comparison of the electron configurations of palladium (Pd) and silver (Ag) indicates that: 

• A. Ag has 2 more d electrons and the same number of s electrons as Pd.
• B. Ag has 1 more d electron and the same number of s electrons as Pd.
• C. Ag has 2 more d electrons and 1 less s electron than Pd.
• D. Ag has 1 more d electron and 1 less s electron than Pd.
• E. Ag has 1 more d electron and 1 more s electron than Pd.

Answer: (C)

Here’s what students ask on this topic:

Why are d subshells more stable when they are half-filled or fully filled?

d subshells are more stable when they are half-filled or fully filled due to electron-electron repulsion and exchange energy. When d orbitals are half-filled, each electron occupies a separate orbital, minimizing repulsion. This configuration also maximizes exchange energy, which is the energy released when electrons with parallel spins exchange positions. Fully filled d subshells are stable because they achieve a symmetrical distribution of electrons, reducing repulsion and increasing stability. This stability is a key factor in the exceptions observed in electron configurations for elements like chromium and copper.

What is the electron configuration exception for chromium?

The electron configuration exception for chromium (Cr) occurs because it is more stable for the d subshell to be half-filled. Instead of the expected configuration of [Ar] 4s² 3d⁴, one electron from the 4s orbital is promoted to the 3d orbital. This results in the configuration [Ar] 4s¹ 3d⁵. This half-filled d subshell configuration is more stable due to reduced electron-electron repulsion and increased exchange energy.

What is the electron configuration exception for copper?

The electron configuration exception for copper (Cu) occurs because it is more stable for the d subshell to be fully filled. Instead of the expected configuration of [Ar] 4s² 3d⁹, one electron from the 4s orbital is promoted to the 3d orbital. This results in the configuration [Ar] 4s¹ 3d¹⁰. This fully filled d subshell configuration is more stable due to reduced electron-electron repulsion and increased exchange energy.

How does Hund's rule relate to electron configuration exceptions?

Hund's rule states that electrons will fill degenerate orbitals (orbitals with the same energy) singly before pairing up. This rule is crucial in understanding electron configuration exceptions because it explains why half-filled and fully filled d subshells are more stable. In elements like chromium and copper, electrons are promoted from the 4s orbital to the 3d orbital to achieve a half-filled or fully filled d subshell, respectively. This minimizes electron-electron repulsion and maximizes exchange energy, leading to a more stable configuration.

Why do elements like chromium and copper have electron configuration exceptions?

Elements like chromium and copper have electron configuration exceptions because achieving half-filled or fully filled d subshells provides greater stability. For chromium, the configuration changes from [Ar] 4s² 3d⁴ to [Ar] 4s¹ 3d⁵ to achieve a half-filled d subshell. For copper, the configuration changes from [Ar] 4s² 3d⁹ to [Ar] 4s¹ 3d¹⁰ to achieve a fully filled d subshell. These configurations reduce electron-electron repulsion and increase exchange energy, leading to a more stable electron arrangement.