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 begin with chromium (Cr, Z=24) and include copper (Cu). For chromium, the configuration is adjusted from argon 4s²3d⁴ to 4s¹3d⁵ to achieve half-filled stability. Similarly, copper's configuration shifts from argon 4s²3d⁹ to 4s¹3d¹⁰ for full stability, illustrating the drive for optimal electron arrangements.

The Electron Configuration: Exceptions (Simplified) Concept 1

Understanding the stability of electron orbitals is crucial in chemistry, particularly when examining d subshells. The stability of these orbitals is maximized when they are either half filled or completely filled. This phenomenon is largely attributed to the symmetry and distribution of electrons within the orbitals.

According to Hund's rule, when dealing with orbitals of the same energy level, known as degenerate orbitals, electrons will first occupy each orbital singly before pairing up. This means that in a half-filled configuration, each of the five d orbitals will contain one electron, all with parallel spins, represented as ↑ ↑ ↑ ↑ ↑. This arrangement minimizes electron-electron repulsion and maximizes stability.

In contrast, a completely filled d subshell involves filling each orbital with two electrons, following the sequence of filling: ↑ ↑ ↑ ↑ ↑ ↓ ↓ ↓ ↓ ↓. Here, each orbital is filled to capacity, which also contributes to stability due to the symmetrical distribution of electrons.

In summary, the half-filled and fully filled configurations of d orbitals are significant for their enhanced stability, which is a key concept in understanding electron configurations and their implications in chemical behavior.

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. Notably, these exceptions can be observed in six key elements: chromium, manganese, iron, cobalt, nickel, and copper.

To remember the sequence of these exceptions, one can visualize starting with chromium and then skipping the next four elements in the periodic table: manganese (Mn), iron (Fe), cobalt (Co), and nickel (Ni). This leads us to copper (Cu), where additional exceptions may occur. Understanding these exceptions is crucial for accurately determining the electron configurations of these elements.

In summary, the primary elements to focus on regarding exceptions in electron configurations are chromium, manganese, iron, cobalt, nickel, and copper. Recognizing these exceptions helps in grasping the complexities of electron arrangements in transition metals.

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 argon 4s₂3d₄. However, this configuration does not reflect the actual arrangement of electrons in chromium. The key concept here is that both s and d subshells have a tendency to achieve either a half-filled or fully filled state, which is energetically favorable.

In the case of chromium, with four electrons in the 3d subshell, there is an opportunity to promote one of the 4s electrons to the 3d subshell. This promotion allows the 3d subshell to become half-filled, which is a more stable configuration. Therefore, the correct electron configuration for chromium is argon 4s₁3d₅. This adjustment reflects the stability gained from having a half-filled d subshell, as it now contains five electrons.

Understanding this exception is crucial, as it highlights the importance of electron arrangements in determining the properties of elements. The drive towards achieving a half-filled set of d orbitals is a significant factor in the electron configuration of transition metals like chromium.

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 argon 4s²3d⁹. However, a closer look at the 3d orbitals reveals that by promoting one electron from the 4s orbital, we can achieve a more stable configuration. This is due to the tendency of d and p subshells to be either half-filled or fully filled, which contributes to the stability of the atom.

In the case of copper, which is classified as a d⁹ element, promoting an electron from the 4s orbital allows the configuration to become 4s¹3d¹⁰. This results in a completely filled d subshell, enhancing the stability of the atom. 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 achieve a more stable electron configuration.

Understanding this concept is crucial, as it highlights the exceptions in electron configurations for transition metals, driven by the desire for stability through half-filled or fully filled d orbitals.

The Electron Configuration: Exceptions (Simplified) Example 1

To determine the condensed electron configuration for a silver atom, we start with its atomic number, which is 47. This indicates that a neutral silver atom has 47 electrons. The initial electron configuration can be derived from the periodic table, where silver is located in the transition metals. The expected configuration would be based on the noble gas preceding silver, which is krypton (Kr), leading to an initial configuration of [Kr] 5s^2 4d^9.

However, silver is known for its electron configuration exception. In this case, to achieve a more stable configuration, one electron is promoted from the 5s orbital to the 4d orbital. This results in a fully filled d subshell, which is energetically favorable. Therefore, the correct condensed electron configuration for silver becomes [Kr] 5s^1 4d^{10}.

This phenomenon is not unique to silver; it occurs in several transition metals where one electron is transferred from the s orbital to the d orbital to achieve either half-filled or fully filled d orbitals, enhancing stability. Understanding these exceptions is crucial for accurately representing the electron configurations of transition metals.

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 correct electron configuration for molybdenum being Krypton 5s¹ 4d⁵. The promotion of the electron allows for a half-filled set of d orbitals, which is energetically favorable.

In an electron orbital diagram, this configuration can be illustrated as follows: the 5s orbital contains one electron, while the 4d orbitals contain five electrons. According to Hund's rule, the electrons in the 4d orbitals are distributed singly before pairing occurs, resulting in the configuration being represented as:

5s: ↑
4d: ↑ ↑ ↑ ↑ ↑

This arrangement highlights the stability gained through the half-filled d subshell, which is a key concept in understanding the electron configurations of transition metals like molybdenum.

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:

What are the exceptions to the electron configuration rules?

Exceptions to the electron configuration rules primarily occur in transition metals, starting with chromium (Cr) and copper (Cu). For chromium, the expected configuration is [Ar] 4s² 3d⁴, but it adjusts to [Ar] 4s¹ 3d⁵ to achieve a half-filled d subshell. Similarly, copper's expected configuration is [Ar] 4s² 3d⁹, but it changes to [Ar] 4s¹ 3d¹⁰ for a fully filled d subshell. These adjustments occur because half-filled and fully filled d subshells are more stable due to electron repulsion and symmetry considerations.

Why is the electron configuration of chromium [Ar] 4s¹ 3d⁵ instead of [Ar] 4s² 3d⁴?

The electron configuration of chromium is [Ar] 4s¹ 3d⁵ instead of [Ar] 4s² 3d⁴ because a half-filled d subshell (3d⁵) is more stable. This stability arises from reduced electron-electron repulsion and increased symmetry. By promoting one electron from the 4s orbital to the 3d orbital, chromium achieves a more stable configuration with half-filled d orbitals.

How does Hund's rule explain the stability of half-filled and fully filled d subshells?

Hund's rule states that electrons will fill degenerate orbitals (orbitals with the same energy) singly before pairing up. This minimizes electron-electron repulsion and maximizes total spin, leading to greater stability. In the context of d subshells, half-filled (d⁵) and fully filled (d¹⁰) configurations are particularly stable because they offer symmetrical electron distribution and reduced repulsion, making the atom more energetically favorable.

What is the electron configuration of copper and why does it differ from the expected configuration?

The electron configuration of copper is [Ar] 4s¹ 3d¹⁰ instead of the expected [Ar] 4s² 3d⁹. This adjustment occurs because a fully filled d subshell (3d¹⁰) is more stable than a partially filled one. By promoting one electron from the 4s orbital to the 3d orbital, copper achieves a more stable configuration with a fully filled d subshell, reducing electron repulsion and increasing symmetry.

What is the significance of the atomic number 24 in electron configuration exceptions?

The atomic number 24 is significant because it marks the beginning of notable exceptions in electron configurations, starting with chromium (Cr). Chromium, with an atomic number of 24, has an electron configuration that deviates from the expected pattern to achieve a more stable half-filled d subshell. This pattern of exceptions continues with other elements, such as copper, highlighting the importance of stability in electron arrangements.