Electron Configurations of Transition Metals: Exceptions: Videos & Practice Problems

Transition metals exhibit exceptions in electron configurations, particularly in chromium (Cr) and palladium (Pd). For Cr (atomic number 24), the expected configuration is altered from ${\mathrm{Ar}}^4{s}^2{d}^4$ to ${\mathrm{Ar}}^4{s}^1{d}^5$ due to electron promotion. Similarly, in period 5, elements like niobium (Nb) also show this behavior, with configurations adjusted to ${\mathrm{Kr}}^5{s}^1{d}^4$. Understanding these exceptions is crucial for mastering transition metal chemistry.

Exceptions (I)

Transition metals exhibit unique electron configurations, particularly starting from chromium (atomic number 24). As the atomic number increases, certain exceptions to the expected electron configurations can be observed, specifically in the d-block elements. These exceptions primarily occur in elements with d₄ and d₉ configurations.

For instance, chromium's expected electron configuration would be [\text{Ar}] 4s^2 3d^4. However, due to the promotion of an electron from the 4s orbital to the 3d orbital, the actual configuration becomes [\text{Ar}] 4s^1 3d^5. This promotion leads to a more stable half-filled d subshell, which is energetically favorable.

Similarly, for elements with a d₉ configuration, such as copper, the electron configuration is adjusted from [\text{Ar}] 4s^2 3d^9 to [\text{Ar}] 4s^1 3d^{10} after the promotion of an electron. This adjustment results in a fully filled d subshell, further enhancing stability.

In summary, the key takeaway is that for transition metals categorized under exception 1, specifically those with d₄ and d₉ configurations, an electron from the s orbital is promoted to the d orbital, leading to configurations of d₅ and d₁₀, respectively. Understanding these exceptions is crucial for predicting the chemical behavior and properties of these transition metals.

Electron Configurations Of Transition Metals: Exceptions Example

To write the condensed electron configuration for the silver ion (Ag⁺), we start with its atomic number, which is 47. The neutral silver atom has an electron configuration of Krypton (Kr) followed by the filling of the 5s and 4d orbitals, specifically Kr 5s¹ 4d¹⁰. However, silver is an exception to the typical filling order due to its electron configuration stability, where one electron from the 5s orbital is promoted to the 4d orbital, resulting in the configuration of Kr 5s¹ 4d¹⁰.

When forming the silver ion (Ag⁺), we lose one electron. This electron is removed from the highest principal energy level, which is the 5th shell in this case. Therefore, the electron configuration for the silver ion becomes Kr 4d¹⁰, as the 5s orbital is no longer filled.

In summary, the condensed electron configuration for the silver ion is Kr 4d¹⁰. This configuration reflects the stability of the filled 4d subshell, which is characteristic of transition metals like silver.

Provide a condensed electron configuration for Au atom and Au (III) ion.

• A. Au: [Xe]6s²4f¹⁴5d⁹
  Au³⁺: [Xe]6s²4f¹⁴5d⁶
• B. Au: [Xe]6s²4f¹⁴5d¹⁰
  Au³⁺: [Xe]4f¹⁴5d⁹
• C. Au: [Xe]6s¹4f¹⁴5d¹⁰
  Au³⁺: [Xe]4f¹⁴5d⁸
• D. Au: [Xe]6s¹4f¹⁴5d¹⁰
  Au³⁺: [Xe]6s¹4f¹⁴5d⁷

Answer: (C)

Exceptions (II)

In the study of electron configurations, particularly for transition metals in period 5 of the periodic table, certain exceptions arise that are crucial for understanding their behavior. A helpful mnemonic to remember these exceptions is "58, the number 8, b," which refers to the elements in groups 5b and 8b. The key elements to focus on are Niobium (Nb), Molybdenum (Mo), Technetium (Tc), and Palladium (Pd).

All these elements typically have one electron in their 5s orbital; however, Palladium stands out as an exception. While Niobium, with an atomic number of 41, starts with an electron configuration of Krypton (Kr) followed by 5s² 4d³, it actually undergoes a modification. To achieve a more stable configuration, one electron from the 5s orbital is promoted to the 4d orbital, resulting in the electron configuration of 5s¹ 4d⁴.

This adjustment is part of a broader pattern observed in transition metals, where electrons are often promoted to d orbitals to achieve greater stability. For Palladium, the configuration is even more unique, as it has no electrons in the 5s orbital, instead adopting a configuration of 4d¹⁰. Understanding these exceptions is essential for accurately predicting the chemical properties and reactivity of these transition metals.

Electron Configurations Of Transition Metals: Exceptions Example

The condensed electron configuration for a neutral ruthenium atom can be derived from its position in the periodic table. Typically, it would be represented as Kr 5s² 4d⁶. However, ruthenium is an exception among transition metals, where one electron from the 5s orbital is promoted to the 4d orbital. This results in the actual electron configuration being Kr 5s¹ 4d⁷. Thus, the final condensed electron configuration for ruthenium is Kr 5s¹ 4d⁷.

Given the following electron configuration, determine the identity of ion with +2 charge.

• A. Pd²⁺
• B. Ru²⁺
• C. Pb²⁺
• D. Cd²⁺

Answer: (A)

Here’s what students ask on this topic:

What are the exceptions to the electron configurations of transition metals?

Exceptions to the electron configurations of transition metals occur primarily in two types. Exception type 1 involves elements starting from chromium (atomic number 24) and follows a pattern of '2, skip, next 4'. In these elements, an electron from the s orbital is promoted to the d orbital, changing configurations like d₄ to d₅ and d₉ to d₁₀. For example, chromium's configuration changes from [Ar] 4s² 3d⁴ to [Ar] 4s¹ 3d⁵. Exception type 2 is observed in period 5, specifically in group 5b and 8b, affecting elements like Niobium. These elements generally have one electron in their 5s orbital, except for palladium, which has none. Niobium's configuration changes from [Kr] 5s² 4d³ to [Kr] 5s¹ 4d⁴ due to electron promotion.

Why does chromium have an unusual electron configuration?

Chromium has an unusual electron configuration because it falls under exception type 1 for transition metals. Normally, we would expect chromium to have the electron configuration [Ar] 4s² 3d⁴. However, to achieve a more stable configuration, one electron from the 4s orbital is promoted to the 3d orbital, resulting in [Ar] 4s¹ 3d⁵. This configuration allows for half-filled d orbitals, which are more stable due to reduced electron-electron repulsion and increased exchange energy.

What is the electron configuration of Niobium and why is it an exception?

Niobium (Nb) has an atomic number of 41 and falls under exception type 2 for transition metals. Its expected electron configuration is [Kr] 5s² 4d³. However, due to the promotion of an electron from the 5s orbital to the 4d orbital, its actual electron configuration is [Kr] 5s¹ 4d⁴. This promotion occurs to achieve a more stable configuration with a half-filled d orbital, which reduces electron-electron repulsion and increases exchange energy.

How does the electron configuration of palladium differ from other transition metals in period 5?

Palladium (Pd) is unique among transition metals in period 5 because it has no electrons in its 5s orbital. While other elements in this period typically have one electron in their 5s orbital, palladium's electron configuration is [Kr] 4d¹⁰. This configuration is achieved by promoting both 5s electrons to the 4d orbital, resulting in a fully filled d orbital, which is highly stable due to minimized electron-electron repulsion and maximized exchange energy.

What is the significance of half-filled and fully filled d orbitals in transition metals?

Half-filled and fully filled d orbitals in transition metals are significant because they provide extra stability to the atom. This stability arises from reduced electron-electron repulsion and increased exchange energy. For example, in chromium, the promotion of an electron from the 4s to the 3d orbital results in a half-filled 3d⁵ configuration, which is more stable than a partially filled d⁴ configuration. Similarly, a fully filled d¹⁰ configuration, as seen in palladium, is highly stable due to the symmetrical distribution of electrons.