Problem 112 - A Complete Resource Book in Chemistry for JEE Main

Question

The transition elements with some exceptions can show a large number of oxidation states. The various oxidation states are related to the electronic configuration of their atoms. The variable oxidation states of a transition metal is due to the involvement of \((\mathrm{n}-1) \mathrm{d}\) and outer \(\mathrm{ns}\)-electrons. For the first five elements of 3 d-transition series. The minimum oxidation state is equal to the number of electrons in 4s shell and the maximum oxidation state is equal to the sum of \(4 \mathrm{~s}\) and \(3 \mathrm{~d}\)-electrons. The relative stability of various oxidation state of a given element can be explained on the basis of stability of \(\mathrm{d}^{0}, \mathrm{~d}^{5}\) and \(\mathrm{d}^{10}\) configurations. In which of the following pair, the first species is more stable than second one (a) \(\mathrm{Mn}^{2+}, \mathrm{Mn}^{3+}\) (b) \(\mathrm{Sc}^{2+}, \mathrm{Sc}^{3+}\) (c) \(\mathrm{Ti}^{3+}, \mathrm{Ti}^{4+}\) (d) \(\mathrm{Fe}^{2+}, \mathrm{Fe}^{3+}\)

Step-by-Step Solution

Answer

(a) \( \text{Mn}^{2+} \) is more stable than \( \text{Mn}^{3+} \).

Step 1: Understanding Oxidation States

Transition metals have variable oxidation states because they can use electrons from both their \( (n-1)d \) and \( ns \) orbitals for bonding. The first 3d-transition series includes Sc, Ti, V, Cr, and Mn, where the 4s orbitals are famously involved in oxidation states.

Step 2: Determine Stability Based on Electron Configurations

The stability of an ion can be predicted based on its electronic configuration, especially stable are \( d^0 \), \( d^5 \), and \( d^{10} \) configurations. Ions with these configurations tend to be more stable.

Step 3: Evaluate the Species in Pair (a): \( \text{Mn}^{2+} \) and \( \text{Mn}^{3+} \)

Manganese has electronic configuration \([Ar] 3d^5 4s^2\). For \( \text{Mn}^{2+} \), the configuration becomes \([Ar] 3d^5\), a stable \( d^5 \) configuration. For \( \text{Mn}^{3+} \), it becomes \([Ar] 3d^4\). The \( d^5 \) configuration of \( \text{Mn}^{2+} \) is more stable than \( d^4 \) of \( \text{Mn}^{3+} \).

Step 4: Evaluate the Species in Pair (b): \( \text{Sc}^{2+} \) and \( \text{Sc}^{3+} \)

Scandium has electronic configuration \([Ar] 3d^1 4s^2\). \( \text{Sc}^{3+} \) has a \( d^0 \) configuration, which is more stable compared to \( \text{Sc}^{2+} \) which would be \([Ar] 3d^1\), making \( \text{Sc}^{3+} \) the more stable ion.

Step 5: Evaluate the Species in Pair (c): \( \text{Ti}^{3+} \) and \( \text{Ti}^{4+} \)

Titanium has electronic configuration \([Ar] 3d^2 4s^2\). For \( \text{Ti}^{3+} \), it becomes \([Ar] 3d^1\). For \( \text{Ti}^{4+} \), it reaches a \( d^0 \) configuration. Therefore, \( \text{Ti}^{4+} \) is more stable due to the \( d^0 \) configuration.

Step 6: Evaluate the Species in Pair (d): \( \text{Fe}^{2+} \) and \( \text{Fe}^{3+} \)

Iron has electronic configuration \([Ar] 3d^6 4s^2\). For \( \text{Fe}^{2+} \), it becomes \([Ar] 3d^6\). For \( \text{Fe}^{3+} \), it becomes \([Ar] 3d^5\), which is a highly stable \( d^5 \) configuration. Thus, \( \text{Fe}^{3+} \) is more stable due to the \( d^5 \) configuration compared to \( \text{Fe}^{2+} \).

Step 7: Conclusion

From the evaluated pairs, only \( \text{Mn}^{2+} \) with \( d^5 \) configuration is found to be more stable than \( \text{Mn}^{3+} \), compared to other pairs where the second species is more stable.

Key Concepts

Transition metals

Transition metals are a unique group in the periodic table, often found in the central block, comprising elements like iron, copper, and manganese. These metals are characterized by their ability to form positive ions with varying oxidation states.
Due to their position, they commonly have incomplete "d" subshells. This property is what allows them to have such diverse chemical behavior.
Transition metals exhibit properties such as:

• High melting points and densities.
• The ability to form colored compounds.
• Conductivity of electricity and heat.
• The capacity to act as catalysts in chemical reactions.

These characteristics make transition metals essential in numerous industrial and biological processes.

Electronic configuration

The electronic configuration of an element describes how electrons are distributed in the atomic orbitals. Transition metals have partially filled "d" orbitals, which are (n-1)d in their electronic configuration.
For example, iron has an electronic configuration of \([Ar] 3d^6 4s^2\). This represents the core electrons and the electrons in the "d" and "s" levels.
Understanding the electronic configuration is crucial as it influences:

• The element's chemical properties.
• Reactivity and bonding behavior.
• Possible oxidation states and stability.

The interplay between "s" and "d" electrons significantly contributes to the complexity observed in transition metals.

3d-transition series

The 3d-transition series consists of ten elements from scandium (Sc) to zinc (Zn), located in the fourth period of the periodic table.
These elements have electrons entering the 3d subshell, contributing to their complex behavior.

• Each element ranges from having a 3d to a 3d10 configuration as you move across the series.
• The "d" subshell becomes more filled, influencing the oxidation states.
• Elements like manganese, with a half-filled 3d5, display preferred stability.

Understanding the 3d-transition series is key for appreciating how these metals behave chemically, from bonding patterns to various oxidation states.

Electron stability

Electron stability in transition metals is determined by their electronic configuration, particularly the "d" orbitals. Generally, certain configurations like \( d^0 \), \( d^5 \), and \( d^{10} \) are highly stable.

• \( d^0 \): No "d" electrons, seen in higher oxidation states.
• \( d^5 \): Half-filled "d" shell, providing symmetric electron distribution.
• \( d^{10} \): Fully filled, associated with low reactivity and high stability.

The preference for these stable configurations explains why certain ions tend to form, dictating the element's chemistry. For instance, Fe^{3+} is more stable than Fe^{2+} due to its \( d^5 \) configuration.

Variable oxidation states

One hallmark of transition metals is their ability to exhibit variable oxidation states. This is largely due to the closeness in energy of their \( (n-1)d \) and \( ns \) subshells. As a result, electrons from both shells can participate in bonding.
Variable oxidation states offer:

• Flexibility in forming compounds with different elements.
• Enhanced catalytic properties due to the ability to change oxidation states during reactions.
• Diverse range in colors for the compounds, influenced by electron transitions among the "d" orbitals.

Understanding these oxidation states is essential for predicting reaction pathways and the formation of complex ions in transition metal chemistry.