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 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) Mn²⁺ is more stable than Mn³⁺.

Step 1: Understand the concept of oxidation states

Transition metals can exhibit multiple oxidation states due to the variable combinations of removing electrons from the \(4s\) and \((n-1)d\)\ shell. The minimum oxidation state corresponds to the electrons in the 4s orbital, and the maximum state corresponds to the sum of electrons in the 4s and 3d orbitals.

Step 2: Review stability of electronic configurations

Metals will tend towards more stable electronic configurations such as \((d^0), (d^5), \text{and} (d^{10})\). Examine this when comparing different ions of the same metal.

Step 3: Analyze Mn ions

For manganese, \(\text{Mn}^{2+} ightarrow [\text{Ar}] 3d^5\) is more stable than \(\text{Mn}^{3+} ightarrow [\text{Ar}] 3d^4\) due to the half-filled \(d^5\) configuration.

Step 4: Analyze Sc ions

For scandium, \(\text{Sc}^{2+}: [\text{Ar}] 3d^2\) and \(\text{Sc}^{3+}: [\text{Ar}] 3d^0\). The \(d^0\) state is more stable than partially filled configurations.

Step 5: Analyze Ti ions

For titanium, \(\text{Ti}^{3+}: [\text{Ar}] 3d^1\) and \(\text{Ti}^{4+}: [\text{Ar}] 3d^0\). The \(d^0\) configuration in \(\text{Ti}^{4+}\) is more stable.

Step 6: Analyze Fe ions

For iron, \(\text{Fe}^{2+} ightarrow [\text{Ar}] 3d^6\) and \(\text{Fe}^{3+} ightarrow [\text{Ar}] 3d^5\). \(\text{Fe}^{3+}\) having a half-filled \(d^5\) configuration is more stable than \(\text{Fe}^{2+}\).

Key Concepts

Variable Oxidation States

Transition elements are known for their capability to form various oxidation states. This versatility is primarily due to the transition metals being able to lose different numbers of electrons from their ns and (n-1) d subshells. In simpler terms, these elements can lose electrons from both the outermost s orbital and the preceding d orbital. This quality allows them to show a range of oxidation states, which is less common in non-transition metals.

For example, in the 3d transition series, the first five elements can have a minimum oxidation state equivalent to the number of electrons present in their 4s orbital. Their maximum oxidation state reflects the total number of electrons from both the 4s and 3d orbitals. This phenomenon explains why we see metals like manganese exhibiting several stable oxidation states, such as +2, +3, +4, +6, and +7. Each state corresponds to a different number of electrons being involved in bond formation. Understanding these states can aid in predicting the chemical behavior of transition elements in reactions.

The ability of transition elements to form multiple oxidation states also influences their roles in numerous industrial and biological processes, where different states are often required. This makes transition metals incredibly versatile and useful in fields ranging from catalysis to materials science.

Electronic Configuration

The electronic configuration of an element describes the distribution of electrons in its orbitals. It is fundamental for predicting the various properties and behaviors of the element. For transition elements, the electronic configuration of their ions can drastically affect their stability. This is because the energy associated with a particular electron configuration can make some states more favored than others.

Transition metals typically fill their d orbitals as they progress across a period. The energy level of these d orbitals allows the distribution of electrons to adjust depending on the oxidation state. For instance, considering a manganese ion with a 3d⁴ configuration being less stable than a 3d⁵ configuration gives insights into why Mn²⁺ is more stable than Mn³⁺. The stability here is due to the half-filled d⁵ subshell, which is energetically favorable.

Understanding electronic configuration is crucial when predicting an element's chemical interactions. It sheds light on why certain ions are more stable, and this knowledge can be applied to predict the behavior of these metals during chemical reactions.

Stability of d0, d5, and d10 Configurations

The stability of specific electron configurations is a fascinating aspect of transition metal chemistry. Certain configurations, namely d⁰, d⁵, and d¹⁰, are particularly stable due to their unique electronic arrangements.

• d⁰ configuration: The absence of d electrons imparts stability due to the lack of electron-electron repulsion within the d subshell. An example is Sc³⁺, which is found to be more stable than Sc²⁺ because its d orbitals are empty, offering minimal repulsive interactions.



• d⁵ configuration: This half-filled configuration is exceptionally stable due to the symmetric distribution of electrons. Mn²⁺ with a 3d⁵ configuration is a prime example, where the half-filled state is more energetically favorable than a filled d⁴.



• d¹⁰ configuration: A fully filled d subshell signifies maximum electron pairing and is considered very stable. An example would be elements like zinc, which frequently maintains this configuration and shows little variation in its oxidation states.

These preferred configurations underpin the stability of metals in various oxidation states, influencing their chemical reactivity and roles in industrial and biological processes. Understanding these principles can lead to more predictable outcomes in chemical synthesis and reactions involving transition metals.