Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

Zinc does not show variable valency, because (a) its d-subshell is incomplete (b) it is relatively soft metal (c) its d-subshell is complete (d) it has two electrons in outermost shell

Step-by-Step Solution

Answer

The correct answer is (c): Zinc's d-subshell is complete.

Step 1: Understanding the Question

The question asks why zinc does not show variable valency. Variable valency occurs when an element can exhibit more than one oxidation state. We need to determine which option correctly explains why zinc shows only one oxidation state.

Step 2: Analyzing Option (a)

Option (a) suggests zinc does not show variable valency because its d-subshell is incomplete. An incomplete d-subshell typically allows for multiple oxidation states, contradicting the premise that zinc does not show variable valency.

Step 3: Analyzing Option (b)

Option (b) mentions zinc is a relatively soft metal. However, the softness of a metal does not directly relate to its ability to exhibit variable valency. Therefore, this option can be ruled out.

Step 4: Analyzing Option (c)

Option (c) suggests that zinc's d-subshell is complete. Upon examining zinc's electronic configuration \( [Ar] \, 3d^{10} \, 4s^2 \), we observe that the 3d-subshell is fully filled with 10 electrons, preventing zinc from using the d-orbitals in bonding and resulting in a single oxidation state.

Step 5: Analyzing Option (d)

Option (d) states zinc has two electrons in its outermost shell. While correct, this information pertains to the outer s-orbital and does not directly explain its inability to exhibit variable valency, which is more influenced by the d-subshell configuration.

Step 6: Selecting the Correct Answer

The correct answer is option (c) since zinc's complete d-subshell prevents it from utilizing multiple oxidation states, hence no variable valency.

Key Concepts

Oxidation States

Oxidation states represent the charge of an element when it forms a compound. This concept is crucial for understanding how and why certain elements bond the way they do. For most elements, different oxidation states arise from their ability to lose or gain electrons and form stable structures.

In general, if an element's valence shell is not completely filled with electrons, it can show multiple oxidation states. This typically involves accessing different subshells to balance charges during bonding. Zinc, with its full d-subshell, is an exception. It predominantly exhibits a single oxidation state, which is usually +2.

Here are key points to remember about oxidation states:

• Oxidation states are often represented by numbers, indicating the total number of electrons lost or gained.
• States can provide insight into the bonding behavior of an element.
• An element with a fully occupied d-subshell, like zinc, may exhibit unique oxidation properties, often limited to a single stable state.

Electronic Configuration

Electronic configuration outlines how electrons are distributed around the nucleus in different shells and subshells. For zinc, the electron configuration is \([Ar] \, 3d^{10} \, 4s^2\), highlighting its full d-subshell. The configuration plays a significant role in determining an element’s chemical properties.

The core ideas about electronic configuration include:

• Electrons fill orbitals starting from the lowest energy level to the highest. This is known as the Aufbau principle.
• Each orbital can hold a specific number of electrons: s-subshells hold 2, p-subshells hold 6, and d-subshells hold 10.
• An element's reactivity often depends on the electron count in its outermost shell.

Zinc’s distribution of electrons, especially the filled d-subshell, restricts it to a single primary oxidation state, signifying its limited variable valency.

d-Subshell

The d-subshell plays a crucial role in the chemistry of transition metals. These elements typically exhibit multiple oxidation states due to incomplete d-subshells. When it’s fully occupied, like in the case of zinc, fewer oxidation states are available due to reduced participation in chemical bonding.

Understanding the d-subshell:

• The d-subshell can hold up to 10 electrons.
• Transition elements with partially filled d-subshells often demonstrate multiple oxidation states, leading to diverse chemical reactivities.
• In zinc’s case, its filled \(3d^{10}\) subshell prevents additional electron interactions, stabilizing it with a +2 oxidation state.

The fully filled nature of zinc’s d-subshell results in its fixed chemical behavior, lacking the flexibility seen in elements with incomplete d-subshells, such as those seen in many other transition metals.