Q.Name a transition element which does not exhibit variable oxidation states.

Concept: Transition Element Definition

A transition element is defined as an element that has an incomplete $d$-subshell in its ground state or in any of its common oxidation states. A transition element that shows NO variable oxidation states must have so few valence electrons that losing all of them is the only accessible, stable option.

Reasoning

1. Scandium ($\text{Sc}$, $Z=21$) has the ground-state configuration $[\text{Ar}]\,3d^1 4s^2$. Its $3d$ subshell is incomplete, so it qualifies as a transition element.
2. Scandium has only three valence electrons ($4s^2\,3d^1$). Losing all three gives $\text{Sc}^{3+} = [\text{Ar}]$, the stable noble-gas configuration — there is no other combination of electrons it can lose to reach a different stable ion. So $+3$ is its only oxidation state. …

Why This Question Is a Trap

It is tempting to answer zinc, since $\text{Zn}$ also shows only one common oxidation state ($+2$). But the question specifically asks for a transition element, and zinc fails that test: by this chapter's own IUPAC definition, a transition element must have an incomplete $d$-subshell in its ground state or in any common oxidation state. Zinc's $d$-subshell is $3d^{10}$ — completely filled — in both its neutral atom ($[\text{Ar}]\,3d^{10}4s^2$) and its only ion, $\text{Zn}^{2+}$ ($[\text{Ar}]\,3d^{10}$). So zinc is a $d$-block element, but it is not classified as a transition element at all, and cannot be the answer.

Step-by-Step Reasoning

1. Recall the definition. A transition element has a partially filled $d$-subshell in its ground state or in at least one common oxidation state.
2. Look for a genuine transition element with only one oxidation state. Scandium ($Z=21$) is the standard example: $[\text{Ar}]\,3d^1 4s^2$. Its $3d$ subshell holds just one electron — incomplete, so scandium qualifies as a transition element. …

Method: Electronic Configuration Analysis

This method uses the Aufbau principle and the definition of transition elements to identify exceptions in variable oxidation states.

Definition (Concept First)

A transition element is an element that has a partially filled $d$ subshell in its ground state or in any of its common oxidation states (IUPAC definition). Variable oxidation states arise because the $(n-1)d$ and $ns$ electrons have similar energies and can all participate in bonding.

Steps

1. Look for a transition element with very few valence electrons.

   An element with only one or two $d$/$s$ electrons beyond its noble-gas core has little room to stop at more than one stable ion — it tends to lose ALL of them and show just one oxidation state.

2. Write the ground-state electronic configuration of scandium (Z = 21).

3. Check its accessible oxidation states.

   Losing all three valence electrons ($4s^2\,3d^1$) gives $\text{Sc}^{3+} = [\text{Ar}]$ — the stable noble-gas configuration. There is no other electron combination that gives a comparably stable ion, so $+3$ is scandium's only oxidation state.

4. Rule out zinc as a trap answer. …

Common Mistakes: Transition Element Without Variable Oxidation States

✗ Mistake 1: Answering "Zinc"

The Error: Students pick Zinc (Zn), reasoning that it only ever shows the $+2$ state.

Why It's Wrong: The question asks for a transition element. By the IUPAC definition (an incomplete $d$-subshell in the ground state or in any common oxidation state), zinc does not qualify: its configuration is $[\text{Ar}]\,3d^{10}4s^2$, and its only common ion, $\text{Zn}^{2+}$, is $[\text{Ar}]\,3d^{10}$ — the $d$-subshell is completely filled in both. Zinc is a $d$-block element, but it is not a transition element, so it cannot be the answer.

✓ How to Avoid:

• First confirm the candidate genuinely qualifies as a transition element (incomplete $d$-subshell somewhere), then check whether it shows only one oxidation state.
• Zinc fails the first test, so it is disqualified before the oxidation-state question is even relevant.

✗ Mistake 2: Picking an Element That Actually Shows Variable Oxidation States

The Error: Students name Titanium (Ti) or Copper (Cu), thinking they show only one state.

Why It's Wrong:

• Titanium ($\text{Ti}$): Configuration $[\text{Ar}]\,3d^2 4s^2$. It shows $+2$, $+3$, and $+4$ — clearly variable.
• Copper ($\text{Cu}$): Configuration $[\text{Ar}]\,3d^{10}4s^1$. It shows $\text{Cu}^{+}$ and $\text{Cu}^{2+}$ — also variable.

✓ How to Avoid:

• Write out the accessible oxidation states for each candidate before answering; don't rely on memory of "which elements are usually stable."

✓ The Correct Answer

The transition element that does not exhibit variable oxidation states is:

Scandium (Sc, $Z=21$) …