Q.Name a transition element which does not exhibit variable oxidation states.
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Transition Elements: From Intuition to Definition
Imagine you're building a house with bricks. Most bricks are identical — you stack them in neat rows. But some bricks are special: they have extra slots on their sides where you can attach hooks, magnets, or other bricks. These special bricks can change the shape of the wall, conduct electricity, or even change colour when you heat them.
In the periodic table, transition elements are those special bricks. They are the metals that sit in the middle block — groups 3 to 12 — and they have a unique ability: they can use their inner electrons (not just the outermost ones) to form bonds, change oxidation states, and create colourful compounds.
The Intuition: Why "Transition"?
The word "transition" comes from the idea that these elements form a bridge between the highly reactive metals on the left (like sodium, magnesium) and the less reactive metals / non-metals on the right (like aluminium, silicon). Their properties are not extreme — they are in-between.
But the real reason they are special lies in their electron configuration.
The Precise Definition (IUPAC)
A transition element is an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.
Let's unpack that.
1. The "d" sub-shell
Electrons are arranged in shells (K, L, M, N...) and sub-shells (s, p, d, f). The d sub-shell can hold a maximum of 10 electrons. In transition elements, the d sub-shell is being filled — but not completely.
For example, consider Iron (Fe):
- Atomic number 26
- Electron configuration: 1s22s22p63s23p64s23d6
- The 3d sub-shell has 6 electrons — it is incomplete (it can hold 10).
So iron is a transition element.
2. The "or" part — cations matter
Some elements have a complete d sub-shell in their neutral atom, but when they lose electrons to form positive ions (cations), the d sub-shell becomes incomplete.
Example: Zinc (Zn)
- Neutral Zn: [Ar]3d104s2 — the 3d sub-shell is full (10 electrons).
- But Zn commonly forms Zn2+: [Ar]3d10 — still full.
- So zinc is NOT a transition element by the IUPAC definition.
Example: Copper (Cu)
- Neutral Cu: [Ar]3d104s1 — 3d is full.
- But Cu2+: [Ar]3d9 — now the 3d sub-shell is incomplete.
- So copper IS a transition element.
A common mistake: thinking that all elements in the d-block (groups 3–12) are transition elements. They are not. Zinc, cadmium, and mercury are d-block elements but NOT transition elements because their common cations have a full d sub-shell.
The "d-block" vs "Transition Elements"
| d-block elements | Transition elements |
|---|---|
| Groups 3 to 12 | Groups 3 to 11 (excluding Zn, Cd, Hg) |
| All have d electrons | Must have incomplete d sub-shell in atom or common cation |
Why this formula?
Transition Element Definition: The "Why" Behind the Definition
The Core Definition
A transition element (IUPAC definition) is an element whose atom has an incomplete d-subshell in its ground state or can form stable ions with an incomplete d-subshell.
Key exam point: This definition covers both the neutral atom and its common ions.
Why This Definition? The Reasoning
1. The d-orbital filling pattern
In the periodic table, transition elements belong to the d-block (Groups 3–12). As we move across a period, electrons fill the (n−1)d orbitals after the ns orbital.
For example, in Period 4:
- Scandium (Sc): [Ar]3d14s2 — has one d-electron → transition element
- Zinc (Zn): [Ar]3d104s2 — d-subshell is full → not a transition element
2. The "incomplete d-subshell" condition
The definition focuses on incompleteness because:
- A full d-subshell (d10) is exceptionally stable (like a noble gas configuration for d-orbitals)
- Elements with d10 configurations do not show the characteristic properties of transition metals (variable oxidation states, coloured compounds, catalytic activity, paramagnetism)
3. Why include ions?
Consider Zinc (Zn):
- Ground state: [Ar]3d104s2 — d-subshell is full → not a transition element
- Common ion: Zn2+: [Ar]3d10 — still full → still not a transition element
Now consider Copper (Cu):
- Ground state: [Ar]3d104s1 — d-subshell is full → by atom definition alone, not a transition element
- But Cu2+: [Ar]3d9 — incomplete d-subshell → is a transition element
Therefore: The definition must include ions to correctly classify elements like Cu, which form stable ions with incomplete d-subshells.
The "Formula" — A Decision Tree
The definition can be expressed as a logical condition:
Transition element⟺(Atom has d1−9)∨(Stable ion has d1−9)
Where:
- d1−9 means incomplete d-subshell (1 to 9 electrons)
- d0 or d10 means complete (empty or full) → not a transition element
Common Exam Exceptions …
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
- Scandium (Sc, Z=21) has the ground-state configuration [Ar]3d14s2. Its 3d subshell is incomplete, so it qualifies as a transition element.
- Scandium has only three valence electrons (4s23d1). Losing all three gives Sc3+=[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. …
The transition element that does not exhibit variable oxidation states is scandium (Sc). Its only accessible oxidation state is +3, because losing all three valence electrons (4s23d1) takes it straight to the stable noble-gas configuration [Ar], with no other electron combination available for a different stable ion.
Why This Question Is a Trap
It is tempting to answer zinc, since 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 3d10 — completely filled — in both its neutral atom ([Ar]3d104s2) and its only ion, Zn2+ ([Ar]3d10). 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
- Recall the definition. A transition element has a partially filled d-subshell in its ground state or in at least one common oxidation state.
- Look for a genuine transition element with only one oxidation state. Scandium (Z=21) is the standard example: [Ar]3d14s2. 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
-
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.
-
Write the ground-state electronic configuration of scandium (Z = 21).
Sc:[Ar]3d14s2
-
Check its accessible oxidation states.
Losing all three valence electrons (4s23d1) gives Sc3+=[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.
-
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 [Ar]3d104s2, and its only common ion, Zn2+, is [Ar]3d10 — 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 (Ti): Configuration [Ar]3d24s2. It shows +2, +3, and +4 — clearly variable.
- Copper (Cu): Configuration [Ar]3d104s1. It shows Cu+ and Cu2+ — 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) …
- COMEDK 2026Set 2026-A1 markMCQQ.Pick out the correct option Assertion(A): Mercury is not considered as a transition element Reason (R): Mercury is a liquid (A) A is false but R is true (B) Both A and R are true but R is the correct explanation of A (C) A is true but R is false (D) Both A and R are true, R is not the correct explanation of A
›Reveal solutionSolution
The assertion is true (mercury is not a transition element) and the reason is true (mercury is a liquid at room temperature), but the reason does not explain the assertion — the correct classification depends on electron configuration, not physical state.
Concept & Intuition
Transition elements are defined by having an incomplete d-subshell in their neutral atom or common oxidation states. Mercury (Hg) has the electron configuration [Xe]4f145d106s2. Its 5d subshell is completely filled, so it does not meet the definition of a transition element. The fact that mercury is a liquid at room temperature is a physical property related to relativistic effects and weak metallic bonding — it has nothing to do with its electronic classification. The question tests whether you can separate a true statement (mercury is liquid) from its relevance to a different true statement (mercury is not a transition element).
Step-by-step reasoning
-
Check the Assertion (A):
Mercury’s ground-state electron configuration ends in 5d106s2. The d-subshell is full. In its common oxidation states (e.g., Hg²⁺, which loses the 6s electrons), the d-subshell remains 5d10 — still full. Since a transition element requires an incomplete d-subshell in the atom or a common ion, mercury does not qualify.
→ Assertion (A) is true.
-
Check the Reason (R):
Mercury is indeed a liquid at standard temperature and pressure (melting point −38.8 °C).
→ Reason (R) is true.
-
Evaluate the connection between A and R:
The reason given for why mercury is not a transition element is “mercury is a liquid.” But the actual reason is its filled d-subshell. The liquid state is irrelevant to the electronic definition. Therefore, R is not the correct explanation of A.
-
Match to the options: …
-
- COMEDK 2026Set 2026-M1 markMCQQ.The statements given below contains assertion and reason. Choose the correct option Assertion (A): Cr, Mo and W possess the highest melting point in their respective series of elements. Reason (R): Cr, Mo and W have stable half-filled electrons in the 'd' shell resulting in strong metallic bonding. (A) A is true but R is false (B) Both A and R are true, R is the correct explanation of A (C) Both A and R are true but R is not the correct explanation of A (D) A is false but R is true
›Reveal solutionSolution
[!TLDR]
The assertion (Cr, Mo, W have the highest melting points in their series) and the reason (their stable half-filled d configuration gives the strongest metallic bonding) are both true, and the reason is the correct explanation, so option (B).
Concept
From CBSE Class 12 d- and f-block chemistry: the melting point of a transition metal depends on the strength of metallic bonding, which rises with the number of unpaired (n−1)d and ns electrons taking part in bonding. This number is greatest near the middle of each series.
Solution …
- COMEDK 2024Set 2024-A1 markMCQQ.The correct order of increasing melting point is (A) Cr<Ti<V<Mn (B) Mn<Ti<V<Cr (C) Ti<V<Cr<Mn (D) V<Ti<Mn<Cr
›Reveal solutionSolution
The melting points of transition metals depend on the number of unpaired d-electrons available for metallic bonding. For Cr, Ti, V, and Mn, the order of increasing melting point is Mn < Ti < V < Cr, which corresponds to option (B).
The key concept here is metallic bonding strength in transition metals. In a transition metal, the melting point reflects how strongly the atoms are held together. The strength of metallic bonding depends on the number of electrons that can participate in bonding — specifically, the number of unpaired d-electrons. More unpaired d-electrons mean stronger bonding and a higher melting point.
For the first-row transition metals from Ti to Mn, the d-orbital filling is:
- Ti: [Ar] 3d² 4s² → 2 unpaired d-electrons
- V: [Ar] 3d³ 4s² → 3 unpaired d-electrons
- Cr: [Ar] 3d⁵ 4s¹ → 6 unpaired electrons (because of the half-filled stability, one 4s electron moves to the 3d)
- Mn: [Ar] 3d⁵ 4s² → 5 unpaired d-electrons
Watch outA common mistake is to think that Mn, with 5 unpaired d-electrons, should have a very high melting point. But Mn has an unusually low melting point because its half-filled d-subshell is stable and symmetrical, leading to weaker metallic bonding — the atoms don't "stick" as strongly.
Now, let's work through the reasoning step by step.
-
Understand the trend in metallic bonding strength.
Melting point in transition metals generally increases with the number of unpaired d-electrons, because these electrons form a "sea" that binds the positive metal ions together. However, when the d-subshell is exactly half-filled (d⁵) or fully filled (d¹⁰), the extra stability reduces the tendency to share electrons, lowering the melting point.
-
List the elements and their unpaired d-electron counts:
- Ti: 2 unpaired
- V: 3 unpaired
- Cr: 6 unpaired (due to 4s¹ 3d⁵ configuration)
- Mn: 5 unpaired
-
Predict the order based on unpaired electrons (ignoring the half-filled anomaly): …
- COMEDK 2023Set 2023-E1 markMCQQ.Match the following characteristics of transition metals given in Column I with the examples listed in Column II .tg {border-collapse:collapse;border-spacing:0;} .tg td{border-color:black;border-style:solid;border-width:1px;font-family:Arial, sans-serif;font-size:14px; overflow:hidden;padding:10px 5px;word-break:normal;} .tg th{border-color:black;border-style:solid;border-width:1px;font-family:Arial, sans-serif;font-size:14px; font-weight:normal;overflow:hidden;padding:10px 5px;word-break:normal;} .tg .tg-c3ow{border-color:inherit;text-align:center;vertical-align:top} .tg .tg-7btt{border-color:inherit;font-weight:bold;text-align:center;vertical-align:top} .tg .tg-0pky{border-color:inherit;text-align:left;vertical-align:top} S. No. Characteristic properties S. No. Examples W Chemically inert, non-stoichiometric compound A WO3 X Higher oxidation states are more stable B TiCl4 Y Due to inert pair effect, lower oxidation state is more stable C Mn4N Z. Colourless compound of transition metal D SnCl2 (A) W=BX=CY=DZ=A (B) W=CX=DY=AZ=B (C) W=CX=AY=DZ=B (D) W=AX=DY=CZ=B
›Reveal solutionSolution
[!TLDR]
Using standard d-block trends, W=Mn4N (C), X=WO3 (A), Y=SnCl2 (D), Z=TiCl4 (B), which is option (C).
Concept
This is the CBSE/NCERT Class 12 "d- and f-block" chapter: transition metals form hard, chemically inert interstitial (non-stoichiometric) compounds; heavier elements show stable high oxidation states; the inert-pair effect stabilises lower oxidation states for post-transition heavy elements; and d0/d10 ions are colourless.
Solution
W — chemically inert, non-stoichiometric compound: interstitial nitrides/carbides such as Mn4N have small atoms trapped in the metal lattice, are hard, inert and non-stoichiometric. So W=C.
X — higher oxidation states more stable: among 5d metals the highest oxidation state is very stable; tungsten is stable as W(VI) in WO3. So X=A. …
- COMEDK 2021Set 20211 markMCQQ.In 3d-transmission series, which one has the least melting point? (A) V (B) Zn (C) Mn (D) Cu
›Reveal solutionSolution
Compare: V ~ 2175 K, Mn ~ 1519 K, Cu ~ 1358 K.
Concept: Melting points across the 3d series depend on the number of unpaired d electrons available for metallic bonding.
Zn has the configuration 3d10 4s2 - a completely filled d subshell. Its d electrons are not available for metallic bonding, so only the two 4s electrons participate; metallic bonding is weakest. Zn melts …
- COMEDK 2021Set 2021-B1 markMCQQ.Which of the following statements is NOT correct with respect to interstitial compounds of transition metals? (A) They have comparatively low melting points than those of the pure metals. (B) They are chemically inert. (C) They retain metallic conductivity (D) They are very hard and rigid
›Reveal solutionSolution
Interstitial compounds are known for high (not low) melting points, so statement (A) is the incorrect one.
Properties of interstitial compounds:
- High melting points, often exceeding those of the parent metal — so (A) is wrong.
- (B) They are chemically fairly inert — correct.
- (C) They retain metallic conductivity — correct. …
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