All transition elements are d-block elements, but all d-block elements are not transition elements. Explain.

NCERT Exemplar · Q29

Q. All transition elements are d-block elements, but all d-block elements are not transition elements. Explain.

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Concept understanding — Transition Element Definition

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: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6$
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: $[Ar] 3d^{10} 4s^2$ — the 3d sub-shell is full (10 electrons).
But Zn commonly forms $Zn^{2+}$ : $[Ar] 3d^{10}$ — still full.
So zinc is NOT a transition element by the IUPAC definition.

Example: Copper (Cu)

Neutral Cu: $[Ar] 3d^{10} 4s^1$ — 3d is full.
But $Cu^{2+}$ : $[Ar] 3d^9$ — now the 3d sub-shell is incomplete.
So copper IS a transition element.

Watch out

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): $[Ar]\,3d^1\,4s^2$ — has one d-electron → transition element
Zinc (Zn): $[Ar]\,3d^{10}\,4s^2$ — d-subshell is full → not a transition element

2. The "incomplete d-subshell" condition

The definition focuses on incompleteness because:

A full d-subshell ( $d^{10}$ ) is exceptionally stable (like a noble gas configuration for d-orbitals)
Elements with $d^{10}$ 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: $[Ar]\,3d^{10}\,4s^2$ — d-subshell is full → not a transition element
Common ion: $\text{Zn}^{2+}$ : $[Ar]\,3d^{10}$ — still full → still not a transition element

Now consider Copper (Cu):

Ground state: $[Ar]\,3d^{10}\,4s^1$ — d-subshell is full → by atom definition alone, not a transition element
But $\text{Cu}^{2+}$ : $[Ar]\,3d^9$ — 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:

\text{Transition element} \iff \left( \text{Atom has } d^{1-9} \right) \lor \left( \text{Stable ion has } d^{1-9} \right)

Where:

$d^{1-9}$ means incomplete d-subshell (1 to 9 electrons)
$d^{0}$ or $d^{10}$ means complete (empty or full) → not a transition element

Common Exam Exceptions …

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A transition element must have a partially filled d-subshell in at least one common oxidation state. While all transition elements sit in the d-block, some d-block elements (like Zn, Cd, Hg) have completely filled d-orbitals in their atom and all stable ions, so they fail the definition.

The heart of this distinction lies in what we mean by "transition." The term captures the idea of an element that uses its d-electrons in bonding and chemistry—electrons that are neither completely absent nor completely paired away. A transition element must exhibit the characteristic properties that come from having unpaired d-electrons available: variable oxidation states, colored compounds, catalytic activity, and paramagnetism. These properties vanish when the d-subshell is fully occupied.

The d-block is a positional label: it includes every element whose differentiating electron (the last electron added according to the Aufbau principle) enters a d-orbital. That spans groups 3 through 12 on the periodic table. But position alone doesn't guarantee transition-element behavior.

Let's see why some d-block members are excluded:

Zinc (Zn), Cadmium (Cd), and Mercury (Hg) have the electronic configurations $[\text{noble gas}]\,d^{10}s^2$ . The d-subshell holds its maximum of ten electrons.
When these elements form ions, they lose the two s-electrons first, giving $[\text{noble gas}]\,d^{10}$ configurations. For example, $\mathrm{Zn}^{2+}$ is $[\mathrm{Ar}]\,3d^{10}$ —still a completely filled d-subshell.
Because the d-orbitals remain full in both the atom and the common ions, there are no unpaired d-electrons. The chemistry of Zn, Cd, and Hg resembles that of main-group metals: they show a single dominant oxidation state (+2), form colorless or white compounds (no d–d transitions), and are diamagnetic.
By the IUPAC definition, a transition element is one that has an incomplete d-subshell in the neutral atom or in any of its common oxidation states. Zn, Cd, and Hg fail this test in every relevant form.

Watch out

A common mistake is to assume that being in the d-block automatically makes an element a transition element. Always check whether a partially filled d-subshell exists in at least one oxidation state.

In contrast, consider Scandium (Sc): $[\mathrm{Ar}]\,3d^1 4s^2$ . The neutral atom has one d-electron. Even though $\mathrm{Sc}^{3+}$ is $[\mathrm{Ar}]$ (d-subshell empty), the neutral atom satisfies the definition, so Sc is a transition element. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

AP EAPCET 2025Set eng-2025-05-26-FN1 markMCQ
$Q. Identify the incorrect statement regarding the interstitial compounds (A) They have high melting points (B) They lose electrical conductivity during the formation from metal (C) They are chemically inert (D) They are very hard.$
›Reveal solutionSolution
This tests properties of interstitial compounds (metal lattices with small non-metal atoms in the voids). The answer is (B): they retain, not lose, metallic conductivity.

Concept and Intuition

Interstitial compounds (e.g., TiC, TiN, Fe $_3$ C, VH $_{0.56}$ ) form when small atoms such as H, C, N, or B fit into the interstitial (empty) spaces of a metal's crystal lattice without drastically disrupting the metallic bonding framework. Because the delocalised electron sea of the metal lattice is largely preserved, these compounds keep several metal-like characteristics: high melting point, hardness, and — crucially — metallic electrical conductivity.

Step-by-Step Solution

(A) High melting points: interstitial compounds are known for even higher melting points than the parent metal (interstitial atoms strengthen the lattice). TRUE.

(B) Loses electrical conductivity: since the metallic bonding/electron sea is retained, these compounds actually conduct electricity like the parent metal — they do NOT lose conductivity. FALSE — this is the incorrect statement.

(C) Chemically inert: interstitial compounds are indeed chemically quite inert/unreactive. TRUE. …
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AP EAPCET 2024Set eng-2024-05-20-AN1 markMCQ
$Q. The transition metal with highest melting point is (A) Re (B) Cr (C) Mo (D) W$
›Reveal solutionSolution
Tungsten (W) has the highest melting point of all the transition metals (and of all metals), around 3422°C.

Concept and Intuition

Melting points of the d-block transition metals rise toward the middle of each series (peaking around Group 6) because of strong metallic bonding reinforced by (n−1)d electron participation, then fall off toward both ends. Among all transition metals, tungsten holds the record for the highest melting point.

Step-by-Step Solution

Compare typical high melting points: W ≈ 3422°C, Re ≈ 3186°C, Mo ≈ 2623°C, Cr ≈ 1907°C.

Tungsten's melting point is the highest among these (and the highest of any metal), due to very strong metallic/covalent-like bonding involving its d-electrons.

Common Mistakes …
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AP EAPCET 2023Set eng-2023-05-15-AN1 markMCQ
$\mathrm{MnO_2}$
›Reveal solutionSolution
TiO $_2$ -pigment and MnO $_2$ -dry cell are standard correct facts; the Cu/Ni-"copper coins" pairing is a mismatch (Cu/Ni is used for the UK's "silver" coins, not its "copper" ones), so only i and ii are correct.

Concept and Intuition

This is a fact-recall matching question about industrially important compounds/alloys and their real-world uses.

Step-by-Step Solution

i. TiO $_2$ – pigment industry: True. Titanium dioxide is the most widely used white pigment (titanium white) in paints, plastics, and paper.

ii. MnO $_2$ – dry battery cells: True. In the Leclanché dry cell, MnO $_2$ acts as a depolarizer, oxidizing the hydrogen gas produced at the cathode. …
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AP EAPCET 2023Set eng-2023-05-17-FN1 markMCQ
$Q. Among V, Cr, Zn, Fe, the metal having lowest enthalpy of atomization is (A) V (B) Cr (C) Zn (D) Fe$
›Reveal solutionSolution
This tests why enthalpy of atomization varies across the 3d transition series. Answer: Zn has the lowest enthalpy of atomization.

Concept and Intuition

Enthalpy of atomization reflects the strength of metallic bonding, which comes largely from the overlap of unpaired d-orbital electrons between neighbouring metal atoms (in addition to the delocalized s-electrons). Metals with more unpaired d-electrons form stronger, more extensive metallic bonds and so have higher atomization enthalpies. Zinc has the electronic configuration $[\mathrm{Ar}]3d^{10}4s^2$ — its d-subshell is completely filled, leaving no unpaired d-electrons to participate in interatomic bonding, so its metallic bonding is comparatively weak.

Step-by-Step Solution

Write electron configurations: V = $[\mathrm{Ar}]3d^3 4s^2$ (3 unpaired d-electrons), Cr = $[\mathrm{Ar}]3d^5 4s^1$ (6 unpaired electrons total incl. 4s, exceptionally high atomization enthalpy), Fe = $[\mathrm{Ar}]3d^6 4s^2$ (4 unpaired d-electrons), Zn = $[\mathrm{Ar}]3d^{10}4s^2$ (0 unpaired d-electrons). …
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AP EAPCET 2023Set eng-2023-05-18-FN1 markMCQ
$\mathrm{V^{2+}}$
›Reveal solutionSolution
This tests recall of d- and f-block facts from NCERT: reducing power of $\mathrm{V^{2+}}$ , which metal the early lanthanoids resemble, coinage alloys, and actinoid oxidation states. Three of the four statements (i, iii, iv) are correct.

Concept and Intuition

Statement (i): A metal ion liberates $\mathrm{H_2}$ from a dilute acid when its reduction potential is more negative than that of the $\mathrm{H^+/H_2}$ couple (taken as $0\,V$ ). For vanadium, $E^\circ(\mathrm{V^{3+}/V^{2+}}) = -0.26\,V$ . Since this is negative, the reverse reaction ( $\mathrm{V^{2+}} \to \mathrm{V^{3+}} + e^-$ ) coupled with $2\mathrm{H^+} + 2e^- \to \mathrm{H_2}$ is spontaneous — so $\mathrm{V^{2+}}$ is a strong enough reducing agent to liberate hydrogen gas from dilute acid.

Statement (ii): Lanthanoid contraction means ionic radii shrink steadily across the series. The early members (La, Ce, Pr…) have relatively large $\mathrm{Ln^{3+}}$ radii, close in size to $\mathrm{Ca^{2+}}$ — this is exactly why rare-earth minerals substitute for calcium in nature. They do not behave like aluminium (aluminium chemistry — small, highly charge-dense $\mathrm{Al^{3+}}$ — is a different comparison used elsewhere, e.g. for beryllium/diagonal relationships). So (ii) is false as stated.

Statement (iii): Historically 'silver' coins in the UK were sterling silver, but since 1947 they have been struck in cupro-nickel (75% Cu, 25% Ni) — a genuine transition-metal alloy fact.

Statement (iv): Actinoids show a wider range of oxidation states than lanthanoids because 5f, 6d and 7s levels are close in energy. Np, Pu, and Am can all reach $+7$ (e.g. as $\mathrm{NpO_5^{3-}}$ ) under strongly oxidising alkaline conditions, though $+5$ / $+6$ are more common. So Np's maximum oxidation state of $+7$ is correct.

Step-by-Step Solution …
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AP EAPCET 2022Set eng-2022-07-04-AN1 markMCQ
$Q. Assertion (A): Transition metals and their complexes show catalytic activity. Reason (R): The activation energy of a reaction is lowered by the catalyst. (A) Both (A) and (R) are correct and (R) is the correct explanation of (A). (B) Both (A) and (R) are correct but (R) is not the correct explanation of (A). (C) (A) Is correct but (R) is incorrect. (D) (A) Is incorrect but (R) is correct.$
›Reveal solutionSolution
The key idea is that while both statements are factually correct, the Reason (R) is a general definition of a catalyst and does not specifically explain why transition metals and their complexes are particularly good at catalysis. The correct option is (B).

Concept and Intuition (Transition Element Definition)

Transition metals (like Fe, Ni, Pt, Pd) and their complexes are famous for their catalytic activity. This is not just because they lower activation energy — all catalysts do that. The special reason lies in their unique electronic structure: they have partially filled d-orbitals, which allow them to:

adopt multiple oxidation states,

form temporary bonds with reactants,

provide a surface or coordination site where reactants can come together in the right orientation.

The Reason (R) simply states the universal property of any catalyst. It is true, but it does not explain why transition metals in particular are so effective. So (R) is not the correct explanation of (A).

Step-by-step reasoning:

Check Assertion (A):

Transition metals and their complexes are indeed widely used as catalysts — e.g., iron in the Haber process, platinum in catalytic converters, nickel in hydrogenation. This is a well-known fact.

→ So (A) is correct.

Check Reason (R):

A catalyst, by definition, lowers the activation energy of a reaction, thereby increasing the rate without being consumed. This is a fundamental principle of catalysis.

→ So (R) is also correct.

Determine if (R) explains (A): …
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AP EAPCET 2022Set eng-2022-07-07-AN1 markMCQ
$Q. Which of the following elements are not regarded as transition elements? (A) Zn, Cd, Hg (B) Cu, Zn, Hg (C) Ag, Zn, Hg (D) Ag, Cd, Hg$
›Reveal solutionSolution
Group 12 elements (Zn, Cd, Hg) have a fully filled $d^{10}$ configuration and so fail the IUPAC definition of a transition element.

Concept and Intuition

IUPAC defines a transition element as one whose atom (in the ground state) or common ion has an incompletely filled d-subshell. Zinc, cadmium and mercury all have the configuration $(n-1)d^{10}ns^2$ and lose only the $ns^2$ electrons to form $M^{2+}$ , which is still $d^{10}$ — no partially filled d-orbital ever appears, so they are excluded from the transition series even though they sit in the d-block.

Step-by-Step Solution

Write electron configurations: Zn = $[Ar]3d^{10}4s^2$ ; Cd = $[Kr]4d^{10}5s^2$ ; Hg = $[Xe]4f^{14}5d^{10}6s^2$ .

In each case the d-subshell is completely filled ( $d^{10}$ ), both in the atom and in the common $M^{2+}$ ion. …
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AP EAPCET 2022Set eng-2022-07-08-FN1 markMCQ
$Q. Assertion (A): Transition elements have higher enthalpies of atomization. Reason (R): Large number of unpaired electrons present in transition elements facilitate strong interatomic interaction and strong bonding between atoms. (A) Both (A) and (R) are correct and (R) is the correct explanation of (A) (B) Both (A) and (R) are correct and (R) is not the correct explanation of (A). (C) (A) Is correct and (R) is incorrect. (D) (A) Is incorrect and (R) is correct.$
›Reveal solutionSolution
Both statements are true, and the reason genuinely explains the assertion: transition metals have high atomization enthalpies precisely because their unpaired d-electrons enable extra interatomic (covalent-like) bonding on top of ordinary metallic bonding. Answer: (A).

Concept and Intuition

Enthalpy of atomization measures the energy needed to convert one mole of metal atoms in the solid state into gaseous atoms — essentially, the strength of the metallic bonding holding the solid lattice together. Transition metals show unusually high atomization enthalpies compared to their neighbouring s- and p-block metals. NCERT explains this by noting that in transition metals, in addition to the delocalized valence-electron ('electron sea') metallic bonding common to all metals, the partially filled (n-1)d orbitals allow additional localized, covalent-like overlap between neighbouring atoms' d-orbitals. The greater the number of unpaired d-electrons available for this extra overlap, the stronger the overall interatomic bonding — which is exactly why atomization enthalpies of transition metals peak somewhere in the middle of each series (where the number of unpaired d-electrons is often highest) and are generally much larger than for s-/p-block metals.

Step-by-Step Solution

Check Assertion (A): transition elements have higher enthalpies of atomization — this is a well-established, textbook-supported fact (compare, e.g., atomization enthalpies of 3d transition metals to those of Ca, K, or Ga/Ge). True.

Check Reason (R): a large number of unpaired electrons facilitate strong interatomic interaction and strong bonding between atoms — also a textbook-supported mechanistic explanation. True. …
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AP EAPCET 2021Set eng-2021-10-05-FN1 markMCQ
$Q. The general trend of enthalpies of atomisation of d-block elements is ______ (A) Series-1 > Series-2 > Series-3 (B) Series-1 > Series-3 > Series-2 (C) Series-3 > Series-2 > Series-1 (D) Series-2 > Series-1 > Series-2$
›Reveal solutionSolution
This tests the periodic trend in enthalpies of atomisation across the three transition series; the answer is Series-3 (5d) > Series-2 (4d) > Series-1 (3d).

Concept and Intuition

Enthalpy of atomisation measures the energy needed to convert one mole of metal atoms in the solid (metallic) state into gaseous atoms — essentially a measure of the strength of metallic bonding. In transition metals, metallic bonding strength depends on the number of unpaired $d$ electrons and how well the $d$ -orbitals overlap between neighbouring atoms.

Step-by-Step Solution

Across a transition series, atomisation enthalpy is influenced by the number of unpaired electrons — it rises to a maximum near the middle of the series (where the number of unpaired electrons is highest) and falls off toward both ends.

Comparing the same group across the three transition series (3d, 4d, 5d), the outer $d$ -orbitals become progressively larger and more diffuse — $5d$ orbitals overlap more effectively with neighbouring atoms' orbitals than $4d$ , which in turn overlap better than $3d$ . …
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Q.All transition elements are d-block elements, but all d-block elements are not transition elements. Explain.

Concept understanding — Transition Element Definition

Transition Elements: From Intuition to Definition

The Intuition: Why "Transition"?

The Precise Definition (IUPAC)

1. The "d" sub-shell

2. The "or" part — cations matter

The "d-block" vs "Transition Elements"

Why this formula?

Transition Element Definition: The "Why" Behind the Definition

The Core Definition

Why This Definition? The Reasoning

1. The d-orbital filling pattern

2. The "incomplete d-subshell" condition

3. Why include ions?

The "Formula" — A Decision Tree

Common Exam Exceptions …

Transition Elements vs d-Block Elements

Concept and Intuition

Step-by-Step Solution

Concept and Intuition

Step-by-Step Solution

Common Mistakes …

Concept and Intuition

Step-by-Step Solution

Concept and Intuition

Step-by-Step Solution

Concept and Intuition

Step-by-Step Solution …

Concept and Intuition

Step-by-Step Solution

Concept and Intuition

Step-by-Step Solution

Concept and Intuition

Step-by-Step Solution

Q.All transition elements are d-block elements, but all d-block elements are not transition elements. Explain.

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