What are inner transition elements? Decide which of the following atomic numbers are the atomic numbers of the

Exercises · 4.28

Q. What are inner transition elements? Decide which of the following atomic numbers are the atomic numbers of the inner transition elements: 29, 59, 74, 95, 102, 104.

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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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Inner transition elements are the f-block elements (lanthanoids and actinoids) where the last electron enters the (n–2)f subshell. Among the given atomic numbers, 59 (Pr, a lanthanoid), 95 (Am, an actinoid), and 102 (No, an actinoid) are inner transition elements.

Why This Approach Works

The periodic table is built on the principle of electron configuration. Elements are grouped into blocks (s, p, d, f) based on which subshell receives the last electron. Inner transition elements are the f-block elements — specifically, the two rows that sit apart from the main table: the lanthanoids (Ce to Lu, atomic numbers 58–71 — the fourteen elements following lanthanum) and the actinoids (Th to Lr, atomic numbers 90–103 — the fourteen elements following actinium).

The key identifier: the last electron enters the penultimate-outermost f-subshell, i.e., the $(n-2)f$ orbital. For lanthanoids, $n = 6$ (so $4f$ is filled); for actinoids, $n = 7$ (so $5f$ is filled).

So to decide which atomic numbers belong here, we need to check if the element’s electron configuration ends in an f-orbital filling — not d or p.

Step-by-Step Reasoning

Atomic number 29 (Copper, Cu)

Copper’s configuration is $[Ar] 3d^{10} 4s^1$ . The last electron enters the 3d subshell. This is a d-block element (transition metal), not an inner transition element.

→ Not an inner transition element.
Atomic number 59 (Praseodymium, Pr)

Pr lies in the lanthanoid series (atomic numbers 58–71). Its configuration is $[Xe] 4f^3 6s^2$ . The last electron enters the $4f$ subshell.

→ Yes, it is an inner transition element (lanthanoid).
Atomic number 74 (Tungsten, W)

Tungsten’s configuration is $[Xe] 4f^{14} 5d^4 6s^2$ . The last electron enters the $5d$ subshell. This is a d-block element (transition metal).

→ Not an inner transition element.
Atomic number 95 (Americium, Am)

Am is an actinoid (atomic numbers 90–103). Its configuration is $[Rn] 5f^7 7s^2$ . The last electron enters the $5f$ subshell.

→ Yes, it is an inner transition element (actinoid).
Atomic number 102 (Nobelium, No) …

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Method: Electronic Configuration Analysis

This method uses the Aufbau principle and the definition of inner transition elements to identify them by their atomic numbers.

Step 1: Recall the definition

Inner transition elements are those in which the last electron enters the f-orbital of the penultimate shell (the (n–2) shell). They belong to:

Lanthanoids (4f series): atomic numbers 58 to 71
Actinoids (5f series): atomic numbers 90 to 103

Step 2: Write the electronic configuration for each given atomic number

We only need to check where the last electron goes.

Atomic number	Element	Expected configuration (last subshell)	f-orbital?
29	Cu	$[Ar]\,3d^{10}\,4s^1$	No (d-block)
59	Pr	$[Xe]\,4f^3\,6s^2$	Yes
74	W	$[Xe]\,4f^{14}\,5d^4\,6s^2$	No (d-block, 5d)
95	Am	$[Rn]\,5f^7\,7s^2$	Yes
102	No	$[Rn]\,5f^{14}\,7s^2$	Yes

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

COMEDK 2026Set 2026-A1 markMCQ
$A$
›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 $[Xe] 4f^{14} 5d^{10} 6s^2$ . 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 $5d^{10}6s^2$ . The d-subshell is full. In its common oxidation states (e.g., Hg²⁺, which loses the 6s electrons), the d-subshell remains $5d^{10}$ — 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: …
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COMEDK 2026Set 2026-M1 markMCQ
$A$
›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 …
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COMEDK 2024Set 2024-A1 markMCQ
$\mathrm{Cr}<\mathrm{Ti}<\mathrm{V}<\mathrm{Mn}$
›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 out

A 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): …
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COMEDK 2023Set 2023-E1 markMCQ
$\mathrm{WO}_3$
›Reveal solutionSolution
[!TLDR]

Using standard d-block trends, W= $\mathrm{Mn_4N}$ (C), X= $\mathrm{WO_3}$ (A), Y= $\mathrm{SnCl_2}$ (D), Z= $\mathrm{TiCl_4}$ (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 $d^0$ / $d^{10}$ ions are colourless.

Solution

W — chemically inert, non-stoichiometric compound: interstitial nitrides/carbides such as $\mathrm{Mn_4N}$ have small atoms trapped in the metal lattice, are hard, inert and non-stoichiometric. So $\mathrm{W}=\mathrm{C}$ .

X — higher oxidation states more stable: among 5d metals the highest oxidation state is very stable; tungsten is stable as $\mathrm{W(VI)}$ in $\mathrm{WO_3}$ . So $\mathrm{X}=\mathrm{A}$ . …
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COMEDK 2021Set 20211 markMCQ
$Q. In 3d-transmission series, which one has the least melting point? (A) V (B) Zn (C) Mn (D) Cu$
›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 …
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COMEDK 2021Set 2021-B1 markMCQ
$Q. Which of the following statements is NOT correct with respect to interstitial compounds of transition metals? (A) They have comparatively low melting points than those of the pure metals. (B) They are chemically inert. (C) They retain metallic conductivity (D) They are very hard and rigid$
›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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←Previous question4.27 — What are alloys? Name an important alloy which contains some of the la…→Next question4.29 — The chemistry of the actinoid elements is not so smooth as that of the…

Q.What are inner transition elements? Decide which of the following atomic numbers are the atomic numbers of the inner transition elements: 29, 59, 74, 95, 102, 104.

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 …

Why This Approach Works

Step-by-Step Reasoning

Method: Electronic Configuration Analysis

Step 1: Recall the definition

Step 2: Write the electronic configuration for each given atomic number

Common Mistakes: Inner Transition Elements

Mistake 1: Confusing "Transition" with "Inner Transition"

Mistake 2: Forgetting the Two Series

Mistake 3: Misidentifying Atomic Numbers Near the Boundaries

Mistake 4: Guessing Based on "Looks Like a Metal"

Concept

Solution …

Concept

Solution

Q.What are inner transition elements? Decide which of the following atomic numbers are the atomic numbers of the inner transition elements: 29, 59, 74, 95, 102, 104.

Why This Approach Works

Step-by-Step Reasoning

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