Q.Silver atom has completely filled d orbitals (4d10) in its ground state. How can you say that it is a transition element?
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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 …
Silver is classified as a transition element not by its ground-state configuration alone, but by the definition based on its ions.
Key idea: A transition element is defined as an element whose atom in the ground state or in any of its common oxidation states has an incomplete d subshell.
Reasoning:
- In the ground state, silver has the configuration [Kr]4d105s1 — the d subshell is completely filled.
- However, silver commonly forms the Ag+ ion by losing the single 5s electron. The configuration of Ag+ is [Kr]4d10, which is still completely filled. …
Silver’s ground-state configuration is [Kr]4d105s1. Although the 4d subshell is fully filled, silver is a transition element because in its common +1 oxidation state it loses the 5s electron, leaving a 4d10 configuration — but the definition of a transition element hinges on having an incomplete d subshell either in the ground state or in any common oxidation state. Silver qualifies because it can form Ag2+ (with 4d9), which is an incomplete d subshell. The key is that the definition does not require the ground state itself to have an incomplete d subshell.
Why This Question Trips Students Up
The confusion is natural. You see 4d10 — a completely filled d subshell — and think “that’s not a transition element.” But the official IUPAC definition (and the one used in Indian board exams) is broader:
Transition element: An element whose atom has an incomplete d subshell or which can give rise to cations with an incomplete d subshell.
This means we must check both the ground state and the common oxidation states of the element. Silver’s ground state is 4d105s1 — the d subshell is full. But that’s not the end of the story.
Step-by-Step Reasoning
-
Write the ground-state electronic configuration of silver (Ag, atomic number 47).
Following the Aufbau principle:
1s22s22p63s23p63d104s24p64d105s1
In condensed form: [Kr]4d105s1.
The 4d subshell is completely filled — no doubt about that.
-
Check the ground state against the definition.
The ground state has 4d10 — a full d subshell. So by the first part of the definition (incomplete d subshell in the atom), silver does not qualify.
But the definition has a second part: “or which can give rise to cations with an incomplete d subshell.”
-
Examine the common oxidation states of silver.
Silver most commonly forms Ag+ (as in AgNO3, AgCl, etc.).
Ag+: remove the 5s1 electron → configuration becomes [Kr]4d10.
That’s still a full d10 — so Ag+ does not have an incomplete d subshell either.
At this point, many students stop and conclude silver is not a transition element. But that’s a mistake — because we haven’t checked all common oxidation states.
-
Look for a less common but still important oxidation state: Ag2+.
Silver can also form Ag2+ compounds (e.g., AgF2, AgO). This is a stable oxidation state, though less common than +1.
Ag2+: remove the 5s1 electron and one 4d electron → configuration becomes [Kr]4d9.
4d9 is incomplete — it has one vacancy in the d subshell. …
Method: Electronic Configuration Analysis Based on IUPAC Definition
Step 1 – Recall the IUPAC definition of a transition element
A transition element is defined as an element whose atom has an incomplete d-subshell in its ground state or can form stable ions with an incomplete d-subshell.
Step 2 – Examine the ground state of silver
Silver (Ag, atomic number 47) has the ground state configuration:
[Kr]4d105s1
Here, the 4d subshell is completely filled (4d10). So in the ground state, silver does not have an incomplete d-subshell.
Step 3 – Check the ions formed by silver
Silver commonly forms the Ag⁺ ion by losing the single 5s electron:
Ag→Ag++e−
The electronic configuration of Ag⁺ is:
[Kr]4d10
This still has a completely filled 4d subshell — no incompleteness.
Step 4 – Check higher oxidation states
Silver can also form Ag²⁺ (e.g., in AgF₂). The configuration of Ag²⁺ is: …
Common Mistakes & How to Avoid Them
Mistake 1: Confusing "Completely Filled" with "Not a Transition Element"
The error: Students see 4d10 and immediately conclude silver cannot be a transition element because transition elements require incomplete d-orbitals.
Why it's wrong: The definition of a transition element (as per IUPAC and most Indian exam boards) is:
An element whose atom has an incomplete d-subshell either in its ground state or in any of its common oxidation states.
Silver's ground state is [Kr]4d105s1. The 4d subshell is full — but in its common +1 oxidation state (Ag+), the configuration becomes [Kr]4d10 (still full). However, in the +2 oxidation state (Ag2+), the configuration is [Kr]4d9 — incomplete. That qualifies silver as a transition element.
How to avoid: Always check both the ground state and common oxidation states. Never judge solely from the ground state configuration.
Mistake 2: Forgetting to Check Oxidation States
The error: Students only look at the ground state and ignore the "or in any of its common oxidation states" part of the definition.
Why it's wrong: Many transition elements (like Cu, Ag, Au) have fully filled d-orbitals in the ground state but show incomplete d-orbitals in higher oxidation states.
How to avoid: Memorise the full definition word-for-word. When given a configuration, always ask: "Does this element form any common ion where the d-subshell becomes incomplete?"
Mistake 3: Misidentifying "Common Oxidation States"
The error: Students think Ag+ (the most common oxidation state) has incomplete d-orbitals — it doesn't (4d10). So they wrongly conclude silver is not a transition element.
Why it's wrong: The definition says any common oxidation state. For silver, Ag2+ (though less common than Ag+) is still a known oxidation state, and it has 4d9.
How to avoid: Know the common oxidation states of d-block elements:
- Silver: +1 (most common), +2 (less common but known)
- Copper: +1, +2 (both common)
- Zinc: +2 only — hence not a transition element
Mistake 4: Confusing "Transition Elements" with "d-Block Elements" …
Showing the 12 most recent of 36 on this concept.
- CBSE 2026Set 56/1/11 markMCQQ.Which of the following is not a transition metal ? (A) Sc (B) Ag (C) Hg (D) Cu
›Reveal solutionSolution
Transition metals are defined by having partially filled d-orbitals in their common oxidation states. Mercury (Hg) has a full d¹⁰ configuration in both its elemental and common +2 state, so it is not a transition metal. The correct answer is (C).
Why This Definition Matters
The classification of transition metals isn't about being a metal or having d-electrons — it's about partially filled d-orbitals in the atom or in any common ion. This is the IUPAC definition. If an element's d-subshell is completely full (d¹⁰) in all its common forms, it doesn't qualify, even if it sits in the d-block of the periodic table.
Let's check each option against this rule.
-
Scandium (Sc, Z=21)
Electronic configuration: [Ar]3d14s2.
Its common ion is Sc³⁺: [Ar] — the 3d orbital is empty.
But the atom has a partially filled d-orbital (3d¹). The definition says "in the atom or in any common oxidation state." Since the atom itself has an incomplete d-subshell, Sc is a transition metal.
-
Silver (Ag, Z=47)
Configuration: [Kr]4d105s1.
Common ion: Ag⁺ → [Kr]4d10 — full d-subshell.
However, silver also forms Ag²⁺ (e.g., in AgF₂), which has 4d9 — a partially filled d-orbital. Because at least one common oxidation state (Ag²⁺) has an incomplete d-subshell, Ag qualifies as a transition metal.
-
Mercury (Hg, Z=80)
Configuration: [Xe]4f145d106s2.
Common ions: Hg²⁺ → [Xe]4f145d10 (full d¹⁰); Hg₂²⁺ (dimeric) also has each Hg with a full d¹⁰ core.
Mercury does not form any stable ion with a partially filled d-orbital. Its d-subshell is always full. Therefore, Hg is not a transition metal.
-
Copper (Cu, Z=29)
Configuration: [Ar]3d104s1. …
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- CBSE 2026Set 56/2/11 markMCQQ.Assertion (A) : Zinc, cadmium and mercury are not considered as transition elements. Reason (R) : These elements have completely filled orbitals in their ground state as well as in their common oxidation states.
›Reveal solutionSolution
The key idea is that transition elements are defined by having partially filled d-orbitals in their ground state or common oxidation states. Since Zn, Cd, and Hg have completely filled d¹⁰ configurations in both, they fail this criterion. Thus, both Assertion (A) and Reason (R) are true, and (R) correctly explains (A).
Why this approach works
The definition of a transition element is not arbitrary — it stems from the unique chemistry of d-block elements. The IUPAC defines a transition element as an element whose atom has a partially filled d-subshell, or which can give rise to cations with an incomplete d-subshell. This means we must check two things: the ground state electron configuration of the neutral atom, and the configurations of its common oxidation states. If either has a partially filled d-orbital, the element qualifies. If both are completely filled, it does not.
Zinc, cadmium, and mercury sit at the end of their respective d-block series (Group 12). Their neutral atoms have the configuration (n−1)d10ns2 — the d-subshell is full. Their common oxidation state is +2, formed by losing the two s-electrons, leaving (n−1)d10 — still full. No partially filled d-orbital appears anywhere. Hence, they are not transition elements.
Let’s walk through the reasoning step by step.
-
Recall the defining criterion for transition elements.
A transition element must have an atom or a common ion with an incomplete d-subshell. This is the official IUPAC definition. Elements that have completely filled d-orbitals in both the ground state and all common oxidation states are excluded.
-
Examine the ground state configurations of Zn, Cd, and Hg.
- Zinc (Z=30): [Ar]3d104s2
- Cadmium (Z=48): [Kr]4d105s2
- Mercury (Z=80): [Xe]4f145d106s2 In each case, the d-subshell is completely filled (d10). No partially filled d-orbital exists in the neutral atom.
-
Check their common oxidation states.
The most stable and common oxidation state for all three is +2. For example:
- Zn loses its two 4s electrons to form Zn2+: [Ar]3d10
- Cd loses its two 5s electrons to form Cd2+: [Kr]4d10
- Hg loses its two 6s electrons to form Hg2+: [Xe]4f145d10 In every case, the d-subshell remains completely filled. No partially filled d-orbital appears in any common oxidation state. …
-
- CBSE 2026Set DZ1 markMCQQ.Which of the following is not a transition metal?(a) Cu(b) Cr(c) Fe(d) Na
›Reveal solutionSolution
A transition metal has a partially filled d-subshell (in the atom or a common ion). Cu, Cr and Fe qualify; Na does not — so option (d).
A transition element is defined as one whose atom or one of its stable ions has an incompletely filled d-orbital.
- Cu ([Ar]3d104s1): forms Cu2+ (3d9) — partially filled d → transition metal.
- Cr ([Ar]3d54s1) — partially filled d → transition metal. …
- CBSE 2026Set ANNUAL1 markMCQQ.Transition element among the following is(a) Zinc(b) Cadmium(c) Cerium(d) Rutherfordium
›Reveal solutionSolution
A transition element is defined as one having a partially (incompletely) filled d-subshell in the ground state or in any of its common oxidation states.
- Zinc (3d10 4s2) and Cadmium (4d10 5s2) have a completely filled d-subshell in the atom AND in their only common +2 ion, so they are NOT classified as transition elements (they are 'post-transition'/group 12 metals).
- Cerium is an f-block (lanthanide) element, not d-block. …
- CBSE 2026Set ANNUAL1 markQ.Why is Zn not considered as transition element? (Z = 30)
›Reveal solutionSolution
Transition elements must have partly filled d-orbitals in the elemental or common ionic state; zinc's d-subshell is always completely full, so it does not qualify.
Zinc (Z = 30) has the electronic configuration [Ar] 3d10 4s2. When it forms its common oxidation state, Zn2+, it loses the two 4s electrons, giving [Ar] 3d10 - the d-subshell remains completely filled (all 10 electrons present) in both the metal and its ion.
…
- CBSE 2026Set ANNUAL1 markMCQQ.Chromium belongs to which block?(a) s-block(b) p-block(c) d-block(d) f-block
›Reveal solutionSolution
Chromium (Z=24) has the configuration [Ar] 3d⁵ 4s¹, placing it in the d-block.
Chromium has atomic number 24 and electronic configuration [Ar] 3d⁵ 4s¹ (an exception to the expected 3d⁴4s², arising from the extra stability of a half-filled d-subshell). Since its last-entering electron occupies a …
- CBSE 2026Set ANNUAL1 markQ.______ block elements are called transition elements.
›Reveal solutionSolution
d-block elements are called transition elements.
The periodic table is divided into four blocks (s, p, d, f) based on which subshell receives the last (differentiating) electron. Elements in which the last electron enters a d-orbital form the d-block, spanning groups 3 to 12. These are called transition elements because most of them have partially filled d-orbitals in their elemental or common ionic states, giving them properties 'transitional' between the highly reactive s-b …
- CBSE 2026Set ANNUAL1 markMCQQ.Transition metals have incomplete(a) s-orbital(b) p-orbital(c) d-orbital(d) f-orbital
›Reveal solutionSolution
Transition metals have incomplete d-orbitals.
Transition elements are defined as those whose atoms or stable ions have partially filled (incomplete) d-orbitals. This partly filled d-subshell is responsible for their characteristic properties: var …
- CBSE 2026Set ANNUAL1 markMCQQ.Total number of elements are in d-block is ______(a) 39(b) 40(c) 38(d) 36
›Reveal solutionSolution
Four transition series of 10 elements each -> 40 d-block elements.
The d-block (transition elements) consists of elements in which the last electron enters a d-subshell. There are four such series:
- 3d series (Sc to Zn),
- 4d series (Y to Cd),
- 5d series (La/Hf to Hg),
- 6d series (Ac/Rf onward). …
- CBSE 2026Set ANNUAL1 markQ.Fill in the blank: Zn, Cd and Hg generally do not considered as a ______ elements.
›Reveal solutionSolution
Zn, Cd and Hg are not true transition elements because their d-orbitals are completely filled.
A transition element is defined as one that has a partially filled d-subshell in its atomic state or in one of its common oxidation (ionic) states. Zinc, cadmium and mercury have the configuration (n-1)d10 ns2, and even in their common +2 ions they are (n-1)d10 - the d-subshell is completely filled. Since they …
- CBSE 2025Set 56/6/11 markMCQQ.Which of the following is the softest metal ? (A) Zn (B) Sc (C) Cu (D) Fe
›Reveal solutionSolution
Softness in metals is determined by how easily their atoms slide past one another; among the transition elements listed, scandium (Sc) has the weakest metallic bonding due to having only one d-electron available for bonding, making it the softest. The answer is (B).
Understanding Metallic Softness
Hardness in metals arises from the strength of metallic bonding—the electrostatic attraction between the "sea" of delocalized electrons and the positive metal ions. The more electrons available for delocalization (especially from d-orbitals in transition metals), and the smaller the atomic radius, the stronger the bonding and the harder the metal.
Conversely, a soft metal has weaker metallic bonds. Its atoms can slide past one another more easily under stress, making it malleable and easy to deform.
Comparing the Four Metals
Let's examine each candidate by looking at their electronic configurations and the number of electrons contributing to metallic bonding:
-
Zinc (Zn): Electronic configuration [Ar]3d104s2
All ten d-electrons are paired in filled orbitals. While the two 4s electrons participate in bonding, the filled d10 subshell contributes relatively little to directional bonding. Zinc is moderately soft but not the softest here.
-
Scandium (Sc): Electronic configuration [Ar]3d14s2
Only one d-electron is available. This is the first transition metal in the series, with minimal d-orbital participation in bonding. The metallic bond is weak because there are fewer electrons to delocalize and the d-orbitals are only just beginning to fill. Scandium is notably soft and can be cut with a knife.
-
Copper (Cu): Electronic configuration [Ar]3d104s1 …
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- CBSE 2025Set D1 markMCQQ.Lutetium (Lu) is(a) a transition element(b) a lanthanide(c) an actinide(d) a p-block element
›Reveal solutionSolution
Lutetium (Z = 71) is the last of the fourteen lanthanide (4f-series) elements.
The lanthanoids (lanthanides) are the fourteen elements from cerium (Z = 58) to lutetium (Z = 71) in which the 4f subshell is progressively filled. Lutetium has the configuration [Xe] 4f14 5d1 6s2, comp …
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