Transition Elements Revision Notes — 9701 Paper 4

Defining a transition element — configurations and d orbitals

Syllabus requirement · §28

“

explain what is meant by a transition element, in terms of electronic configuration · describe the shapes and degeneracy of the d orbitals

”

The gateway definition

Paper 4 opens its transition-element questions with the definition more often than any other topic opens — define transition element appeared as a one-marker in at least six papers in our window alone. It is worth one mark every time, and it unlocks everything downstream, because the definition is really a statement about electron configurations: which ions have a part-full set of d orbitals.

The precise wording to memorise:

A transition element is a d-block element that forms one or more stable ions with an incomplete d subshell (partially filled d orbitals).

Every phrase carries weight. d-block element: it sits in the middle block of the Periodic Table, Sc → Zn in Period 4. Forms … ions: we judge the element by the ions it makes, not by the atom. Stable: an ion that persists in ordinary compounds, not a fleeting species. Incomplete d subshell: somewhere between $3d^1$ and $3d^9$ — neither empty nor full.

Configurations: 4s fills first, and empties first

Across Period 4 the $4s$ subshell fills before $3d$ (potassium $[\text{Ar}]4s^1$ , calcium $[\text{Ar}]4s^2$ , then titanium $[\text{Ar}]3d^2 4s^2$ …). But when a cation forms, the $4s$ electrons are removed first:

element	atom	common ion	ion
Ti	$[\text{Ar}]\,3d^2 4s^2$	$\text{Ti}^{3+}$	$[\text{Ar}]\,3d^1$
V	$[\text{Ar}]\,3d^3 4s^2$	$\text{V}^{2+}$	$[\text{Ar}]\,3d^3$
Cr	$[\text{Ar}]\,3d^5 4s^1$	$\text{Cr}^{3+}$	$[\text{Ar}]\,3d^3$
Mn	$[\text{Ar}]\,3d^5 4s^2$	$\text{Mn}^{2+}$	$[\text{Ar}]\,3d^5$
Fe	$[\text{Ar}]\,3d^6 4s^2$	$\text{Fe}^{3+}$	$[\text{Ar}]\,3d^5$
Co	$[\text{Ar}]\,3d^7 4s^2$	$\text{Co}^{2+}$	$[\text{Ar}]\,3d^7$
Ni	$[\text{Ar}]\,3d^8 4s^2$	$\text{Ni}^{2+}$	$[\text{Ar}]\,3d^8$
Cu	$[\text{Ar}]\,3d^{10} 4s^1$	$\text{Cu}^{2+}$	$[\text{Ar}]\,3d^9$
Zn	$[\text{Ar}]\,3d^{10} 4s^2$	$\text{Zn}^{2+}$	$[\text{Ar}]\,3d^{10}$

Two rows deserve a second look. Chromium and copper are the familiar "half-full / full-subshell" exceptions in the ATOM ( $3d^5 4s^1$ and $3d^{10} 4s^1$ ). And zinc's ion, $\text{Zn}^{2+}$ , has $3d^{10}$ — a complete d subshell — so by the definition zinc is a d-block metal but not a transition element. Scandium fails the same way from the other end: its only stable ion, $\text{Sc}^{3+}$ , has $3d^0$ . Every metal between them qualifies.

Ionise 4s before 3d — always

The single most-dropped mark in this topic: writing $\text{Fe}^{2+}$ as $[\text{Ar}]\,3d^4 4s^2$ . Once $3d$ is occupied, it lies BELOW $4s$ in energy, so ionisation strips the two $4s$ electrons away first: $\text{Fe}^{2+} = [\text{Ar}]\,3d^6$ , $\text{Fe}^{3+} = [\text{Ar}]\,3d^5$ , $\text{Cu}^{+} = [\text{Ar}]\,3d^{10}$ . Filling order and removal order are different rules — learn them as separate facts.

The five d orbitals and their shapes

A subshell is a set of orbitals, and the d subshell contains five. Four of them — $3d_{xy}$ , $3d_{yz}$ , $3d_{xz}$ , $3d_{x^2-y^2}$ — are four-lobed "cloverleaf" shapes lying between or along the axes; the fifth, $3d_{z^2}$ , is two lobes along the z-axis with a doughnut (torus) of electron density around the nucleus in the xy-plane.

In an isolated ion all five sit at exactly the same energy. Orbitals of equal energy are called degenerate — a one-mark definition that pairs naturally with the transition-element definition on the same paper. Degeneracy is precisely what §07 breaks: put ligands around the ion and the five orbitals split into two sets of different energy — the origin of all transition-element colour.

The two shape families within the 3d subshell. Left: 3d_xy — four lobes sitting between the x and y axes (its siblings 3d_yz and 3d_xz lie between the other axis pairs; 3d_x²₋y² has the same cloverleaf but points ALONG the axes). Right: 3d_z² — two lobes along the z-axis plus a torus around the nucleus. All five orbitals of the isolated subshell are degenerate (equal energy).

Sorting invented metals into the definition

4 marks

Two Period 4 d-block metals have the following stable ions.

Metal J forms the stable ion $\text{J}^{2+}$ , with configuration $[\text{Ar}]\,3d^{10}$ .
Metal K forms two stable ions: $\text{K}^{2+}$ ( $[\text{Ar}]\,3d^7$ ) and $\text{K}^{3+}$ ( $[\text{Ar}]\,3d^6$ ).

(a) State whether each metal is a transition element, explaining both decisions.

(b) Write the full electron configuration of the ATOM of K if its ion $\text{K}^{2+}$ is formed by removing two $4s$ electrons.

Show full working

1
(a) J is not a transition element: its only stable ion has a COMPLETE $d$ subshell ( $3d^{10}$ ), so no stable ion with incomplete $d$ subshell exists.
Apply the definition clause by clause — 'incomplete' kills J immediately, exactly as it kills real zinc.
2
K is a transition element: it forms stable ions ( $3d^7$ , $3d^6$ ) with incompletely filled $d$ subshells.
One qualifying stable ion is enough — the definition says 'one or more', so K passes via either ion.
3
(b) Remove the two $4s$ electrons in reverse: $\text{K}$ atom $= [\text{Ar}]\,3d^7 4s^2$ .
Work backwards through the same door: if the 4s electrons left last-in-first-out to make K²⁺, the neutral atom must have had them.

Answer

(a) J — no (3d¹⁰ complete); K — yes (stable ions with incomplete d subshell) · (b) [Ar]3d⁷4s²

Examiner phrasing for the 'no' case mirrors zinc: 'does not form a stable ion with an incomplete d subshell'. Say which clause fails, not just 'it isn't one'.

The definition, word for word

9701/43 M/J 2024 Q2(a)(i)1 mark

Define transition element.

Show full working

1
A transition element is a d-block element which forms one or more stable ions with an incomplete d subshell (incomplete d orbitals).
The mark scheme's exact chain: d-block element + forms stable ion(s) + incomplete d subshell. Dropping 'stable' or 'd-block' still usually earns the mark, but 'incomplete d subshell' is non-negotiable.

Answer

a d-block element that forms one or more stable ions with an incomplete d subshell

Write it as one sentence with all three ingredients. 'Has incomplete d orbitals' alone is not a definition of the ELEMENT.

Degenerate — the companion one-marker

9701/41 M/J 2024 Q2(b)(i)1 mark

Define degenerate d orbitals.

Show full working

1
Degenerate d orbitals are d orbitals of the same energy.
One clause, one mark. The context (an isolated ion, before ligands arrive) is what makes the five orbitals equal — §07 shows what happens when that context ends.

Answer

orbitals of the same energy

Zinc's ion in boxes — why Zn fails the test

9701/42 F/M 2023 Q1(a)(i)1 mark

Complete the electrons-in-boxes diagram for the $3d$ subshell in a $\text{Zn}^{2+}$ ion.

The printed answer frame: five empty boxes labelled for the 3d subshell, awaiting the electrons of Zn²⁺.

Show full working

1
$\text{Zn}^{2+}$ is $[\text{Ar}]\,3d^{10}$ : all five boxes are filled with paired electrons — ten electrons, two per box, drawn as paired arrows ↑↓.
Hund's rule fills boxes singly before pairing, but ten electrons in five boxes forces full pairing everywhere.
2
A full $3d^{10}$ subshell means $\text{Zn}^{2+}$ has no incomplete d subshell — so zinc, though a d-block metal, is not a transition element.
The diagram IS the argument: the examiner shows you the full subshell so you can see why the definition excludes it.

Answer

five boxes, each ↑↓ (3d¹⁰) — complete d subshell, so Zn is not a transition element

Sketching a d orbital

9701/41 M/J 2022 Q2(b)1 mark

Sketch the shape of a $3d_{z^2}$ orbital.

The printed answer space (Fig. 2.1): blank axes awaiting your sketch of the orbital.

Show full working

1
Draw two lobes along the $z$ -axis, one above and one below the nucleus, plus a doughnut (torus) of electron density encircling the nucleus in the $xy$ -plane.
The torus is what distinguishes 3d_z² from its four cloverleaf siblings — omitting it usually forfeits the mark.

Answer

two z-axis lobes + torus in the xy-plane around the nucleus

The mark scheme's drawing shows exactly this dumbbell-plus-ring. A plain dumbbell alone is the classic near-miss.

Common mistakes

Writing ion configurations as if $4s$ leaves last: $\text{Fe}^{2+} = [\text{Ar}]\,3d^4 4s^2$
Remove $4s$ first: $\text{Fe}^{2+} = [\text{Ar}]\,3d^6$ .
Once occupied, 3d lies below 4s in energy. Filling order (4s first) and removal order (4s first out) are different rules — this trap appears on nearly every paper.
Saying zinc (or scandium) is not in the d block
Zn and Sc ARE d-block elements; they are just NOT transition elements, because their only stable ions (3d¹⁰ and 3d⁰) have complete or empty d subshells.
The definition sorts d-block elements into transition and non-transition. Confusing the two categories costs the explanation mark even when the classification is right.
Counting $\text{Cu}^{+}$ ( $3d^{10}$ ) as evidence copper is not a transition element
Copper qualifies via its OTHER stable ion, $\text{Cu}^{2+} = [\text{Ar}]\,3d^9$ .
'One or more stable ions' — a single qualifying ion suffices. Cu⁺ being d¹⁰ only explains why Cu(I) compounds are typically colourless (§07).

Your turn

Configurations first, then the definition applied both ways — the two moves §01 owns.

1
(i) Write the electron configuration of the $\text{Cr}^{3+}$ ion.

(ii) Vanadium forms stable ions $\text{V}^{2+}$ , $\text{V}^{3+}$ , $\text{V}^{4+}$ and $\text{V}^{5+}$ . Explain, using configurations, why $\text{V}^{5+}$ is not evidence that vanadium is a transition element, and why vanadium is nevertheless one.
Stuck? Show hint
Strip 4s first, then count what remains of 3d for each ion.
Show solution
1. 1
  (i) Cr atom $=[\text{Ar}]\,3d^5 4s^1$ (the half-filled-subshell exception). Remove the $4s$ electron first, then one $3d$ : $\text{Cr}^{3+} = [\text{Ar}]\,3d^3$ .
  Both removals pass through the 4s-before-3d door — the exception in the ATOM does not change the removal order.
2. 2
  (ii) $\text{V}^{5+} = [\text{Ar}]\,3d^0$ — an EMPTY d subshell, so this ion alone would not satisfy the definition (just like $\text{Sc}^{3+}$ ).
  'Incomplete' means between 1 and 9 d electrons; 0 and 10 both fail.
3. 3
  Vanadium is still a transition element because it ALSO forms stable ions with incomplete d subshells — $\text{V}^{2+}$ ( $3d^3$ ) and $\text{V}^{3+}$ ( $3d^2$ ) among them.
  'One or more' — the highest oxidation state failing the test never disqualifies the element.
Answer
(i) [Ar]3d³ · (ii) V⁵⁺ is 3d⁰ (empty, not incomplete); V qualifies via V²⁺/V³⁺ (incomplete 3d)
2
Element Q is in the d block of Period 4. Its atom has configuration $[\text{Ar}]\,3d^8 4s^2$ , and its only stable ion is $\text{Q}^{2+}$ .

(a) Is Q a transition element? Justify your answer.

(b) State the configuration of $\text{Q}^{2+}$ and sketch the shape of ONE of the four-lobed d orbitals in this ion.
Stuck? Show hint
Which clause of the definition does Q²⁺ satisfy?
Show solution
1. 1
  (a) $\text{Q}^{2+} = [\text{Ar}]\,3d^8$ — an incomplete d subshell — so yes, Q is a transition element (it behaves like real nickel).
  8 electrons sit between 1 and 9: comfortably 'incomplete'.
2. 2
  (b) $[\text{Ar}]\,3d^8$ . Sketch any cloverleaf: four lobes, e.g. $3d_{xy}$ with lobes between the x and y axes.
  Any of the four cloverleaves earns the shape mark; labelling the axes keeps the drawing honest.
Answer
(a) yes — Q²⁺ is 3d⁸, incomplete · (b) [Ar]3d⁸; four-lobed cloverleaf between the axes

Practise definitions, configurations and d orbitalsReal past-paper questions · Defining transition elements; 3d/4s; d orbitals

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Can you do all of these?

I can define a transition element and decide whether a given element or ion qualifies
I can write configurations for 3d-block atoms and their ions, removing 4s electrons first
I can sketch d orbital shapes and explain 'degenerate'
I can list typical transition-element properties and explain variable oxidation states via 3d/4s energy proximity
I can describe heterogeneous catalysis mode of action and write a homogeneous redox catalyst cycle
I can define ligand classes by denticity and deduce formula, charge, coordination number and geometry of any complex
I can draw cis/trans and optical isomers in 3D and use dipole cancellation to assign polarity
I can write equations and observations for aqua-ions with NaOH, excess NH₃ and concentrated HCl, and classify each reaction
I can run full MnO₄⁻ and EDTA titration calculations including % purity and water of crystallisation
I can predict whether a redox reaction occurs using E⦵ values and recognise disproportionation
I can give the four-step colour explanation and say why d⁰/d¹⁰ ions are colourless
I can write Kstab expressions with correct units and compute concentrations or combined constants from them
I can state how cisplatin damages cancer cells and why it is square planar

Now do the questions

475 real Paper 4 parts from 2021–2025, sorted by difficulty, with mark schemes

Transition Elements

Defining a transition element — configurations and d orbitals

The gateway definition

Configurations: 4s fills first, and empties first

The five d orbitals and their shapes

Typical properties, variable oxidation states and catalysis

Ligands, complexes and coordination number

Can you do all of these?

Defining a transition element — configurations and d orbitals

The gateway definition

Configurations: 4s fills first, and empties first

The five d orbitals and their shapes

The rest of this note

Can you do all of these?