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9701 A Level Chemistry/ Topic 28 Transition elements/ 6 interactive simulations/ drag to rotate 3D scenes

Transition elements, made interactive

Six simulations covering Topic 28: an orbital box builder for the d-block atoms and ions with the 4s-first rule and the definition test; complex ions in 3D with ligands labelled, coordination number and overall charge worked out; a ligand exchange bench for copper(II) and cobalt(II) with colours, species and equations; d-orbital splitting with the colour wheel, so an absorbed wavelength gives the colour you see; cis-trans and optical isomers side by side in 3D; and a redox titration calculator with the half-equations combined.

1. Configurations and the definition 2D · 28.1

A transition element is a d-block element that forms at least one stable ion with a partially filled d subshell. Build the atom or ion in orbital boxes, notice that chromium and copper break the simple filling pattern, and watch the 4s electrons leave first when an ion forms. Then check the definition: scandium and zinc are in the d-block but fail the test.

Element

Charge on the particle

Filling rule

PARTICLE
ELECTRONS
CONFIGURATION
UNPAIRED d ELECTRONS
DEFINITION TEST
Cambridge writes the configuration as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ (3d before 4s); [Ar] is shorthand for the first five sub-shells. In the atom the 4s sub-shell fills before the 3d, but when a transition element forms an ion the 4s electrons are lost first.

2. Complexes, ligands and shapes 3D · 28.2

A complex is a central metal ion surrounded by ligands, each of which donates a lone pair to the metal through a dative covalent bond. The number of donor atoms is the coordination number, and it fixes the shape. Pick a complex, rotate it, and check that the overall charge is the oxidation state of the metal plus the charges on the ligands.

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Complex ion

SHAPE
COORDINATION NUMBER
OVERALL CHARGE
BOND ANGLE
Cambridge shapes: octahedral for coordination number 6, tetrahedral for 4 (for example [CuCl₄]²⁻), square planar for 4 in platinum(II) complexes such as cisplatin, and linear for 2 in [Ag(NH₃)₂]⁺. Hydrogen atoms on en, ethanedioate and EDTA are left out so the rings are easier to see.

3. Ligand exchange bench 2D · 28.3, 28.4

Swap the ligands round a metal ion and the colour changes. Add reagents to the aqueous copper(II) or cobalt(II) ion drop by drop, or in excess, and read off the species that forms, the colour, and the equation. Hydroxide and ammonia first make a precipitate, ammonia in excess redissolves it, and concentrated hydrochloric acid brings in chloride ligands. The stability constants decide which swap goes ahead.

Metal ion

Add a reagent

OBSERVATION
LAST CHANGE
Kstab AND PREDICTION
Reagents: dilute or concentrated ammonia solution, sodium hydroxide solution, concentrated hydrochloric acid, all at room temperature. Water and ammonia are similar in size, so 6 stays 6 when one replaces the other; chloride is larger, so the coordination number falls from 6 to 4. Kstab values are rounded textbook values; use the figures given in your question or Data Booklet.

4. Why complexes are coloured 2D · 28.3

In a complex the ligands split the five 3d orbitals into two groups with an energy gap ΔE between them. An electron can jump across the gap by absorbing a photon of exactly that energy, ΔE = hν. The light that is left over is the colour you see, the complement of the colour absorbed. Choose a complex, or drag the wavelength yourself.

Complex ion

FREQUENCY ν
ΔE PER ELECTRON
ΔE PER MOLE
COLOUR SEEN
ΔE = hν = hc/λ, with h = 6.63 × 10−34 J s, c = 3.00 × 108 m s−1 and L = 6.02 × 1023 mol−1. Absorption wavelengths are the approximate centres of broad bands, chosen to show the idea. Four things change the colour: the metal, its oxidation state, the ligands, and the coordination number. Ions with an empty (3d⁰) or full (3d¹⁰) subshell have no d-d transition and are colourless.

5. Isomerism in complexes 3D · 28.5

Complexes with the same formula can have the same ligands arranged differently. In an octahedral or square planar complex the ligands can sit next to each other (cis) or opposite each other (trans). Some octahedral complexes have no mirror plane at all and exist as a pair of non-superimposable mirror images, which rotate plane-polarised light in opposite directions.

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Complex

TYPE OF ISOMERISM
SHAPE
THE TWO ISOMERS
Cis-trans isomerism needs two or more different ligands that can swap positions. Optical isomerism needs the complex to lack a plane of symmetry; the usual cases are an octahedral complex with three bidentate ligands, or two bidentate ligands and two monodentate ones in a cis arrangement.

6. Redox titrations 2D · 28.6

Transition metal ions change oxidation state easily, so manganate(VII) and dichromate(VI) are used to measure the amount of a reducing agent by titration. The half-equations are combined so the electrons cancel, which gives the mole ratio that the calculation needs. Set the unknown concentration, run the titration, and follow the working.

Titration

TITRE (END-POINT)
MOLE RATIO
WORKING
25.0 cm³ of the solution of unknown concentration is pipetted into the flask and acidified with dilute sulfuric acid (not hydrochloric, which MnO₄⁻ would oxidise to chlorine, and not nitric, which is itself an oxidising agent). Manganate(VII) is its own indicator: the end-point is the first permanent pale pink.
TheLucidSTEM · Cambridge International A Level Chemistry 9701 · Topic 28 Transition elements. Every scene runs in the browser; drag to rotate 3D scenes, wheel to zoom. Values are standard textbook data rounded for teaching.

About this lab: Transition elements

Six interactive simulations for Cambridge A Level Chemistry 9701 topic 28 Transition elements: electron configurations of titanium to copper and their ions in orbital boxes with the 4s-first rule and the d-block definition test, octahedral, tetrahedral, square planar and linear complexes in 3D with ligand charge, denticity, coordination number and overall charge, ligand exchange and precipitation of copper(II) and cobalt(II) with colours and equations plus Kstab, d-orbital splitting with delta E equals h nu and the complementary colour wheel, cis-trans and optical isomers of cobalt, platinum and nickel complexes in 3D, and redox titrations with permanganate and dichromate with the half-equations combined.

Six simulations covering Topic 28: an orbital box builder for the d-block atoms and ions with the 4s-first rule and the definition test; complex ions in 3D with ligands labelled, coordination number and overall charge worked out; a ligand exchange bench for copper(II) and cobalt(II) with colours, species and equations; d-orbital splitting with the colour wheel, so an absorbed wavelength gives the colour you see; cis-trans and optical isomers side by side in 3D; and a redox titration calculator with the half-equations combined.

Transition elements is a free, browser-based simulation for Cambridge International AS & A Level Chemistry 9701 (topic 28 Chemistry of transition elements). The course is assessed in Paper 1 (Multiple Choice (AS)), Paper 2 (AS Structured), Paper 3 (Advanced Practical), Paper 4 (A Level Structured), Paper 5 (Planning, Analysis, Evaluation). Syllabus for examination in 2025-2027. There are 6 tabs, each a self-contained experiment with its own controls and live readings, and nothing to install: it runs on a laptop, tablet or classroom projector.

Syllabus coverage

Full syllabus map: A Level Chemistry 9701 topic 28.

How to use this simulation

  1. 1. Configurations and the definition. A transition element is a d-block element that forms at least one stable ion with a partially filled d subshell.
  2. 2. Complexes, ligands and shapes. A complex is a central metal ion surrounded by ligands, each of which donates a lone pair to the metal through a dative...
  3. 3. Ligand exchange bench. Swap the ligands round a metal ion and the colour changes.
  4. 4. Why complexes are coloured. In a complex the ligands split the five 3d orbitals into two groups with an energy gap ΔE between them.
  5. 5. Isomerism in complexes. Complexes with the same formula can have the same ligands arranged differently.
  6. 6. Redox titrations. Transition metal ions change oxidation state easily, so manganate(VII) and dichromate(VI) are used to measure the...
  7. The buttons at the end of the tab strip give the scene the whole window (Full screen, Hide controls) and, on a dark stage, a projector-friendly palette (Light scene). Many scenes also have drag handles: drag them to change the set-up directly.

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