AS & A-Level Chemistry 14 — Group 2 and transition-metal chemistry

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Independent Deckloop A Level Chemistry study material aligned with Cambridge International 9701 (2025–2027). Deck 14 of 18: Group 2 and transition-metal chemistry. Original explanations, worked applications and practice. Not affiliated with or endorsed by Cambridge International Education.

Chemistry EN A-Level

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Properties and Definition of Transition Elements

For this Cambridge syllabus, a transition element is defined as a d-block element that forms one or more stable ions with an incomplete d sub-shell. This syllabus definition explains why titanium to copper are transition elements, whereas zinc (which only forms

\text{Zn}^{2+}

with a full

3\text{d}^{10}

configuration) and scandium (which typically forms

\text{Sc}^{3+}

with an empty

3\text{d}^0

configuration) are frequently excluded from this specific chemical classification.

The unique electronic structure of transition elements gives rise to four defining characteristic properties: they exhibit variable oxidation states, they act as effective catalysts in both homogeneous and heterogeneous systems, they readily form complex ions with a variety of ligands, and their compounds or ions are typically highly coloured.

Understanding the spatial geometry of the d orbitals is fundamental to explaining these properties, particularly complex formation and colour. The

3 d

sub-shell comprises five distinct orbitals. Students must be able to sketch the

3\text{d}_{xy}

orbital, which features four teardrop-shaped lobes directed directly between the Cartesian x and y axes, and the

3\text{d}_{z^2}

orbital, which consists of a principal dumbbell lying along the z-axis combined with a concentric torus (or doughnut shape) situated in the xy plane.

Key points

A transition element is a d-block element that forms one or more stable ions with an incomplete d sub-shell.
Transition elements exhibit variable oxidation states.
Transition elements frequently behave as catalysts.
Transition elements form complex ions with ligands.
Compounds and ions of transition elements are typically coloured.

Worked example

Question

Evaluate whether zinc and copper fit the definition of a transition element based on their most common stable ions.

Solution

1. Write the electronic configuration of the parent atoms: neutral copper is

[\text{Ar}] \, 3\text{d}^{10} \, 4\text{s}^1

and neutral zinc is

[\text{Ar}] \, 3\text{d}^{10} \, 4\text{s}^2

.

2. Identify their stable ions by removing

4 \mathrm{s}\,\text{electrons}

first: copper primarily forms

\text{Cu}^{2+}

(

[\text{Ar}] \, 3\text{d}^9

) and

\text{Cu}^+

(

[\text{Ar}] \, 3\text{d}^{10}

). Zinc forms only

\text{Zn}^{2+}

(

[\text{Ar}] \, 3\text{d}^{10}

).

3. Apply the definition: a transition element must form at least one stable ion with an incomplete d sub-shell.

4. Evaluate the ions:

\text{Cu}^{2+}

has a

3\text{d}^9

configuration, which is an incomplete sub-shell.

\text{Zn}^{2+}

has a

3\text{d}^{10}

configuration, which is a completely full sub-shell.

Copper is a transition element because its stable

\text{Cu}^{2+}

ion has an incomplete d sub-shell (

3\text{d}^9

). Zinc is not a transition element because its only stable ion,

\text{Zn}^{2+}

, has a completely

\text{full}\,d

sub-shell (

3\text{d}^{10}

Common pitfalls

Treating all d-block elements as transition elements under the syllabus definition. Apply the stated stable-ion criterion to the common ions in this course: scandium $(\mathrm{III})$ is d0 and zinc $(\mathrm{II})$ is d10. Definitions used in wider chemistry can differ; the syllabus here focuses on titanium to copper.
Drawing the $3\text{d}_{xy}$ orbital lobes directly on the Cartesian axes. Correct reasoning: The lobes of the $3\text{d}_{xy}$ orbital must point explicitly between the x and y axes; orbitals with lobes lying on the axes are $3\text{d}_{x^2-y^2}$ .

Prerequisites

Study Periodic patterns and inorganic reactions first.
Study Thermodynamics and electrochemistry first.
Study Quantitative equilibria and kinetics first.