Define transition elements and identify their characteristic properties
Apply Cambridge's definition to d-block elements and connect incomplete d orbitals with the characteristic properties of transition elements.
Section 28 · Chemistry of transition elements
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 28. Use the first-row transition elements titanium to copper and the species explicitly named in the syllabus. Where redox potentials or stability constants are needed, use the values supplied in the question. Colour descriptions can depend on conditions and concentration. Written practice is self-checked
Before you begin
- Use electron configurations, oxidation numbers, ionic equations, equilibrium expressions and the AS redox and bonding models.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which feature is required by the syllabus definition of a transition element?
What you will learn
- Define a transition element using a stable ion with an incomplete d subshell
- Identify variable oxidation states, catalysis, complex ions and coloured compounds as characteristic properties
- Explain variable oxidation states through the similar energies of 3d and 4s subshells
Use the stable-ion definition
For this syllabus, a transition element is a d-block element that forms one or more stable ions with incomplete d orbitals. Check the ion, not only the neutral atom. The definition makes the incompletely filled d subshell in a stable ion the deciding feature.
The first-row scope is titanium to copper. Do not automatically label every d-block element a transition element: zinc commonly forms Zn²⁺ with a complete 3d subshell, so it does not meet this definition.
Connect the definition to recurring properties
The first-row transition elements show variable oxidation states, catalytic behaviour, complex-ion formation and coloured compounds. These are related to the energies and availability of 3d electrons and orbitals, but each property needs its own explanation in later lessons.
For variable oxidation states, the 3d and 4s subshells have similar energies. Electrons from both can be involved in ion formation, so more than one stable oxidation state can occur. In forming cations, remove 4s electrons before 3d electrons.
Pause and explain
Why can a first-row transition element show more than one oxidation state?
Worked example
Explain why Fe is a transition element and why Zn is not, using the syllabus definition.
Show the worked solution
- Check stable ions rather than only the neutral atoms.
- Fe²⁺ has an incomplete 3d subshell.
- Zn²⁺ has a complete 3d subshell.
Answer Fe is a transition element because Fe²⁺ has an incomplete 3d subshell. Zn²⁺ has a complete 3d¹⁰ subshell, so zinc does not meet the stated definition.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Use the stable-ion test to classify Fe²⁺ (3d⁶) and Zn²⁺ (3d¹⁰) for the transition-element definition.
Give me a hint
Look for an incomplete d subshell in a stable ion.
Compare my reasoning
- Fe²⁺ has six d electrons in a capacity of ten.
- Zn²⁺ has ten d electrons.
- Apply the definition to each ion separately.
Fe qualifies because 3d⁶ is incomplete. Zn does not qualify because 3d¹⁰ is complete.
Look for these in your work
- Stable ions checked
- Incomplete subshell identified
- Full subshell distinguished
Write the d-electron configurations of Fe²⁺ and Fe³⁺ from Fe: [Ar] 3d⁶ 4s².
Give me a hint
Remove 4s electrons before removing a 3d electron.
Compare my reasoning
- Remove two 4s electrons for Fe²⁺.
- Remove one further 3d electron for Fe³⁺.
- Keep the argon core unchanged.
Fe²⁺: [Ar] 3d⁶. Fe³⁺: [Ar] 3d⁵.
Look for these in your work
- 4s electrons removed first
- Correct electron counts
- Core retained
A student says ‘copper is not a transition element because Cu²⁺ has lost its 4s electron.’ Explain why this conclusion does not follow.
Give me a hint
Check the d subshell of the stable ion after ion formation.
Compare my reasoning
- Cu²⁺ has configuration [Ar] 3d⁹.
- The 3d subshell is incomplete.
- The definition is met even though the 4s electron was removed first.
Cu²⁺ has an incomplete 3d⁹ subshell, so copper meets the definition. Removing a 4s electron does not make the 3d subshell complete.
Look for these in your work
- Ion configuration used
- d occupancy assessed
- Definition applied correctly
Common mistakes
- Defining a transition element as any d-block element
- Checking only the neutral atom's d subshell
- Saying variable oxidation states come from large energy gaps between 3d and 4s
- Removing 3d electrons before 4s when forming the first cations
What can you explain now?
Which feature is required by the syllabus definition of a transition element?
Compare with the explanation
A stable ion with an incomplete d subshell
The definition is based on at least one stable ion, not just block position.
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Make it stick
What does the stable-ion definition require, and why are 3d and 4s energies relevant to variable oxidation states?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.