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📌 Illustrated Diagram: Transition-Metal Properties

d- and f-Block Elements – Complete CBSE Class 12 Chemistry Notes

Unit 4 | d- and f-Block Elements | CBSE 2026–27

1. Transition Elements

d-block elements occupy groups 3–12. A transition element is characterised by a partially filled d subshell in its atom or in a commonly occurring oxidation state, with standard exceptions considered in the syllabus.

2. Why Do Transition Elements Show Special Properties?

3. Important Compounds

KMnO₄ and K₂Cr₂O₇ are important oxidising agents. Their oxidising behaviour depends on the reaction medium and oxidation state. Balanced equations must always be written for the specified medium.

4. Lanthanoid Contraction

Across the lanthanoid series, atomic/ionic size decreases gradually because 4f electrons shield nuclear charge imperfectly. This is called lanthanoid contraction.

Consequences

• Similarities in size and properties of some 4d/5d element pairs.
• Difficulty in separating lanthanoids because of similar properties.
• Systematic change in basic character of lanthanoid hydroxides.

5. Actinoids

Actinoids involve 5f orbitals and generally display greater variation in oxidation state. Many are radioactive and have nuclear applications.

🧠 Property Map

📝 Competency Practice

1. Why are many transition-metal ions coloured while d⁰ ions are often colourless?
2. Why can transition metals act as catalysts?
3. Explain why lanthanoids are difficult to separate.
4. Compare the electronic involvement of lanthanoids and actinoids.

🎯 Board Practice

1. Explain variable oxidation states with suitable examples.
2. Explain magnetic behaviour in terms of unpaired electrons.
3. Explain lanthanoid contraction and two consequences.
4. Write and balance specified oxidation reactions of KMnO₄/K₂Cr₂O₇.

MCQs

1. Variable oxidation states are common among transition elements. True.
2. All d-block ions are coloured. False.
3. Lanthanoid contraction is associated with imperfect 4f shielding. True.
4. Many actinoids are radioactive. True.

Deep-Dive: Transition Elements

Transition elements have partially filled d-orbitals in their atoms or common oxidation states. Their characteristic properties include variable oxidation states, coloured ions, complex formation, catalytic activity and magnetic behaviour.

Oxidation-State Pattern

Many transition metals show multiple oxidation states because ns and (n−1)d electrons can participate in bonding.

Lanthanoids and Actinoids

Know the general electronic configurations, common oxidation states and the lanthanoid contraction. Lanthanoid contraction influences similarities between elements and properties of their compounds.

Important Comparisons

Common Mistakes

• Assuming every d-block element is a transition element.
• Ignoring exceptional electronic configurations.
• Confusing lanthanoid contraction with ordinary atomic-size trends.

Practice

1. Write electronic configurations of specified elements/ions.
2. Explain variable oxidation states.
3. Explain colour and magnetic behaviour.
4. Explain lanthanoid contraction and its consequences.

Worked Application: Variable Oxidation State

Transition elements can exhibit multiple oxidation states because the energies of ns and (n−1)d electrons are relatively close. For example, manganese forms compounds containing Mn(II), Mn(IV) and Mn(VII). The oxidation state should always be determined from the chemical formula and charge balance rather than memorised in isolation.

Exam Calculation Practice

1. Determine the oxidation state of the central metal in a given permanganate or dichromate compound.
2. Use electronic configuration to explain why a transition metal can show more than one oxidation state.

d- and f-Block Elements — Trend Map

Why Variable Oxidation States?

The energies of ns and (n−1)d electrons are relatively close, allowing different numbers of electrons to participate in bonding. This explains multiple oxidation states in many transition metals.

Magnetism

Unpaired d-electrons produce paramagnetism. The number of unpaired electrons therefore helps explain magnetic behaviour.

Colour

Many transition-metal ions are coloured because electronic transitions can absorb particular wavelengths of visible light. The exact colour depends on the electronic structure and environment.

Worked Application

In MnO₄⁻, oxygen contributes −8 overall. Since the ion has charge −1, Mn must be +7: x−8=−1, so x=+7.

Lanthanide Contraction

Gradual decrease in atomic/ionic size across the lanthanide series influences similarities among elements and the properties of later transition elements.

CBSE Practice

1. Determine oxidation states from formulae.
2. Explain paramagnetism using unpaired electrons.
3. Explain colour in transition-metal compounds.
4. Discuss catalytic behaviour.
5. Explain consequences of lanthanide contraction.