Lanthanoids and Actinoids Class 12 Chemistry | The d- and f-Block Elements

Understand it

Lanthanoids have similar chemistry because the +3 state is strongly favoured and their sizes decrease gradually due to lanthanoid contraction. Actinoids show more variable oxidation states because 5f, 6d, and 7s orbitals have comparable energies, especially in earlier actinoids. Many actinoids are radioactive, so school-level discussion focuses on periodic and chemical trends, not handling procedures.

Example

Misch metal is an alloy containing lanthanoid metals, commonly with iron, and is used in lighter flints. Actinoids such as uranium can show oxidation states beyond +3, unlike most lanthanoids.

Rule to remember

Trend rule: lanthanoids mainly show +3 oxidation state and exhibit lanthanoid contraction due to poor 4f shielding.

Actinoids show variable oxidation states because 5f, 6d, and 7s orbitals are close in energy.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Compare Ce and a typical lanthanoid.

1Most lanthanoids favour +3, but Ce can also show +4 because Ce4+ attains a stable 4f0 configuration.
2This is an important exception to the simple +3 pattern.

Method you can reuse

Identify whether the element belongs to 4f or 5f series, state the common oxidation state, add the reason based on orbital energy and shielding, then mention one example or exception if asked.

Visual tip

A comparison chart of lanthanoids and actinoids is useful; a detailed orbital-energy diagram is usually beyond the required answer depth.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why do actinoids show more variable oxidation states than lanthanoids?

Think first, then show answer

In actinoids, 5f, 6d, and 7s orbitals have comparable energies, so different numbers of electrons can participate in bonding. Lanthanoids more commonly stabilise the +3 state.

Common wrong answer

A common wrong answer is saying every lanthanoid shows only +3. Some lanthanoids also show +2 or +4 when extra stability is gained, such as Ce(IV) or Eu(II).

Don't mix this up

Students often assume lanthanoids and actinoids behave identically because both are f-block series. Actinoids have more variable oxidation states and greater complexity than lanthanoids.

Exam tip

For comparison questions, write three points clearly: orbital filled, common oxidation state, and reason for variability or contraction.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask for lanthanoid contraction, consequences of contraction, comparison of lanthanoids and actinoids, common oxidation states, exceptions, and uses such as misch metal.

1
Enough for 1 mark
Lanthanoids and actinoids are f-block elements in which differentiating electrons enter 4f and 5f orbitals respectively. Lanthanoids commonly show +3 oxidation state, while actinoids show greater oxidation-state variability.
2
Add for 2 marks
Example: Misch metal is an alloy containing lanthanoid metals, commonly with iron, and is used in lighter flints. Actinoids such as uranium can show oxidation states beyond +3, unlike most lanthanoids.
3
Add for 3 marks
Lanthanoids have similar chemistry because the +3 state is strongly favoured and their sizes decrease gradually due to lanthanoid contraction.

Use the concept graph around this idea instead of revising it in isolation.

Trends in Atomic and Ionic Radii

Across a transition series, atomic and ionic radii generally decrease at first due to increasing nuclear charge, but the decrease becomes small because added d-electrons shield each other poorly. In f-block elements, poor shielding by f-electrons causes lanthanoid contraction.

Position of d-Block Elements in the Periodic Table

d-block elements are elements in which the differentiating electron enters the d-subshell of the penultimate shell. They are placed between the s-block and p-block elements in the periodic table.

General Properties of Transition Metals

Transition metals show characteristic properties such as variable oxidation states, coloured ions, paramagnetism, complex formation, catalytic activity, high enthalpy of atomisation, high melting points, and alloy formation due to involvement of d-electrons.

Magnetic Properties and the Spin-Only Formula

Magnetic behaviour of transition-metal ions depends mainly on the number of unpaired electrons. Paramagnetic species have unpaired electrons, while diamagnetic species have all electrons paired.

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Lanthanoids and Actinoids

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Lanthanoids and Actinoids

Understand it

See it worked out

Check yourself

Write it in the exam

Related concepts

Practice this concept with focused MCQs

Help improve this page