Comprehensive Guide to f-block Elements in the Periodic Table for Class 12

Position and Electronic Configuration of f-block Elements

The f-block elements are located separately at the bottom of the periodic table to maintain its structure. They consist of two series:

• Lanthanoids (4f series): Elements from Cerium (Ce) to Lutetium (Lu) filling the 4f orbitals.
• Actinoids (5f series): Elements from Thorium (Th) to Lawrencium (Lr) filling the 5f orbitals.

Their general electronic configuration is:

• Lanthanoids: $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$
• Actinoids: $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$

The filling of f orbitals imparts unique chemical and physical properties distinct from d-block elements. These elements are also called inner transition metals because the f orbitals are filled inside the atom.

Occurrence and Extraction of f-block Elements

Lanthanoids are relatively abundant in the Earth's crust, often found in minerals like monazite and bastnäsite. Actinoids, however, are mostly synthetic or radioactive, with uranium and thorium being naturally occurring.

Extraction methods:

• Lanthanoids are extracted by separating them from their ores using solvent extraction or ion-exchange methods.
• Actinoids like uranium are extracted from ores such as pitchblende by chemical methods involving leaching.

The separation of lanthanoids is challenging due to their similar chemical properties but is essential for obtaining pure elements for industrial use.

General Properties of f-block Elements

The f-block elements show several distinctive properties:

• Variable oxidation states: Lanthanoids mostly exhibit +3 oxidation state, while actinoids show +3 and higher states (up to +6).
• Magnetic properties: Many f-block elements are paramagnetic due to unpaired electrons in f orbitals.
• Complex formation: They form coordination compounds with various ligands.
• Colour: Lanthanoids form coloured compounds, often used in phosphors and lasers.
• Radioactivity: Most actinoids are radioactive, influencing their handling and applications.

These properties make f-block elements important in various technological fields such as nuclear energy, electronics, and catalysis.

Comparison of Lanthanoids and Actinoids

Here is a comparison table highlighting key differences between lanthanoids and actinoids:

This comparison helps clarify their distinct roles and chemical behaviours in the periodic table.

Chemical Behaviour and Oxidation States of f-block Elements

The chemical behaviour of f-block elements is largely influenced by their electron configurations:

• Lanthanoids generally show +3 oxidation state due to the loss of two 6s and one 4f electron.
• Some lanthanoids like cerium and europium show +4 and +2 states respectively.
• Actinoids exhibit a wider range of oxidation states (+3 to +6) because 5f, 6d, and 7s electrons participate in bonding.

Example:

• Uranium commonly shows +6 oxidation state in compounds like $UO_3$.

The highest oxidation states are usually stabilised in oxides and fluorides because oxygen and fluorine are highly electronegative and can stabilise the positive charge.

Worked example: Calculate the magnetic moment of $Mn^{2+}$ (atomic number 25) using the spin-only formula.

• Electronic configuration of $Mn^{2+}$ is $[Ar] 3d^5$ with 5 unpaired electrons.
• Magnetic moment, $$BC = sqrt{n(n+2)} = sqrt{5 imes 7} = sqrt{35} approx 5.92 ext{ BM}$$

Applications of f-block Elements in Technology

f-block elements have significant applications:

• Lanthanoids: Used in manufacturing strong permanent magnets, phosphors in TV screens and LEDs, catalysts in petroleum refining, and glass polishing.
• Actinoids: Uranium and plutonium are crucial as nuclear fuels in reactors and weapons.
• Radioactive isotopes from actinoids are used in medical treatments and radiography.

Their unique electronic and magnetic properties make them indispensable in modern technology and industry.