Understanding f-block Elements in the Periodic Table for Class 12 Chemistry

Introduction to f-block Elements in the Periodic Table

The f-block elements in the periodic table consist of two series: the lanthanoids (elements 57 to 71) and the actinoids (elements 89 to 103). These elements are placed separately at the bottom of the periodic table to maintain its structure.

• Lanthanoids fill the 4f orbitals progressively.
• Actinoids fill the 5f orbitals.

These elements are characterised by their unique electronic configurations, where electrons enter the inner f-subshells rather than the outer d or s orbitals. This leads to distinctive chemical and physical properties compared to other blocks.

Understanding f-block elements is essential for Class 12 NCERT chemistry as they play important roles in advanced materials, magnets, catalysts, and nuclear reactors.

Electronic Configuration and Position in the Periodic Table

The general electronic configuration of f-block elements is:

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

Here, electrons fill the 4f or 5f orbitals gradually, which are inner orbitals shielded by outer electrons. This shielding effect causes unique trends in atomic size and chemical behaviour.

Position in the Periodic Table:

The placement of f-block elements below the main table helps keep the periodic table compact and highlights their special properties.

Key Physical and Chemical Properties of f-block Elements

f-block elements show several characteristic properties:

• Atomic and Ionic Size: Due to poor shielding by 4f and 5f electrons, there is a gradual decrease in size across the lanthanoids, known as the lanthanoid contraction. This affects the chemistry of subsequent elements.

• Oxidation States: Lanthanoids mostly exhibit a +3 oxidation state, while actinoids show multiple oxidation states from +3 to +6 due to variable 5f electron involvement.

• Magnetic Properties: Many f-block ions have unpaired electrons, resulting in paramagnetism. The magnetic moment can be calculated using the spin-only formula:

$$\mu = \sqrt{n(n+2)}$$

where $n$ is the number of unpaired electrons.

• Reactivity: Lanthanoids are reactive metals that tarnish easily, while actinoids are mostly radioactive and show complex chemistry.

• Colour and Complex Formation: f-block ions often form coloured compounds due to f-f electronic transitions, though these are less intense than d-d transitions in transition metals.

Lanthanoid Contraction and Its Effects

One of the most important concepts in the chemistry of f-block elements is the lanthanoid contraction. It refers to the steady decrease in atomic and ionic radii of lanthanoid elements from La (57) to Lu (71).

Reason: Poor shielding by 4f electrons causes the effective nuclear charge to increase, pulling electrons closer to the nucleus.

Consequences:

• The atomic size of 5d transition metals is similar to their 4d counterparts, despite being in a higher period.
• Influences the chemical properties and hardness of alloys.
• Affects the separation and extraction of lanthanoids due to similar sizes.

This contraction is a key topic in Class 12 NCERT chemistry and explains many periodic trends.

Comparison of Lanthanoids and Actinoids

While both series belong to the f-block, lanthanoids and actinoids differ in several aspects:

This comparison helps Class 12 students understand the diversity within the f-block and their practical applications.

Applications and Importance of f-block Elements

f-block elements have many industrial and technological uses:

• Lanthanides: Used in strong permanent magnets (NdFeB magnets), phosphors in TV and LED screens, catalysts in petroleum refining, and alloys to improve strength.

• Actinides: Uranium and thorium are vital nuclear fuels. Plutonium is used in nuclear reactors and weapons. Some actinoids are used in radiation sources and medical treatments.

• Other Uses: Lanthanoids help in glass polishing and as additives in ceramics.

Understanding these applications is important for Class 12 NCERT students to relate theoretical concepts to real-world uses.

Worked Example: Calculating Magnetic Moment of Mn2+ Ion

Manganese (Mn) has atomic number 25. The electronic configuration of Mn2+ ion is:

$$[Ar] 3d^5$$

Number of unpaired electrons, $n = 5$.

Using the spin-only magnetic moment formula:

$$\mu = \sqrt{n(n+2)} = \sqrt{5(5+2)} = \sqrt{35} \approx 5.92 \text{ BM}$$

So, the magnetic moment of Mn2+ ion is approximately 5.92 Bohr Magnetons.

This calculation is typical in Class 12 NCERT exams to test understanding of electronic configuration and magnetism.