JEE Chemistry F-Block Elements Complete Guide

F-block elements, comprising the lanthanide and actinide series, form a vital part of the JEE Chemistry syllabus. These elements are placed in the inner transition metal section of the periodic table and exhibit unique electronic configurations, oxidation states, and chemical behavior. This guide delves deep into their properties, electronic structure, common compounds, separation methods, and applications to help you excel in your JEE preparation.

1. Introduction to F-Block Elements

F-block elements include the lanthanides (atomic numbers 57 to 71) and actinides (atomic numbers 89 to 103). They are called inner transition elements because their differentiating electron enters the 4f or 5f orbitals respectively, which are inner orbitals relative to the outer s and d shells.

The lanthanides are also called rare earth elements, although some are fairly abundant in the earth's crust. Actinides are mostly radioactive and have important applications in nuclear chemistry.

2. Position in the Periodic Table and Electronic Configuration

The general electronic configuration for f-block elements is:

(n-2)f^{1-14} (n-1)d^{0-1} ns^2

- Lanthanides fill the 4f orbitals: from \( \mathrm{Ce} (4f^1) \) to \( \mathrm{Lu} (4f^{14}) \).
- Actinides fill the 5f orbitals: from \( \mathrm{Th} (5f^0) \) to \( \mathrm{Lr} (5f^{14}) \), though actinide configurations can be more complex.

Example:

• Cerium (Ce): \( [\mathrm{Xe}]\, 4f^1\, 5d^1\, 6s^2 \)
• Uranium (U): \( [\mathrm{Rn}]\, 5f^3\, 6d^1\, 7s^2 \)

3. Lanthanide Series

3.1 Electronic Configuration and Lanthanide Contraction

Lanthanides show gradual filling of 4f orbitals with increasing atomic number. Due to poor shielding of 4f electrons, effective nuclear charge increases steadily causing lanthanide contraction — a decrease in ionic radii across the series.

Lanthanide contraction affects:

• Similarity in size of 4th and 5th period elements.
• Complex formation and chemical behavior of heavier elements.

3.2 Common Oxidation States

The common oxidation state of lanthanides is +3. However, some exhibit +2 and +4 states in specific cases.

• Examples: \( \mathrm{Eu}^{2+} \), \( \mathrm{Sm}^{2+} \), and \( \mathrm{Ce}^{4+} \).

3.3 Physical Properties

• Silvery-white metals, soft and malleable.
• High melting points.
• Paramagnetic due to unpaired 4f electrons.

3.4 Chemical Properties

• Reactivity decreases with increasing atomic number.
• Form compounds mainly in +3 oxidation state like oxides (\( \mathrm{Ln}_2\mathrm{O}_3 \)), halides, and sulfides.
• React with water slowly to form hydroxides and hydrogen gas.

3.5 Important Lanthanide Compounds

• \( \mathrm{Lanthanide} \, \mathrm{Oxides} \) — Important in glass making and catalysts.
• \( \mathrm{Lanthanide} \, \mathrm{Halides} \) — Used in lighting and lasers.
• \( \mathrm{Lanthanide} \, \mathrm{Sulfides} \) — Semiconductor materials.

4. Actinide Series

4.1 Electronic Configuration and Radioactivity

Actinides fill the 5f orbitals. Most are radioactive due to instability in nucleus, and exhibit multiple oxidation states. Electronic configurations are more complex and involve 5f, 6d, and 7s electrons.

Example configurations:

• Uranium (U): \( [\mathrm{Rn}]\, 5f^3\, 6d^1\, 7s^2 \)
• Plutonium (Pu): \( [\mathrm{Rn}]\, 5f^6\, 7s^2 \)

4.2 Common Oxidation States

• Wide range from +3 to +6, with +3, +4, +5, and +6 common for uranium and other actinides.
• Example: \( \mathrm{U}^{6+} \) in \( \mathrm{UO}_2^{2+} \) (uranyl ion).

4.3 Physical Properties

• Silvery metals, radioactive, dense and heavy.
• High melting points.

4.4 Chemical Properties

• Highly reactive, especially with oxygen and halogens.
• Form oxides, halides, and other compounds exhibiting multiple oxidation states.
• Actinides form complexes with ligands such as carbonate, phosphate, and nitrate ions.

4.5 Important Actinide Compounds

• \( \mathrm{UO}_2 \) (uranium dioxide) — nuclear fuel.
• \( \mathrm{PuO}_2 \) (plutonium dioxide) — nuclear fuel and weapon material.
• Actinide halides — used in research and specialized industrial processes.

5. Oxidation State Trends in F-Block Elements

Lanthanides primarily exhibit +3 oxidation state, whereas actinides exhibit variable oxidation states due to the involvement of 5f, 6d, and 7s electrons. Higher oxidation states are more common in early actinides.

6. Separation Techniques of Lanthanides

Due to their similar chemical properties and close atomic sizes (lanthanide contraction), separation is difficult but vital for practical uses.

6.1 Ion Exchange Method

Uses resin columns to separate lanthanides based on their differing affinities and exchange rates.

6.2 Solvent Extraction

Organic solvents selectively extract certain lanthanides from aqueous solutions using complexing agents.

6.3 Fractional Crystallization

Based on slight differences in solubility of their salts, used for large scale separation historically.

7. Applications of F-Block Elements

7.1 Lanthanides

• Used in strong permanent magnets (Nd-Fe-B magnets).
• Catalysts in petroleum refining.
• Phosphors in color TV and LED displays.
• Glass polishing powders.

7.2 Actinides

• Uranium and plutonium used as nuclear fuels.
• Radioisotopes in medicine and industry.
• Research in nuclear chemistry and radiopharmaceuticals.

8. Important Notes for JEE

• Remember general electronic configuration of f-block elements.
• Understand the cause and effects of lanthanide contraction.
• Focus on common oxidation states, especially +3 for lanthanides and variable for actinides.
• Practice reactions and separation techniques.
• Memorize important compounds and their applications.

Mastery of f-block elements provides a competitive advantage in JEE Chemistry, especially for inorganic chemistry and coordination compounds sections.