Short Answer

Inner transition elements are the elements in the f-block of the periodic table, including lanthanides and actinides.

• They have electrons filling the inner f-orbitals, which gives them unique properties like variable oxidation states, magnetic behavior, and colored compounds.
• Examples include cerium (Ce), uranium (U), thorium (Th), and neodymium (Nd), used in magnets, nuclear fuel, and alloys.

Detailed Explanation :

Definition of Inner Transition Elements

Inner transition elements are f-block elements located at the bottom of the periodic table. They are called “inner” because their d-electrons are in an outer shell, but the f-electrons are being filled in the penultimate shell.

• Lanthanides: 14 elements from atomic numbers 57 (La) to 71 (Lu). They fill the 4f orbitals.
• Actinides: 14 elements from atomic numbers 89 (Ac) to 103 (Lr). They fill the 5f orbitals.

These elements are chemically similar within their series due to similar f-orbital configuration.

Physical Properties

1. Metallic Nature:
   • Inner transition elements are shiny metals and generally hard and dense.
2. High Melting and Boiling Points:
   • Lanthanides and actinides have high melting and boiling points because of strong metallic bonding.
3. Conductivity:
   • Good conductors of heat and electricity due to mobile electrons.
4. Atomic and Ionic Size:
   • Lanthanides exhibit lanthanide contraction, a gradual decrease in size across the series.
   • Actinides show similar trend, though radioactive elements may differ slightly.
5. Density:
   • Increases across the series as atomic number increases.

Chemical Properties

1. Variable Oxidation States:
   • Lanthanides mostly show +3 oxidation state, but some can exhibit +2 or +4.
   • Actinides show multiple oxidation states (+3 to +6) due to involvement of 5f, 6d, and 7s electrons.
2. Formation of Compounds:
   • Form oxides, halides, and salts with non-metals.
   • Example: UO₂ (uranium dioxide), CeCl₃ (cerium chloride).
3. Magnetic and Spectral Properties:
   • Partially filled f-orbitals give rise to paramagnetism and colored compounds.
   • Used in lasers, phosphors, and magnets.
4. Reactivity:
   • Lanthanides react slowly with water and more rapidly with acids to produce hydrogen.
   • Actinides are radioactive → chemical reactivity may also involve nuclear reactions.

Trends in Inner Transition Elements

1. Atomic and Ionic Size:
   • Lanthanide contraction → decrease in size across the series.
   • Similar trend in actinides.
2. Density and Melting Point:
   • Gradually increases across the series due to increasing atomic mass.
3. Reactivity:
   • Lanthanides are less reactive than alkali metals but more reactive than transition metals.
   • Actinides vary due to radioactivity; early actinides are more reactive.
4. Complex Formation:
   • Inner transition metals can form complexes with ligands due to partially filled f-orbitals.

Occurrence and Uses

1. Lanthanides:
   • Found in minerals like monazite and bastnasite.
   • Used in magnets, catalysts, phosphors, and alloys.
2. Actinides:
   • Found in uranium and thorium ores.
   • Used as nuclear fuel, radioactive tracers, and in research.
3. Industrial and Technological Applications:
   • Lanthanides in permanent magnets (NdFeB), lasers, and fluorescent lamps.
   • Actinides in nuclear reactors and radiopharmaceuticals.

Conclusion

Inner transition elements are f-block elements, including lanthanides and actinides, with electrons filling inner f-orbitals. They are dense, metallic, and exhibit variable oxidation states, magnetic properties, and colored compounds. Lanthanides show lanthanide contraction, while actinides are radioactive with multiple oxidation states. These elements are important in industrial, technological, and nuclear applications, making them critical in modern chemistry and materials science.