Short Answer

Actinides are the f-block elements from atomic number 89 (Actinium, Ac) to 103 (Lawrencium, Lr) in the periodic table.

• They are characterized by filling of 5f orbitals and are mostly radioactive metals.
• Actinides show multiple oxidation states, form compounds with non-metals, and are important in nuclear energy, medicine, and research. Examples include uranium (U), thorium (Th), and plutonium (Pu).

Detailed Explanation :

Definition of Actinides

Actinides are f-block elements located below the main body of the periodic table. They follow actinium (Ac) and precede rutherfordium (Rf).

• General electronic configuration: [Rn] 5f¹⁻¹⁴6d⁰⁻¹7s², depending on the element and oxidation state.
• They are called “actinides” because they start with actinium.
• All actinides are metallic and mostly radioactive, unlike lanthanides which are stable.

Physical Properties

1. Metallic Nature:
   • Shiny, silvery, and dense metals.
2. Melting and Boiling Points:
   • Relatively high due to strong metallic bonding, though some like uranium have moderate melting points.
3. Density:
   • High density; increases down the series due to atomic mass increase.
4. Conductivity:
   • Good conductors of heat and electricity because of delocalized valence electrons.
5. Radioactivity:
   • Most actinides are unstable and radioactive, with short to long half-lives.

Chemical Properties

1. Multiple Oxidation States:
   • Unlike lanthanides, actinides show variable oxidation states, typically +3, +4, +5, and +6.
   • Example: Uranium → U³⁺, U⁴⁺, U⁶⁺.
2. Formation of Compounds:
   • React with non-metals to form oxides, halides, and hydrides.
   • Example: UO₂ (uranium dioxide), ThCl₄ (thorium chloride).
3. Reactivity:
   • React with oxygen, water, and acids, usually more reactive than lanthanides.
4. Complex Formation:
   • Can form coordination compounds, useful in nuclear chemistry.
5. Radioactive Decay:
   • Some elements undergo alpha or beta decay, producing other elements.

Trends in Actinides

1. Atomic and Ionic Size:
   • Atomic size decreases slightly across the series due to actinide contraction.
2. Density and Melting Points:
   • Generally increase down the series due to heavier nuclei and stronger metallic bonds.
3. Reactivity:
   • Highly reactive, especially early actinides like thorium and uranium.
   • Reactivity decreases for later elements, partly due to radioactivity.
4. Oxidation States:
   • Variable oxidation states decrease across the series; earlier actinides show more +4 to +6 states, later actinides mostly +3.

Occurrence and Uses

1. Occurrence:
   • Found in uranium and thorium ores, like uraninite and monazite.
   • Some actinides are synthetic (e.g., plutonium, americium).
2. Uses:
   • Uranium and plutonium → nuclear fuel for reactors and weapons.
   • Thorium → nuclear fuel and high-temperature ceramics.
   • Americium → smoke detectors.
   • Actinides play a role in medical isotopes and research.

Conclusion

Actinides are f-block elements from Ac to Lr, characterized by 5f electron filling, metallic nature, and radioactivity. They show multiple oxidation states, high density, and reactivity, making them important in nuclear energy, research, and industrial applications. Actinides’ unique properties distinguish them from lanthanides and other transition metals.