The actinoids are also known as actinides and a group of 15 metallic elements with atomic numbers 89–103 that range from actinium (Ac) to lawrencium (Lr) in the periodic table. These elements are recognized for their radioactive characteristics and are found in the periodic table’s f-block.
Position in the Periodic Table
Like lanthanoids, actinoids belong to the inner transition elements and are positioned in the seventh period. In Group 3 of the periodic chart, they come after actinium. Despite the fact that their overall electronic setup is [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²,
the filling of electrons occurs in the 5f orbital, making them unique.
General Properties of Actinoids
Radioactivity: As actinoids break down into lighter elements, they all release radiation. Some are used in nuclear energy, such as plutonium and uranium.
Variable Oxidation States: Because of the tiny energy difference between their 5f, 6d, and 7s orbitals, actinoids display a variety of oxidation states in distinction to lanthanoids, which primarily display a +3 oxidation state. Uranium exhibits +3, +4, +5, and +6 oxidation states, for example.
Metallic Nature: Although they have a lower melting point than lanthanoids, actinoids are metals with a high density. They appear silvery and are flexible, but they tarnish easily in the air.
Complex Formation: Actinoids combine with ligands like fluoride, sulfate, and chloride ions to form complexes because of their larger atomic sizes and availability of unoccupied orbitals.
Actinoid Contraction: Because of inadequate shielding of the 5f electrons, the actinoid series likewise exhibits a progressive decrease in atomic and ionic radious throughout the series, much like lanthanoid contraction.
Coloured ions: Are coloured due to f-f transition except Ac3+ (5f0), Cm3+ (5f7) and Th4+ (5f0) which are colourless.
Reducing Agents: All actionid are strong reducing agents.
Density: All actionid except thorium and americium have high densities.
Ionisattion energy: They have low ionisation energies.
Electroposition character: They are highly electropositive metals.
Important Actinoids and Their Uses
Uranium (U, atomic number 92): A nuclear fuel used in atomic bombs and power plants.
Nuclear weapons and reactors use plutonium (Pu, atomic number 94).
Thorium (Th, atomic number 90): A possible nuclear energy substitute for uranium.
Smoke detectors use amerium (Am, atomic number 95).
Curium (Cm, Atomic number 96): A power source for space travel.
Comparison: Actinoids vs. Lanthanoids
Property | Actinoids | Lanthanoids |
Electron Filling | 5f-orbitals | 4f-orbitals |
Oxidation States | Multiple (+3 to +6) | Mostly +3 |
Radioactivity | All radioactive | Mostly non-radioactive |
Complex Formation | Strong tendency | Comparatively less |
Reactivity | More reactive | Less reactive |