- The f-block elements are found in two rows placed separately at the bottom of the periodic table, known as the lanthanides and actinides. They are called the f-block because their valence electrons enter the (n–2)f orbitals, giving them the general electronic configuration of (n–2)f¹–¹⁴ (n–1)d⁰–¹ ns².
- There are 28 elements in total: 14 lanthanides (atomic numbers 57–71, from lanthanum to lutetium) and 14 actinides (atomic numbers 89–103, from actinium to lawrencium). These elements are also referred to as inner transition elements, as they are positioned between the s-block and d-block elements in terms of electron filling.
- The lanthanides are often called the rare earth elements, although most are not particularly rare in the Earth’s crust. They are soft, silvery-white metals with high reactivity, especially in finely divided form. Their ions are almost always +3 oxidation state, although some can exhibit +2 and +4 in certain compounds. Lanthanides are known for their ability to form colored ions and complexes, owing to f–f electronic transitions. They also display a phenomenon known as the lanthanide contraction, a steady decrease in atomic and ionic radii from lanthanum to lutetium caused by poor shielding of nuclear charge by the 4f electrons. This contraction influences the chemical similarities between lanthanides and some transition metals.
- The actinides are all radioactive elements, with many of them not occurring naturally but instead being synthesized artificially. The most familiar actinides are uranium (U) and thorium (Th), which occur in nature and serve as important fuels in nuclear power. Plutonium (Pu), neptunium (Np), and americium (Am) are man-made and used in nuclear reactors and weapons. Actinides exhibit greater variability in oxidation states compared to lanthanides, commonly ranging from +3 to +6, with uranium showing +4 and +6 as its most stable states. Because of the large size and availability of 5f, 6d, and 7s orbitals, actinides form complex compounds and often display covalent character.
- The physical properties of f-block elements include high melting and boiling points, metallic luster, and good conductivity, although they are softer compared to d-block metals. They are generally reactive, tarnishing quickly in air and reacting with water and acids to form hydrogen. Their magnetic and optical properties are significant—lanthanides produce sharp absorption and emission spectra, making them crucial in lasers, phosphors, and optical devices. Actinides, due to radioactivity, have special nuclear and medical uses but require careful handling.
- The applications of f-block elements are wide-ranging. Lanthanides are extensively used in modern technology: neodymium in strong permanent magnets, europium and terbium in color television screens and LED displays, cerium in catalytic converters and glass polishing, and gadolinium in MRI contrast agents. Actinides, particularly uranium and plutonium, are vital in nuclear power generation and weapons. Americium is used in smoke detectors, while thorium is being explored as an alternative nuclear fuel.
- In summary, the f-block elements are characterized by their placement in the inner transition region, their partially filled f-orbitals, variable oxidation states, and unique physical and chemical properties. Lanthanides are known for their magnetic, optical, and catalytic roles, while actinides are central to nuclear science and energy. Despite their position at the bottom of the periodic table, the f-block elements play an outsized role in technology, energy, medicine, and industry, making them some of the most strategically important elements in the modern world.
