F-block

The f-block is one of the four main sections of the periodic table. It is defined by the fact that the elements in it have their outermost electrons filling f orbitals. An orbital is a region where an electron is likely to be found; an f-type orbital can hold up to 14 electrons.

The f-block comprises two rows: the lanthanides (from lanthanum to lutetium) and the actinides (from actinium to lawrencium). Altogether there are 28 elements. They are usually displayed as two rows placed below the periodic table to make it manageable to view; in reality they belong within the middle of the 6th and 7th rows.

Electronic structure

The f-block elements are described as having the 4f (lanthanides) and 5f (actinides) orbitals progressively filling. Because the energy levels of the f, d, and s orbitals are very close to one another, many f-block elements have an electronic structure that does not follow the expected pattern. For example, some elements donate one electron to the d orbital instead of entering the f orbital.

Chemical properties

The lanthanides have very similar chemical properties, because the 4f orbital sits below the outermost electron shells and does not participate significantly in chemical bonding. The most common oxidation state is +3. There is also a phenomenon called 'lanthanide contraction'—the gradual decrease in atomic radius as atomic number increases—which affects the properties of subsequent elements. The actinides have a 5f orbital that is more extended, so they exhibit varying oxidation states, particularly the lighter elements such as uranium and plutonium.

Radioactivity and occurrence

All actinides are radioactive. Uranium and thorium occur naturally in significant amounts; those beyond uranium (transuranium elements) are mostly produced in reactors or accelerators. Except for promethium, the lanthanides are found in rocks and minerals and are used in high-strength magnets, lasers, and display technology.