4f

The 4f subshell refers to the f-type electron shell with principal quantum number n = 4. It comprises seven degenerate orbitals (m_l = −3, −2, −1, 0, 1, 2, 3), each capable of holding two electrons, for a total capacity of 14 electrons. In atomic structure, the 4f orbitals lie in the inner part of the atom and are more contracted and shielded than the outer valence orbitals.

In practice, 4f electrons are added across the lanthanide series, from cerium to lutetium. The common ground-state configurations are described as [Xe] 4f^n 6s^2 (with occasional 5d involvement in some elements). Because the 4f electrons are well shielded by the filled 5s and 5p shells, they interact weakly with chemical bonds and with the surrounding environment, giving rise to relatively uniform chemistry across the lanthanides and oxidation states typically around +3.

Physically, 4f electrons are largely localized and contribute to magnetic properties and luminescence rather than to covalent bonding. The shielding leads to sharp, well-defined f–f electronic transitions, which produce characteristic absorption and emission lines. This shielding also makes the lanthanide ions valuable in optical applications, where doped materials serve as phosphors, lasers, and scintillators.

In the periodic table, the 4f block corresponds to the lanthanides, often referred to as the inner transition metals. The behavior of 4f electrons underpins many of the distinctive magnetic and optical properties associated with this series, while their limited bonding participation generally results in predictable chemical trends across the group.