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Gadolinium is a chemical element with the symbol Gd and atomic number 64. It belongs to the lanthanide series, a group of rare earth elements. Gadolinium was identified in 1880 by Jean Charles Galissard de Marignac from gadolinite, a mineral named after the Finnish chemist Johan Gadolin. The element is typically found as trivalent ions (Gd3+) in compounds and minerals.

In its pure form, gadolinium is a soft, silvery‑white metal that tarnishes in air. In compounds, it most commonly exhibits a +3 oxidation state. Gadolinium compounds are often oxides like gadolinium oxide (Gd2O3) and various halides and chelates. The metal is highly paramagnetic, a property that underpins many of its practical uses.

Gadolinium occurs in minerals such as gadolinite, monazite, and bastnasite. Commercial production arises from the processing of mixed rare‑earth concentrates, with major sources historically centered in regions of China and other rare-earth producing areas. The element is separated from other rare earths through complex hydrometallurgical and solvent‑extraction processes.

The most widespread modern use of gadolinium is in magnetic resonance imaging (MRI) as gadolinium‑based contrast agents (GBCAs). When chelated to organic compounds, gadolinium enhances image contrast by altering the relaxation times of nearby water protons. Gadolinium also has a high neutron capture cross‑section and is used in neutron shielding and control materials in nuclear applications. It appears in phosphors for some lighting and display technologies as well as certain magnetic materials and dopants.

Safety and regulatory considerations are important; free gadolinium ions are toxic, so medical applications rely on stable chelated forms. Some concerns about gadolinium deposition in tissues have led to safety guidance, with preference for more stable (macrocyclic) chelates in vulnerable patients. Natural gadolinium has several stable isotopes, including Gd‑152, ‑154, ‑155, ‑156, ‑157, ‑158, and ‑160, along with numerous radioactive isotopes produced for research.