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The actinides, also known as actinoids, is series of 15 metallic chemical elements from the 7th period of the f-block in the periodic table, right below the lanthanides. It comprises the elements from actinium to lawrencium, with atomic numbers from 89 to 103.

All the elements display similar chemical properties but also display various oxidation states (the lighter actinoids display "quasi-groups", their maximum valence and oxidation state increase with each element, e.g. Th +4, Pa +5, U +6, etc; on plutonium this trend breaks, and heavier actinoids display properties similar to lanthanoids). They react with air and in finely divided form, they are pyrophoric. Actinides are highly reactive towards halogens and chalcogens. All actinides are radioactive.

Of all the actinides, the most important are thorium and uranium. Plutonium and americium are also often encountered in several applications.



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