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Any of the series of 15 consecutive chemical elements in the periodic table from actinium to lawrencium (atomic numbers 89–103). All are radioactive heavy metals; and only the first four (actinium, thorium, protactinium, and uranium) occur in nature in appreciable quantities. The other 11 (the transuranium elements) are unstable and are produced only artificially. Actinides are transition elements, so their atoms have similar configurations and similar physical and chemical behaviour; the most usual valences are 3 and 4.
This entry comes from Encyclopædia Britannica Concise. For the full entry on actinide, visit Britannica.com.