Any of the series of 15 consecutive chemical element s in the periodic table from actinium to lawrencium ( atomic number s 89–103).
All are radioactive heavy metal s; and only the first four (actinium, thorium, protactinium, and uranium ) occur in nature in appreciable quantities. The other 11 (the transuranium element s) are unstable and are produced only artificially. Actinides are transition element s, so their atom s have similar configurations and similar physical and chemical behaviour; the most usual valence s are 3 and 4.