Actinons (also called actinoids or actinides) describe the series of 14 chemical elements that follow actinium in group 3; actinium is usually considered a member of this series. Excluding actinium, the 14-member series, known collectively as the actinons, consists of thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium and lawrencium. All the actinons are radioactive and most, particularly the transuranic elements, are produced artificially. In the elements of the actinon series the outer f orbital, the 5f subshell, is filled as atomic number increases. The actinons exhibit properties similar to the lanthanons respective to the number of electrons that fill their outermost shell. Curium, for example, possesses chemistry similar to that of gadolinium. The actinons as a group are strongly electropositive, shiny, hard metals that tarnish in air.