The actinide elements are the fifteen metallic elements from actinium to lawrencium, although actinium is not always included within the series. The actinides were so-named because they possess similar properties to actinium in Group 3 and would be fitting members of that group. They are designated a separate division, however, because their electrons fill a different shell than do those of actinium.
All the actinides are radioactive. Those following uranium are also known as the transuranium elements, and are produced artificially. It was once thought that only the first four actinides - actinium, thorium, protactinium and uranium - occur naturally, but very small quantities of neptunium and plutonium have recently been discovered.
The dominant characteristic of the series is its members' radioactivity. Progressing left to right across the series, the actinides' half-lives shorten, and the elements are increasingly prone to spontaneous fission. This makes their properties increasingly difficult to determine.
Those actinides that have been seen - and some, such as mendelevium, nobelium and lawrencium have never been produced in visible quantities - are silver in appearance.
From the limited data available, it is known that the actinides are reactive elements, oxidising rapidly on exposure to air, and that they increase in reactivity as the series progresses.
