Actinide, also called an actinoid, is any of the 15 chemical elements that follow the element radium in the periodic table. In order of atomic number, the actinides are actinium (atomic number 89), thorium (90), protactinium (91), uranium (92), neptunium (93), plutonium (94), americium (95), curium (96), berkelium (97), californium (98), einsteinium (99), fermium (100), mendelevium (101), nobelium (102), and lawrencium (103). Scientists sometimes do not include actinium among the actinides.
Most periodic tables show the actinides and a series called the lanthanides separate from the rest of the elements in two long rows. If the rows were shown in place, the table would be too wide for easy display.
Actinide atoms have an increasing number of electrons in a region called the 5f subshell. In general, each actinide has one more electron in the 5f subshell than the previous actinide. But several actinides do not follow this pattern, leading to unique electron arrangements.
In forming compounds, most actinides tend to lose three electrons, gaining a 3+ charge. However, some actinides are known to take on charges as high as 7+.
Only two actinide elements, thorium and uranium, are present on Earth in significant quantities. The other elements are present only in minute quantities or are made exclusively by human beings. All of the actinide elements are radioactive, limiting their use.
Uranium and plutonium are used to produce power in nuclear power plants and atomic bombs. Uranium is also used to make dense, armor-penetrating artillery shells. Americium is widely used as part of the electrical circuit by which smoke detectors operate.
