ElementsWhat are the actinides?
The actinides are the fifteen elements from actinium to lawrencium, drawn as the second separate row below the main table. All of them are radioactive. Only thorium and uranium occur in nature in large amounts. Those heavier than uranium are made in reactors or particle accelerators. Uranium and plutonium are the fuels of nuclear power.
The family in numbers
Melting points in this family run from 639.9 °C (plutonium) to 1,750 °C (thorium). Among its solid members, the least dense is actinium (10.07 g/cm³) and the densest is neptunium (20.25 g/cm³). The most recently discovered member is lawrencium (1961).
Radioactivity
An atom is radioactive when its nucleus is unstable and breaks down by itself, giving out radiation and turning into another element. Every actinide is radioactive. Thorium and uranium break down so slowly, over billions of years, that plenty remains from the time the Earth formed. The others break down far faster, so any that the Earth started with are long gone, and the elements beyond uranium have to be made. They are called the transuranium elements.
Fission and nuclear power
The nucleus of one kind of uranium atom can be split by a neutron into two smaller nuclei. This is fission. It releases a great deal of energy and more neutrons, which can split further atoms in a chain reaction. A nuclear power station runs this reaction under control and uses the heat to make steam. Plutonium, made from uranium inside reactors, can be used in the same way. India has large reserves of thorium in its beach sands, and its nuclear programme is planned in stages that lead towards using thorium as fuel.
Names
Several actinides are named after scientists and places: curium after Marie and Pierre Curie, einsteinium after Albert Einstein, fermium after Enrico Fermi, mendelevium after Dmitri Mendeleev, the maker of the first periodic table, and americium, berkelium and californium after the places where they were first made. A tiny amount of americium is inside many household smoke detectors.
All actinides compared
| No. | Symbol | Name | Atomic mass | State | Melts at | Boils at | Density g/cm³ | Discovered |
|---|---|---|---|---|---|---|---|---|
| 89 | Ac | Actinium | 227.02775 | Solid | 1,051 °C | 3,198 °C | 10.07 | 1899 |
| 90 | Th | Thorium | 232.038 | Solid | 1,750 °C | 4,788 °C | 11.72 | 1828 |
| 91 | Pa | Protactinium | 231.03588 | Solid | 1,572 °C | no value | 15.37 | 1913 |
| 92 | U | Uranium | 238.0289 | Solid | 1,135 °C | 4,131 °C | 18.95 | 1789 |
| 93 | Np | Neptunium | 237.048172 | Solid | 643.9 °C | 3,902 °C | 20.25 | 1940 |
| 94 | Pu | Plutonium | 244.06420 | Solid | 639.9 °C | 3,228 °C | 19.84 | 1940 |
| 95 | Am | Americium | 243.061380 | Solid | 1,176 °C | 2,011 °C | 13.69 | 1944 |
| 96 | Cm | Curium | 247.07035 | Solid | 1,345 °C | 3,127 °C | 13.51 | 1944 |
| 97 | Bk | Berkelium | 247.07031 | Solid | 1,050 °C | no value | 14 | 1949 |
| 98 | Cf | Californium | 251.07959 | Solid | 899.9 °C | no value | no value | 1950 |
| 99 | Es | Einsteinium | 252.0830 | Solid | 859.9 °C | no value | no value | 1952 |
| 100 | Fm | Fermium | 257.09511 | Solid | 1,527 °C | no value | no value | 1952 |
| 101 | Md | Mendelevium | 258.09843 | Solid | 826.9 °C | no value | no value | 1955 |
| 102 | No | Nobelium | 259.10100 | Solid | 826.9 °C | no value | no value | 1957 |
| 103 | Lr | Lawrencium | 266.120 | Solid | 1,627 °C | no value | no value | 1961 |
Other families: alkali metals, alkaline earth metals, transition metals, post-transition metals, metalloids, nonmetals, halogens, noble gases, lanthanides.