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).
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.