Actinides
The actinides (atomic numbers 89-103, from actinium to lawrencium) are the lower of the two rows pulled out below the main table. They are all radioactive. Uranium and plutonium power nuclear reactors and weapons; later actinides (americium, curium, einsteinium) exist mostly in research quantities created in particle accelerators or reactor neutron capture.
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| No. | Symbol | Element | Atomic weight (u) | Density (g/cm³) | Melting point (K) | Boiling point (K) | Electronegativity | Discovered |
|---|---|---|---|---|---|---|---|---|
| 89 | Ac | Actinium | 227 | 10 | 1500 | 3500 | 1.1 | 1902 |
| 90 | Th | Thorium | 232.038 | 11.724 | 2023 | 5061 | 1.3 | 1829 |
| 91 | Pa | Protactinium | 231.036 | 15.37 | 1841 | 4300 | 1.5 | 1913 |
| 92 | U | Uranium | 238.029 | 19.1 | 1405.3 | 4404 | 1.38 | 1789 |
| 93 | Np | Neptunium | 237 | 20.45 | 912 | 4447 | 1.36 | 1940 |
| 94 | Pu | Plutonium | 244 | 19.816 | 912.5 | 3505 | 1.28 | 1940 |
| 95 | Am | Americium | 243 | 12 | 1449 | 2880 | 1.13 | 1944 |
| 96 | Cm | Curium | 247 | 13.51 | 1613 | 3383 | 1.28 | 1944 |
| 97 | Bk | Berkelium | 247 | 14.78 | 1259 | 2900 | 1.3 | 1949 |
| 98 | Cf | Californium | 251 | 15.1 | 1173 | 1743 | 1.3 | 1950 |
| 99 | Es | Einsteinium | 252 | 8.84 | 1133 | 1269 | 1.3 | 1952 |
| 100 | Fm | Fermium | 257 | - | 1800 | - | 1.3 | 1952 |
| 101 | Md | Mendelevium | 258 | - | 1100 | - | 1.3 | 1955 |
| 102 | No | Nobelium | 259 | - | 1100 | - | 1.3 | 1966 |
| 103 | Lr | Lawrencium | 262 | - | 1900 | - | 1.3 | 1961 |