Introduction
These are called second inner transition series. The elements in which the last electron enters one of the 5f-orbitals are called as actinides (or) 5f-block elements. They include the elements from actinium (Ac) to Lawrencium (Lr). All these elements are radioactive and the elements beyond uranium are all man-made elements.
Electronic configuration
The last electron enters into 5f-orbital in these elements. The electronic configuration of actinides is given below. The general electronic configuration of these elements is 5f1-14 6d0-1 7s2.
| Element | Symbol | Atomic number | Electronic configuration |
|---|---|---|---|
| Actinium | Ac | 89 | [Rn] 6d17s2 |
| Thorium | Th | 90 | [Rn] 6d2 7s2 |
| Protactinium | Pa | 91 | [Rn] 5f 2 6d1 7s2 |
| Uranium | U | 92 | [Rn] 5f 3 6d1 7s2 |
| Neptunium | Np | 93 | [Rn] 5f 4 6d1 7s2 |
| Plutonium | Pu | 94 | [Rn] 5f 6 7s2 |
| Americium | Am | 95 | [Rn] 5f 7 7s2 |
| Curium | Cm | 95 | [Rn] 5f 7 6d1 7s2 |
| Berkelium | Bk | 97 | [Rn] 5f 8 6d1 7s2 |
| Californium | Cf | 98 | [Rn] 5f10 7s2 |
| Einsteinium | Es | 99 | [Rn] 5f11 7s2 |
| Fermium | Fm | 100 | [Rn] 5f12 7s2 |
| Mendelevium | Md | 101 | [Rn] 5f13 7s2 |
| Nobelium | No | 102 | [Rn] 5f14 7s2 |
| Lawrencium | Lr | 103 | [Rn] 5f14 6d1 7s2 |
