Actinium (symbol Ac, atomic number 89) is the radioactive element that gives the actinide series its name. It is classified as a actinide and sits in the actinide series (f-block), period 7 of the periodic table.
Overview
Actinium is the chemical element with atomic number 89 and the symbol Ac. With a standard atomic weight of [227], it is a actinide. In the periodic table it occupies the actinide series (f-block) and period 7, and under standard conditions it is normally a solid. Actinium is the radioactive element that gives the actinide series its name.
Atomic structure
An atom of Actinium has 89 protons in its nucleus and, when neutral, 89 electrons arranged in the configuration [Rn] 6d1 7s2. Actinium has no stable isotope; the value [227] refers to the mass number of its most stable or most commonly produced isotope.
Discovery and place in the table
Actinium was discovered in 1899 by André-Louis Debierne. It follows Radium (Ra) and precedes Thorium (Th) in order of atomic number.
| Atomic number | 89 |
|---|---|
| Symbol | Ac |
| Atomic mass | [227] |
| Category | Actinide |
| Group | — |
| Period | 7 |
| Electron configuration | [Rn] 6d1 7s2 |
| Phase at STP | Solid |
| Discovered | 1899 (André-Louis Debierne) |