Actinium (Ac): electron configuration, atomic mass and facts
Actinium (Ac) is element 89 of the periodic table, an actinide in period 7 (f-block). Its electron configuration is [Rn] 6d1 7s2, with electrons arranged in shells 2, 8, 18, 32, 18, 9, 2, and its most stable isotope has mass number 227.
▶ Watch Actinium’s electrons orbit in the interactive periodic tableActinium at a glance
| Symbol | Ac |
|---|---|
| Atomic number | 89 |
| Atomic mass | [227] u (most stable isotope) |
| Category | Actinide |
| Group | f-block (no group number) |
| Period | 7 |
| Block | f-block |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d1 7s2 |
| Noble-gas shorthand | [Rn] 6d1 7s2 |
| Electrons per shell | 2, 8, 18, 32, 18, 9, 2 |
| Electronegativity (Pauling) | 1.1 |
| State at 20 °C | solid |
Where is Actinium on the periodic table?
Actinium is in period 7 of the f-block, the actinide row printed below the main table. It has 7 electron shells, and this row is where the 5f subshell fills (its own configuration is an exception, explained below).
How to work out the electron configuration of Actinium
Put 89 electrons into subshells in order of rising energy, filling each before starting the next: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁶ (18), 4s² (20), 3d¹⁰ (30), 4p⁶ (36), 5s² (38), 4d¹⁰ (48), 5p⁶ (54), 6s² (56), 4f¹⁴ (70), 5d¹⁰ (80), 6p⁶ (86), 7s² (88), 5f¹ (89). Actinium is one of the exceptions to this filling order: experiments show its ground state is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d¹ rather than 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f¹, because the neighbouring subshells are so close in energy. In shorthand, [Rn] stands for the 86 electrons of radon, giving [Rn] 6d¹ 7s².
How electronegative is Actinium?
Actinium has a Pauling electronegativity of 1.1, ranking 87 of the 100 elements with a known value (fluorine is highest at 3.98). That is low: it tends to give up electrons and form positive ions.
Frequently asked questions
What is the electron configuration of Actinium?
The electron configuration of Actinium (Ac) is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d1 7s2. In noble-gas shorthand it is written [Rn] 6d1 7s2.
How many electrons does Actinium have in each shell?
Actinium has 89 electrons arranged in shells as 2, 8, 18, 32, 18, 9, 2.
What is the atomic mass of Actinium?
Actinium has no stable isotopes. Its most stable isotope has a mass number of 227.
Is Actinium a metal?
Yes. Actinium is an actinide.
Explore nearby elements
More actinides: Thorium, Protactinium, Uranium, Neptunium, Plutonium, Americium. See all elements on the periodic table.