Thorium (Th): electron configuration, atomic mass and facts
Thorium (Th) is element 90 of the periodic table, an actinide in period 7 (f-block). Its electron configuration is [Rn] 6d2 7s2, with electrons arranged in shells 2, 8, 18, 32, 18, 10, 2, and its standard atomic weight is 232.04 u.
▶ Watch Thorium’s electrons orbit in the interactive periodic tableThorium at a glance
| Symbol | Th |
|---|---|
| Atomic number | 90 |
| Atomic mass | 232.04 u |
| 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 6d2 7s2 |
| Noble-gas shorthand | [Rn] 6d2 7s2 |
| Electrons per shell | 2, 8, 18, 32, 18, 10, 2 |
| Electronegativity (Pauling) | 1.3 |
| State at 20 °C | solid |
Where is Thorium on the periodic table?
Thorium 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 Thorium
Put 90 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), 5f² (90). Thorium 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 6d² rather than 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f², because the neighbouring subshells are so close in energy. In shorthand, [Rn] stands for the 86 electrons of radon, giving [Rn] 6d² 7s².
How electronegative is Thorium?
Thorium has a Pauling electronegativity of 1.3, ranking 62 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 Thorium?
The electron configuration of Thorium (Th) is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d2 7s2. In noble-gas shorthand it is written [Rn] 6d2 7s2.
How many electrons does Thorium have in each shell?
Thorium has 90 electrons arranged in shells as 2, 8, 18, 32, 18, 10, 2.
What is the atomic mass of Thorium?
The standard atomic weight of Thorium is 232.04 u, so one mole of Thorium atoms has a mass of about 232.04 g.
Is Thorium a metal?
Yes. Thorium is an actinide.
Explore nearby elements
More actinides: Actinium, Protactinium, Uranium, Neptunium, Plutonium, Americium. See all elements on the periodic table.