Thorium (Th)
Element 90 · Actinide · atomic mass 232.04 u · f-block, period 7
Electron shells: 2, 8, 18, 32, 18, 10, 2
Thorium is three to four times more abundant than uranium and is being investigated as a reactor fuel that produces less long-lived waste. It once made gas lantern mantles glow brilliantly.
Thorium is common enough that most granite countertops contain a measurable amount.
- Atomic number
- 90
- Atomic mass
- 232.04 u
- Electron configuration
- [Rn] 7s² 6d²
- Group · period · block
- f-block · 7 · f
- Oxidation states
- +4
- Electronegativity
- 1.3 (Pauling)
- Melting point
- 1,750 °C
- Boiling point
- 4,788 °C
- Density
- 11.72 g/cm³
- State at 20 °C
- Solid
- Appearance
- Silvery-white metal
- Discovered
- Jöns Jacob Berzelius, 1828
90 protons, and 90 electrons when neutral
IUPAC standard atomic weight
Shown in the strip below the table
What Thorium is used for
- Experimental thorium reactor fuel
- High-refractive-index optical glass
- Historically, gas mantles and welding rods
- Radiometric dating
Atomic structure of Thorium
A neutral thorium atom has 90 protons and 90 electrons. Its most common isotope carries about 142 neutrons, which is where the mass number 232 comes from. Change the neutron count and you have a different isotope of thorium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 32, 18, 10, 2. In orbital notation that is [Rn] 7s² 6d². As a f-block element its valence count is variable, which is why it can form compounds in several oxidation states.
3D electron orbitals
See where Thorium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Thorium sits in the periodic table
The second f-block row, elements 89-103. All are radioactive, and everything past uranium is essentially man-made.
- One proton lighter: Actinium.
- One proton heavier: Protactinium.
Discovery and naming
Thorium was discovered by Jöns Jacob Berzelius in 1828. Its name comes from Thor, the Norse god of thunder.
Every isotope of thorium is radioactive, so it has no standard atomic weight. Periodic tables show the mass number of its longest-lived isotope in square brackets instead.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Thorium?
The standard atomic weight of Thorium is 232.04 u, as published by IUPAC's Commission on Isotopic Abundances and Atomic Weights.
How many protons, neutrons and electrons does Thorium have?
Thorium has 90 protons, because the atomic number is the proton count and that is what makes an atom Thorium at all. A neutral Thorium atom has the same number of electrons, 90. The neutron count varies by isotope; the most common one has about 142, since 232 − 90 = 142.
What is the electron configuration of Thorium?
The electron configuration of Thorium is [Rn] 7s² 6d². Its electrons are arranged 2, 8, 18, 32, 18, 10, 2 across 7 shells.
Is Thorium a metal?
Yes. Thorium is classified as a actinide, on the metallic side of the table.
What group and period is Thorium in?
Thorium sits in the f-block strip below the main table, in period 7, among the actinides. Elements in that strip are not given a group number in the standard 18-column layout.
What is the charge of Thorium?
Thorium most commonly forms a 4+ ion. Like most transition metals it has several accessible oxidation states, so the charge depends on the compound.
What is the melting point of Thorium?
Thorium melts at 1,750 °C and boils at 4,788 °C. That puts it in the solids at everyday temperatures.
What is Thorium used for?
Experimental thorium reactor fuel, High-refractive-index optical glass, Historically, gas mantles and welding rods, Radiometric dating. Thorium is three to four times more abundant than uranium and is being investigated as a reactor fuel that produces less long-lived waste.
Who discovered Thorium, and where does its name come from?
Thorium (Th) was discovered by Jöns Jacob Berzelius in 1828. The name comes from Thor, the Norse god of thunder.
Is Thorium radioactive?
Yes. Every isotope of Thorium is radioactive — it has no stable form at all, which is why the periodic table shows its mass in square brackets rather than as a standard atomic weight.