Thorium

Actinides

Thorium (Th) is in the f-block of the periodic table — group 3, period 7.

Thorium is a chemical element with symbol Th and atomic number 90.

Key data

Atomic number: 90
Atomic mass: 232.038 u
Group / Period: 3 / 7
Block: f-block
Phase at STP: Solid
Density: 11.724 g/cm³
Melting point: 1749.8 °C
Boiling point: 4787.9 °C
Electron configuration: [Rn] 6d2 7s2

Periodic table block

Thorium is in the f-block (group 3, period 7). Elements land in the f-block when their highest-energy electrons fill an f subshell — these are the lanthanides and actinides shown beneath the main table.

s-blockp-blockd-blockf-block

New to this? Periodic table blocks explained: s, p, d, and f

Common uses

•Camp lantern mantles
•Nuclear fuel research
•High-quality lenses

Discovery

Year: 1828

Discovered by: Jöns Jakob Berzelius

Frequently asked questions

What is the atomic number of Thorium?

Thorium has atomic number 90. Its symbol is Th and its atomic mass is 232.038 u.

What category of element is Thorium?

Thorium is classified as actinides. It sits in group 3, period 7 of the periodic table.

What block of the periodic table is Thorium in?

Thorium is in the f-block (group 3, period 7). Its highest-energy electrons occupy the f subshell — you can see this in its electron configuration, [Rn] 6d2 7s2.

What is Thorium used for?

Common uses of Thorium include: Camp lantern mantles, Nuclear fuel research, High-quality lenses.

When was Thorium discovered?

Thorium was discovered in 1828 by Jöns Jakob Berzelius.

Play the game

Test your knowledge in The Lab

20-round element ID, cryptic 3-clue guesser, and an interactive periodic table.

Open The Lab →