Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder.

It is estimated to be about three times more abundant than uranium in the Earth's crust and is a by-product of the extraction of rare earths from monazite sands.

Thorium was formerly used commonly as (for example) the light source in gas mantles and as an alloying material, but these applications have declined due to concerns about its radioactivity.

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