It reacts slowly with water and is attacked only by hydrochloric acid among the common acids. Thorium forms numerous compounds with other elements. Thorium is widely distributed in small amounts in the earth's crust, being about half as abundant as lead and three times as abundant as uranium. The chief commercial source of thorium is monazite sands obtained from India and Brazil. Vast deposits of low-grade thorium ore in New Hampshire are a potential source.
Thorium is used in magnesium alloys and in tungsten filaments for light bulbs and electronic tubes.
Important uses of the element result from its natural radioactivity. There are 26 known radioactive isotopes, only 12 of which have half-lives greater than 1 sec. One intermediate product is the gas radon -220, also called thorium emanation or thoron. Thorium and its decay products are sometimes used in radiotherapy. Fission of the uranium-233 can provide neutrons to start the cycle again.