Lithium is a metallic element which has an atomic number of 3 and an atomic weight of 6.941. It is classed as an "alkali metal" and in its pure form is silvery white. Its ores were first discovered in 1817 by Johan August Arvedson in Sweden and later isolated by W.T Brande and Humphrey Davy. It is somewhat toxic, and some lithium compounds can cause cancer or birth defects, but lithium carbonate (Li2CO3) is useful as a drug to treat depression and manic-depressive disorders. Lithium is found naturally in the body at about 1.3 parts per million in bone and 0.023 parts per million in muscle tissue.