American chemist and physicist whose work on the radioactive decay of uranium and thorium was important in the development of the theory of isotopes.

Boltwood was a member of the Yale faculty from 1897 until 1900, when he established a consulting firm of mining engineers and chemists in partnership with the American chemist Joseph Hyde Pratt. Boltwood grew interested in radioactivity and, in 1904, showed that many of the radioactive elements decay into other radioactive elements. He discovered ionium, now called thorium-230. In 1905 he proposed that lead is the final decay product of uranium and two years later developed a method of determining the age of some rocks by measuring the ratio of lead and uranium. Additional methods of radiometric dating that followed revolutionized estimates of the age of Earth and proved invaluable in archaeology.

In 1910 Boltwood became professor of radiochemistry at Yale University. Overwork led to a nervous breakdown, and intermittent depression culminated in his suicide.