Gilbert Newton Lewis (October 23, 1875 - March 23, 1946) was an American physical chemist known for the discovery of the covalent bond, his purification of heavy water, his reformulation of chemical thermodynamics in a mathematically rigorous manner accessible to ordinary chemists, his theory of Lewis acids and bases, and his photochemical experiments. In 1916, he also proposed his theory of bonding and added information about electrons in the periodic table of the elements. In 1926, Lewis coined the term "photon" for the smallest unit of radiant energy. In 1933, he started his research on isotope separation. Though he was nominated 35 times, he never won the Nobel Prize in Chemistry. On March 23, 1946, Lewis was found dead in his Berkeley laboratory where he had been working with hydrogen cyanide; many postulated that the cause of his death was suicide. He was 70 years old.