Francis William Aston (September 1, 1877 - November 20, 1945) was a English chemist and physicist who won the 1922 Nobel Prize in Chemistry for his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements. His work on isotopes also led to his formulation of the whole number rule which states that "the mass of the oxygen isotope being defined (16), all the other isotopes have masses that are very nearly whole numbers," a rule that was used extensively in the development of nuclear energy. The exact mass of many isotopes was measured leading to the result that hydrogen has a 1% higher mass than expected by the average mass of the other elements. Aston speculated about the subatomic energy and the use of it in 1936. 'Isotopes' and 'Mass-spectra and Isotopes' are his most well-known books. He died in 1945 at the age of 68.