Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Whole number rule

The Whole Number Rule states that the masses of the elements are whole number multiples of the mass of the hydrogen atom. The rule can be formulated from Prout's hypothesis put forth in 1815. In 1920, Francis W. Aston demonstrated through the use of a mass spectrometer that apparent deviations from the rule are predominantly due to the existence of isotopes; they are secondarily due to binding energy, as mass defect. The modern form of the whole number rule is that the atomic mass of a given isotope is approximately the mass number (number of protons plus neutrons) times an atomic mass unit (approximate mass of a proton, neutron, or hydrogen-1 atom).