Isotope, one of two or more species of atoms of a chemical element with the same atomic number and position in the periodic table and nearly identical chemical behaviour but with different atomic masses and physical properties. An atom is first identified and labeled according to the number of protons in its nucleus. The great importance of the atomic number derives from the observation that all atoms with the same atomic number have nearly, if not precisely, identical chemical properties. In particular, ores of the radioactive elements uranium and thorium had been found to contain small quantities of several radioactive substances never before observed.
A large collection of atoms with the same atomic number constitutes a sample of an element. These substances were thought to be elements and accordingly received special names.
By 1919 he had done so and convincingly argued for the existence of neon-20 and neon-22.
Information from his and other laboratories accumulated rapidly in the ensuing years, and by 1935 the principal isotopes and their relative proportions were known for all but a handful of elements.
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