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.
The unambiguous confirmation of isotopes in stable elements not associated directly with either uranium or thorium followed a few years later with the development of the Francis William Aston. Thomson, Aston had learned that the gaseous element neon produced two positive rays.
A uniform scale of nuclear stability, one that applies to stable and unstable isotopes alike, is based on a comparison of measured isotope masses with the masses of their constituent electrons, protons, and neutrons.
For this purpose, electrons and protons are paired together as hydrogen atoms.
The actual masses of all the stable isotopes differ appreciably from the sums of their individual particle masses.
For example, the isotope C, which has a particularly stable nucleus, has an atomic mass defined to be exactly 12 amu. (Authors who do not wish to use symbols sometimes write out the element name and mass number—hydrogen-1 and uranium-235 in the examples above.)The term is used to describe particular isotopes, notably in cases where the nuclear rather than the chemical properties of an atom are to be emphasized.