Carbon dating chart
Some isotopes/nuclides are radioactive, and are therefore referred to as radioisotopes or radionuclides, whereas others have never been observed to decay radioactively and are referred to as stable isotopes or stable nuclides.For example, of which 286 are primordial nuclides, meaning that they have existed since the Solar System's formation. From left to right, the isotopes are protium ( Isotopes are variants of a particular chemical element which differ in neutron number. The fact that each isotope has one proton makes them all variants of hydrogen: the identity of the isotope is given by the number of neutrons.Of the 253 nuclides never observed to decay, only 90 of these (all from the first 40 elements) are theoretically stable to all known forms of decay.Element 41 (niobium) is theoretically unstable via spontaneous fission, but this has never been detected.However, in the cases of three elements (tellurium, indium, and rhenium) the most abundant isotope found in nature is actually one (or two) extremely long-lived radioisotope(s) of the element, despite these elements having one or more stable isotopes.
However, because isotope is the older term, it is better known than nuclide, and is still sometimes used in contexts where nuclide might be more appropriate, such as nuclear technology and nuclear medicine.Soddy recognized that emission of an alpha particle followed by two beta particles led to the formation of an element chemically identical to the initial element but with a mass four units lighter and with different radioactive properties.Soddy proposed that several types of atoms (differing in radioactive properties) could occupy the same place in the table.Thomson eventually concluded that some of the atoms in the neon gas were of higher mass than the rest. In 1919 Aston studied neon with sufficient resolution to show that the two isotopic masses are very close to the integers 20 and 22, and that neither is equal to the known molar mass (20.2) of neon gas.This is an example of Aston's whole number rule for isotopic masses, which states that large deviations of elemental molar masses from integers are primarily due to the fact that the element is a mixture of isotopes.