The letter m is sometimes appended after the mass number to indicate a nuclear isomer , a metastable or energetically-excited nuclear state as opposed to the lowest-energy ground state , for example m 73Ta The common pronunciation of the AZE notation is different from how it is written: For example, 14 C is a radioactive form of carbon, whereas 12 C and 13 C are stable isotopes. There are about naturally occurring nuclides on Earth,  of which are primordial nuclides , meaning that they have existed since the Solar System 's formation.
Primordial nuclides include 32 nuclides with very long half-lives over million years and that are formally considered as " stable nuclides ",  because they have not been observed to decay. In most cases, for obvious reasons, if an element has stable isotopes, those isotopes predominate in the elemental abundance found on Earth and in the Solar System.
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.
Of the 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. Many other stable nuclides are in theory energetically susceptible to other known forms of decay, such as alpha decay or double beta decay, but no decay products have yet been observed, and so these isotopes are said to be "observationally stable".
The predicted half-lives for these nuclides often greatly exceed the estimated age of the universe, and in fact there are also 27 known radionuclides see primordial nuclide with half-lives longer than the age of the universe. Adding in the radioactive nuclides that have been created artificially, there are 3, currently known nuclides.
See list of nuclides for details. Radioactive isotopes[ edit ] The existence of isotopes was first suggested in by the radiochemist Frederick Soddy , based on studies of radioactive decay chains that indicated about 40 different species referred to as radioelements i. Soddy proposed that several types of atoms differing in radioactive properties could occupy the same place in the table.
Thomson 's photographic plate are the separate impact marks for the two isotopes of neon: Richards found variations between the atomic weight of lead from different mineral sources, attributable to variations in isotopic composition due to different radioactive origins. Thomson in as part of his exploration into the composition of canal rays positive ions.
Each stream created a glowing patch on the plate at the point it struck. Thomson observed two separate patches of light on the photographic plate see image , which suggested two different parabolas of deflection. Thomson eventually concluded that some of the atoms in the neon gas were of higher mass than the rest. Aston subsequently discovered multiple stable isotopes for numerous elements using a mass spectrograph. In 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 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.
Aston similarly showed[ when? Thus different isotopes of a given element all have the same number of electrons and share a similar electronic structure. Because the chemical behavior of an atom is largely determined by its electronic structure, different isotopes exhibit nearly identical chemical behavior. The main exception to this is the kinetic isotope effect: This is most pronounced by far for protium 1 H.