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.
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.
Theory predicts that many apparently "stable" isotopes/nuclides are radioactive, with extremely long half-lives (discounting the possibility of proton decay, which would make all nuclides ultimately unstable).
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.
helium-3, helium-4, carbon-12, carbon-14, uranium-235 and uranium-239). "C" for carbon, standard notation (now known as "AZE notation" because A is the mass number, Z the atomic number, and E for element) is to indicate the mass number (number of nucleons) with a superscript at the upper left of the chemical symbol and to indicate the atomic number with a subscript at the lower left (e.g. 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 as uranium two-thirty-five (American English) or uranium-two-three-five (British) instead of 235-92-uranium.
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.
Isotopes are different kinds of atoms of the same element which have the same numbers of protons (atomic number), but the different numbers of neutrons and different mass number (total number of protons and neutrons).
An isotope and/or nuclide is specified by the name of the particular element (this indicates the atomic number) followed by a hyphen and the mass number (e.g.
Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons.
The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number, and each isotope of a given element has a different mass number.
A nuclide is a species of an atom with a specific number of protons and neutrons in the nucleus, for example carbon-13 with 6 protons and 7 neutrons.