<P> The tabulated atomic masses of elements are averages that account for the presence of multiple isotopes with different masses . Before the discovery of isotopes, empirically determined noninteger values of atomic mass confounded scientists . For example, a sample of chlorine contains 75.8% chlorine - 35 and 24.2% chlorine - 37, giving an average atomic mass of 35.5 atomic mass units . </P> <P> According to generally accepted cosmology theory, only isotopes of hydrogen and helium, traces of some isotopes of lithium and beryllium, and perhaps some boron, were created at the Big Bang, while all other nuclides were synthesized later, in stars and supernovae, and in interactions between energetic particles such as cosmic rays, and previously produced nuclides . (See nucleosynthesis for details of the various processes thought responsible for isotope production .) The respective abundances of isotopes on Earth result from the quantities formed by these processes, their spread through the galaxy, and the rates of decay for isotopes that are unstable . After the initial coalescence of the Solar System, isotopes were redistributed according to mass, and the isotopic composition of elements varies slightly from planet to planet . This sometimes makes it possible to trace the origin of meteorites . </P> <P> The atomic mass (m) of an isotope (nuclide) is determined mainly by its mass number (i.e. number of nucleons in its nucleus). Small corrections are due to the binding energy of the nucleus (see mass defect), the slight difference in mass between proton and neutron, and the mass of the electrons associated with the atom, the latter because the electron: nucleon ratio differs among isotopes . </P> <P> The mass number is a dimensionless quantity . The atomic mass, on the other hand, is measured using the atomic mass unit based on the mass of the carbon - 12 atom . It is denoted with symbols "u" (for unified atomic mass unit) or "Da" (for dalton). </P>

What is the first isotope that was formed