Atomic number and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (making a quantity called the "relative isotopic mass"), is within 1% of the whole number "A". Atoms with the same atomic number "Z" but different neutron numbers "N", and hence different atomic masses, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass)
Even and odd atomic nuclei billion times the age of the universe. For example, the double beta emitter has a half-life of years. This makes for a larger number of stable even–even nuclides, up to three for some mass numbers, and up to seven for some atomic (proton) numbers. For example, the extreme stability of helium-4 due to a double pairing of 2 protons and 2 neutrons prevents "any" nuclides containing five or eight nucleons from existing for long enough to serve as platforms for the buildup of heavier elements via nuclear fusion in Big Bang nucleosynthesis; only in stars is there enough time for