For example, the extreme stability of helium-4 due to a double pairing of two protons and two 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 this (see triple-alpha process). This is also the reason why 4 Be decays so quickly into two alpha particles, making beryllium the only even-numbered element that is monoisotopic.