|MadSci Network: Physics|
They must occur in balance, boron has 5 protons. Five neutrons (Boron-10) is stable, six neutrons is also stable because neutrons have no electric charge and therefore no electrical repulsion. But 7 neutrons and only 5 protons? Too far out of balance for nuclear energy levels, therefore it'll decay to a more stable energy state...namely 12C (6 protons, 6 neutrons... balanced).
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