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The atomic number of Be is only 4, and the only naturally occurring isotope has mass number 9, so the nucleus contains 4 protons and 5 neutrons. The atomic weight is 9.012. The isotope with mass number 8, which might be expected to be quite stable with paired-off protons and neutrons, actually splits with a half-life of less than 4 x 10-16 seconds into two alpha particles, which are even more stable. The decay energy is only 90 keV, however. Be8 is the only light nuclide to undergo alpha decay, but it is a very unusual sort of alpha-decay, that is also fission at the same time. Be7 captures an orbital electron (K-capture) to become Li7, half-life 53 days. Be10 is nearly stable, since its half-life is 2 x 106 years against beta-decay to stable B10. These are the only four beryllium nuclides.
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