Isotopes of beryllium Isotopes of beryllium Beryllium (Be) has 12 known isotopes, but only one of these isotopes () is stable and a primordial nuclide. As such, beryllium is considered a monoisotopic element. It is also a mononuclidic element, because its other isotopes have such short half-lives that none are primordial and their abundance is very low (standard atomic weight is 9.0122). Beryllium is unique as being the only monoisotopic element with both an even number of protons and an odd number of neutrons. There are 25 other monoisotopic elements but all have odd atomic numbers, and even numbers of neutrons. Of the
Mirror nuclei Mirror nuclei Mirror nuclei are nuclei where the number of protons of element one (Z) equals the number of neutrons of element two (N) and the number of protons of element two (Z) equals the number of neutrons in element one (N), such that the mass number is the same (A = N + Z = N + Z). As that Z = N and Z = N, A = N + N = Z + Z. By making the substitution Z = Z and Z = Z − 1, the mass number can be rewritten in the form 2Z