Although beryllium (Be) has multiple isotopes, only one of these isotopes is stable. As such, it is considered a monoisotopic element.
Beryllium is unique as being the only monoisotopic element with an even number of protons. It is also the only one with an odd number of neutrons. There are 25 other monoisotopic elements, but all have odd atomic numbers, and even neutron numbers.
Standard atomic mass: 9.012182(3) u
* Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
* Isotope masses from Ame2003 Atomic Mass Evaluation by G. Audi, A.H. Wapstra, C. Thibault, J. Blachot and O. Bersillon in Nuclear Physics A729 (2003).
* Isotopic compositions and standard atomic masses from Atomic weights of the elements. Review 2000 (IUPAC Technical Report). Pure Appl. Chem. Vol. 75, No. 6, pp. 683–800, (2003) and Atomic Weights Revised (2005).
* Half-life, spin, and isomer data selected from these sources. Editing notes on this article's talk page.
o Audi, Bersillon, Blachot, Wapstra. The Nubase2003 evaluation of nuclear and decay properties, Nuc. Phys. A 729, pp. 3–128 (2003).
o National Nuclear Data Center, Brookhaven National Laboratory. Information extracted from the NuDat 2.1 database (retrieved Sept. 2005).
o David R. Lide (ed.), Norman E. Holden in CRC Handbook of Chemistry and Physics, 85th Edition, online version. CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes.