Beryllium is a chemical element, having the chemical symbol Be. Its atomic number (the number of protons) is 4. It has a standard atomic weight of 9.012182 g•mol −1, and is typically found as a solid in its elemental form.
Beryllium is toxic to living organisms.
It is a strategic material in nuclear weapons, specifically acting as a neutron reflector surrounding the fission core, increasing its neutron density, and decreasing the size of the device. A beryllium reflector was first used operationally in the U.S. Mark 12 (nuclear weapon),