Beryllium

Beryllium 9.012182(3)   Be 4 1s22s2 [ ? ] Alkaline Earth Metal: Properties: Steel-gray, low-density, brittle metal Hazard: Toxic, carcinogenic

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 considered to be a member of the "Alkaline Earth Metal" class of element. At a pressure of 101.325 kPa, it has a boiling point of 2471 °C, and a melting point of 1287 °C. Its density is 1.85 g/cc.

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),