A chemical element, Be, atomic number 4, with an atomic weight of 9.0121. Beryllium, a rare metal, is one of the lightest structural metals, having a density about one-third that of aluminum. Some of the important physical and chemical properties of beryllium are given in the table. Beryllium has a number of unusual and even unique properties.

The largest volume uses of beryllium metal are in the manufacture of beryllium ­copper alloys and in the development of beryllium-containing moderator and reflector materials for nuclear reactors. Addition of 2% beryllium to copper forms a nonmag­netic alloy which is six times stronger than copper. These beryllium-copper alloys find numerous applications in industry as nonsparking tools, as critical moving parts in aircraft engines, and in the key components of precision instruments, mechanical com­puters, electrical relays, and camera shutters. Beryllium-copper hammers, wrenches, and other tools are employed in petroleum refineries and other plants in which a spark from steel against steel might lead to an explosion or fire.

Beryllium has found many special uses in nuclear energy because it is one of the most efficient materials for slowing down the speed of neutrons and acting as a neutron reflector. Consequently, much beryllium is used in the construction of nuclear reactors as a moderator and as a support or alloy with the fuel elements.
The following list shows some of the principal compounds of beryllium. Many of the compounds listed are useful as intermediates in the processes for the preparation of ceramics, beryllium oxide, and beryllium metal. Other compounds are useful in analysis and organic synthesis.
Beryllium is surprisingly rare for a light element, constituting about 0.005% of the Earth's crust. It is about thirty-second in order of abundance, occurring in concentrations approximating those of cesium, scandium, and arsenic. Actually, the abundances of beryllium and its neighbors, lithium and boron, are about 10-5 times those of the next heavier elements, carbon, nitrogen, and oxygen. At least 50 different beryllium-bearing minerals are known, but in only about 30 is beryllium a regular constituent. The only beryllium-bearing mineral of industrial importance is beryl. Bertrandite substitutes for beryl as a domestic source in the United States.