Be9 is the only stable isotope of beryllium, but Be10, which is formed in the atmosphere when oxygen and nitrogen are bombarded by cosmic rays, has a relatively long half-life of 15.1 million years. All remaining isotopes, such as Be13, have very short half-lives.
Beryllium has an atomic number of 4 and is the first element in the alkaline earth metals group of elements. It is lightweight and has the highest melting point (1560 K) of the elements in the group, which makes it useful in components that must function in extreme temperatures.
Because the production of the Be10 isotope is dependent on energy from cosmic rays, the presence of the isotope is an indicator of solar activity. When the amount of Be10 is measured in the layers of ice core samples, it provides an historical record of solar activity.