Beryllium is the fourth element in the periodic table, giving it many special properties. But it’s surprisingly rare, because stars almost always make beryllium-8, an extremely unstable form. And it hasn’t been studied as much as other light elements—because beryllium dust is so poisonous! It can cause fatal scarring of the lungs.
This is a companion discussion topic for the original entry at https://johncarlosbaez.wordpress.com/2023/06/16/diberyllocene/