Electrons are organized according to their energies into sets called shells. Shells are further divided into subsets of electrons called subshells. The first shell has only one subshell, the second shell has two subshells, the third shell has three subshells, and so on. Beryllium, has 4 electrons, so its electron configuration is 1s2 2s2 . As its electron configuration shows, Be has 2 valence electrons located in the 2s subshell.
