Here's why:

* Beryllium (Be) has an atomic number of 4, meaning it has 4 protons and therefore 4 electrons.

* Electron filling order: Electrons fill orbitals in order of increasing energy. The lowest energy level is the 1s orbital, which can hold up to 2 electrons. The next lowest is the 2s orbital, also holding 2 electrons.

* Therefore:

* The first two electrons go into the 1s orbital (1s²).

* The remaining two electrons go into the 2s orbital (2s²).

This gives us the final configuration of 1s² 2s².