Could someon please exsplain electron configurations at ground level? What is the diagonal rule? I don't understand?

Hope this helps! :)

Explanation:

The ground state electron configuration is the arrangement of electrons around the nucleus of an atom with lower energy levels. Electrons occupying the orbitals of varying energy levels naturally fall towards the lowest energy state or ground state. For example, the ground state electron configuration of oxygen is 1s2 2s2 2p4

(But to simplify it, A short hand notation to indicate the electron orbitals which are filled in a particular atom.)

The diagonal rule basically says that electrons fill orbitals in order of increasing "quantum number sum" (n + ℓ). When two orbitals share the same "quantum number sum", they will be filled in order of increasing n.