According to the Bohr model, the quantum number n determines the energies of the allowed states
in a hydrogen atom. We now know that both n and l are factors that determine the energy of a set of
orbitals. In a general way, it is the sum ( n l ) that determines the energy, and the energy increases as
the sum of n and l . However, when there are two or more ways to get a particular sum of ( n l ) the
combination with lower n is normally used fi rst. For example, the 2 p state has ( n l ) (2 1) 3 as
does the 3 s state where ( n l ) (3 0) 3. In this case, the 2 p level fi lls fi rst because of the greater
importance of the lower n value. Table 2.4 shows the scheme used to arrange the orbitals in the order
in which they are normally fi lled. As a result, the general order of fi lling shells is as follows, with the
sum ( n l ) being given directly below for each type of shell.