There are similar irregularities in the filling of 5s and 4d as well as some other pairs of subshells. For example, the 4s and 3d have nearly the same energy level and so, in a way that varies according to which element it is, either of the two subshells could be the first to fill and there is no certainty that one will be completely filled before the other actually starts filling. The actual filling order is more complicated. This pattern will give you the correct configuration for all but about 19 elements. There are no known elements that, in their ground state, have electrons in a subshell beyond 7p.

What you do is you start assigning electrons to the subshells using the following pattern (you completely fill up one subshell before moving onto the next higher) and you keep going until the total number of electrons assigned is equal to the atomic number:ġs, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p However, in the simplified version of these rules that is understandable to a general chemistry student, there are about 19 elements that are exceptions. Given the atomic number, there is a set of rules that allow you to determine the electron configuration. Without that, you cannot determine the electron configuration. You must know the atomic number of the element.

Electron configuration aufbau principle