An Aufbau diagram that illustrates the (n+l) rule. According to rule (b) above, when two orbitals have the same E, such as E=3 for 2p and 3s, the orbital with lower n (2p) is filled first.įigure 2. The orbitals are filled according to the values of E for each orbital: E=1 for 1s, E=2 for 2s, E=3 for 2p and 3s, and so on. The diagram in Figure 1 is the result of these rules.įigure 2 is a version of the diagram that displays the dependence on (n + l) for each orbital, where E represents the relative energy of the orbitals. If two orbitals have the same value of (n + l), they are filled in order of increasing n. Orbitals are filled in order of increasing (n + l), which represents the relative energy.ī. The (relative) energies of the orbitals can be predicted by the sum of n + l for each orbital, according to the following rules:Ī. l is the angular momentum quantum number and is related to the shape of the orbital. n is the principal quantum number and is related to the size of the orbital. The “n” and “l” in the (n + l) rule are the quantum numbers used to specify the state of a given electron orbital in an atom.

How was this diagram constructed in the first place? It turns out that it is a representation of a method of predicting the “order of filling” called the Madelung rule, which is also called the (n + l) rule. Is there a way to connect this diagram to its physical meaning? Yes! That is the goal of this article. The Aufbau Diagram: Atomic orbitals are filled starting at 1s and continuing, from the upper left, in the order indicated by the arrows.

Electron configuration aufbau principle