Aufbau principle Rule stating that

When following the Aufbau principle, the orbitals begin filling at the lowest energy and continue to fill until we account for all the electrons in an atom. Filling begins with the n = 1 level followed by the n = 2 level, and then the n = 3 level. However, there are exceptions in this sequence. In addition, Hund s rule states that the sublevels within a particular orbital will half fill before the electrons pair up in a sublevel. 

All three methods require knowledge of the subshells occupied by electrons in a certain atom. The Aufbau principle or building-up rule states that electrons at ground state fill orbitals starting at the lowest available energy levels. 

Orbital diagrams assign electrons to individual orbitals so the energy state of individual electrons may be found. This requires knowledge of how electrons occupy orbitals within a subshell. Hund s rule states that before any two electrons occupy the same orbital, other orbitals in that subshell must first contain one electron each with parallel spins. Electrons with up and down spins are shown by half-arrows, and these are placed in lines of orbitals (represented as boxes or dashes) according to Hund s rule, the Aufbau principle, and the Pauli exclusion principle. Below is the orbital diagram for vanadium ... 

The Pauli exclusion principle is one rule to help you write an electron configuration for an atom. Another rule is the aufbau principle. Aufbau is the German word for building up. The aufbau principle states that electrons fill orbitals that have the lowest energy first. 

The number of electrons that rotate on a certain level around the nucleus is determined by precise quantum laws. The filling of orbitals with electrons occurs according to the Aufbau principle (in German the construction principle ) so that the electronic configuration leads to the lowest energy state. This principle uses the rule of Hund and the rule of Pauli. 

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