# Aufbau Principle vs Pauli Exclusion Principle: Key Differences

### Pauli Exclusion Principle

An Austrian physicist named Wolfgang Pauli formulated the principle in the year 1925. With this principle, he basically described the behaviour of the electrons. Later in the year 1940, he expanded on the principle to cover all fermions under his spin-statistics theorem. Besides, fermions that are described by the principle include elementary particles such as quarks, electrons, neutrinos, and baryons.

The Pauli exclusion principle states that in a single atom, no two electrons will have an identical set or the same quantum numbers (n, l, ml, and ms). To put it in simple terms, every electron should have or be in its own unique state (singlet state). There are two salient rules that the Pauli exclusion principle follows:

• Only two electrons can occupy the same orbital.
• The two electrons that are present in the same orbital must have opposite spins, or they should be antiparallel.

However, the Pauli exclusion principle does not only apply to electrons, it applies to other particles of half-integer spin, such as fermions. It is not relevant for particles with an integer spin, such as bosons which have symmetric wave functions. Moreover, bosons can share or have the same quantum states, unlike fermions. As far as the nomenclature goes, fermions are named after the Fermi–Dirac statistical distribution that they follow. Bosons, on the other hand, get their name from the Bose-Einstein distribution function.

### Aufbau Principle

The word ‘Aufbau’ has German roots and can be roughly translated as ‘construct’ or ‘build up’. In other words, the Aufbau Principle, also known as the “building-up” principle, is a concept in chemistry, particularly in atomic structure and electronic configuration that describes the order in which electrons fill atomic orbitals within an atom. This principle provides a systematic way to determine the electron configuration of an atom, which in turn influences its chemical properties and behavior.

• According to the Aufbau principle, electrons first occupy those orbitals whose energy is the lowest. This implies that the electrons enter the orbitals having higher energies only when orbitals with lower energies have been completely filled.
• The order in which the energy of orbitals increases can be determined with the help of the (n+l) rule, where the sum of the principal and azimuthal quantum numbers determines the energy level of the orbital.
• Lower (n+l) values correspond to lower orbital energies. If two orbitals share equal (n+l) values, the orbital with the lower n value is said to have lower energy associated with it.
• The order in which the orbitals are filled with electrons is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, and so on.