The Aufbau Principle (from the German word \"aufbauen,\" meaning \"to build up\") is one of the three guiding rules — along with the Pauli Exclusion Principle and Hund's Rule — used to determine the electronic configuration of atoms in their ground state.

According to this principle, electrons are added one at a time to the atomic orbitals of the lowest available energy first, before occupying higher-energy orbitals. This builds up the electron configuration of an atom systematically, orbital by orbital, as the atomic number increases.

The order in which orbitals are filled is:

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s ...

This filling order is predicted by the (n + l) rule (also called the Madelung rule):

• Orbitals are filled in order of increasing (n + l) value, where n is the principal quantum number and l is the azimuthal (subshell) quantum number.
• If two orbitals have the same (n + l) value, the orbital with the lower value of n is filled first. This is why 4s (n + l = 4 + 0 = 4) is filled before 3d (n + l = 3 + 2 = 5), even though 3d has a lower principal quantum number.

This principle correctly predicts why, for example, potassium's 19th electron goes into the 4s orbital rather than the 3d orbital, and it underlies the structure of the periodic table itself (periods correspond to principal energy levels, and blocks correspond to the subshell being filled).

The alternative descriptions given are all incorrect: electrons do not fill the highest-energy orbitals first (that would violate the principle of minimum energy and produce unstable, high-energy atoms); filling is not random, since it strictly follows the (n + l) and Pauli/Hund rules; and orbital filling is very much dependent on relative energy levels — that dependence is precisely what the Aufbau principle describes.