Definition of Aufbau Principle
Aufbau principle states that the electrons are filled in the orbitals of the atom when it is in its ground state. The electrons are filled in the atomic orbitals according to the increasing order of energy level of the orbitals. In simple words, the atomic orbitals which have the lowest energy are occupied by the electrons first and those having higher energies are filled afterward.
The energy level of the atomic orbitals can be determined by the help of the (n+l) rule, where n is the principal and l is the azimuthal quantum number. The orbitals are filled with electrons in the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, and so on.