What is aufbau's principal

Aufbau principle

• The Aufbau Principle is basically a German word “Aufbauen” which means building up.
• According to this principle, the electrons will first occupy the orbitals that have the lowest energy. This means that the electrons enter first orbital and then enter the orbitals which have higher energy but only when the lower- energy orbitals are completely filled.
• The order in which the electrons should be filled is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p…..
• The order of the increasing energy of the orbitals can also be calculated using (n+l) rule.
• The sum of the values of the principal quantum numbers (n) and azimuthal quantum number (l) is used to decide the energy level of an orbital.
• The lower value of the sum of (n+l) implies that the energy of the orbital is low. If the value of n+l for two orbitals is equal then the orbital with a lower value of n will have a lower energy level.