What is the Aufbau principle?

The Aufbau principle is a rule that tells us the order in which electrons fill atomic orbitals in an atom.

Here’s the explanation:

1. Meaning of Aufbau:
   • “Aufbau” is German for “building up”.
   • Electrons build up an atom by filling orbitals from lowest energy to highest energy.
2. Order of Filling:
   • Electrons fill orbitals starting from the lowest energy level:
     1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → …
   • This order can be remembered using the diagonal rule.
3. Rules to Follow:
   • Pauli Exclusion Principle → max 2 electrons per orbital.
   • Hund’s Rule → electrons occupy empty orbitals of the same sublevel first before pairing.

In short:
The Aufbau principle says electrons fill the lowest-energy orbitals first, building up the atom step by step.