Question

Describe the Aufbau principle.

Answer 1

In the ground state of the atoms, the orbitals are filled in the order of their increasing energies. That is the electrons first occupy the lowest energy orbital available to them. Once the lower energy orbitals are completely filled, then the electrons enter the next higher energy orbitals. 

The order of filling of various orbitals as per Aufbau principle is –

1 s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d …

For e.g., K (Z =19) 

The electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ . After filling 4s orbital only we have to fill up 3d orbital.