Question

The orbital diagram in which the Aufbau principle is violated is:

• A.
• B.
• C.
• D.

Answer 1

The correct option is B

According to the Aufbau principle, the available atomic orbitals with the lowest energy levels are occupied before those with higher energy levels.
Energy of orbital can be compare by $n+l$ rule so $2s$ has lower energy than $2p$ so first electrons will first fully fill the $2s$ orbital then the $2p$ orbitals.