Answer
1s $\lt$ 2s $\lt$ 2p $\lt$ 3s $\lt$ 3p $\lt$ 4s $\lt$ 3d $\lt$ 4p $\lt$ 5s
Work Step by Step
1s $\lt$ 2s $\lt$ 2p $\lt$ 3s $\lt$ 3p $\lt$ 4s $\lt$ 3d $\lt$ 4p $\lt$ 5s
This is the order of quantum mechanical orbits through 5s.
The 4s orbital gets filled earlier as compared to the 3d orbital because the 4s orbital has lower energy.
The Aufbau Rule states that the orbitals get filled in order of their increasing energies.
