Answer:
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s
Explanation:
You can predict the order of orbital energies by constructing a diagram as shown below.
Follow the arrows to get the orbitals in order of increasing energy.
The order is
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s