What is energy level diagram

Diagram representing the arrangement of orbitals in order of their increasing energies are called energy level diagrams.

Important observations from energy level diagrams of multi electron atoms are:

1)The sub shell of a particular shell do not have equal energies.For Ex: 2s and 2p have different energies.

2)In a particular shell, sub shell with lower value of l has lower energy. In the second shell, 2s ( l=0) has lower energy than 2p ( l=1) .In the 3rd shell, energies are in the order: 3s < 3p < 3d.

In the 4th shells, they are in the order of 4s <4p< 4d< 4f.

3)For the same value of n, the differences between energies of s and p sub shell is small whereas between p and d sub shell, it is  large and so on.

4)As the value of n increases ,the sub shell of lower shell may have higher energy than that of a higher shell, 3d has higher energy than 4s.