# When is the n+l rule followed when working with energy of different orbitals?

While going through my book, I came across a problem which gave some options about the order of energy of 3s, 3p and 3d orbitals of hydrogen atom. The correct answer was given as:

3s, 3p and 3d orbitals all have the same energy

I also read somewhere that the order of energy of different orbitals increases as the value of $$n+l$$ increases where $$n$$ is the principal quantum number and $$l$$ is the azimuthal quantum number. So my guess was $$3s<3p<3d$$ .

What is the reason for this?

• The former is true for hydrogen atom, the latter for other atoms. Apr 8 at 8:51