Why an atom can have nonzero toal angular momentum in the ground state?

[wdlang]

In its ground state, an atom has no net electric dipole momentum ,but it can have a nonzero angular momentum.

Is this a spontaneous symmetry breaking?

why the ground state is not of zero angular momentum?

 

[mfb]

A classical, charged, rotating ring has no dipole moment, but nonzero angular momentum.
Why do you expect something special to get the same in quantum mechanics? There are orbitals with angular momentum, and if there are is a single electron in one of those orbitals you get a nonzero total angular momentum.

 

[wdlang]

A classical, charged, rotating ring has no dipole moment, but nonzero angular momentum.
Why do you expect that this is something special to get the same in quantum mechanics? There are orbitals with angular momentum, and if there are is a single electron in one of those orbitals you get a nonzero total angular momentum.

intuitively, a non-rotating state is lower in energy than a rotating one

 

[mfb]

Well, this is not true in quantum mechanics, at least not in general. And if the low-energy states with zero angular momentum are filled, electron have to use some with angular momentum.