# Why do atoms have magnetic moments, but no electric dipole moments?

1. Jan 9, 2014

### wdlang

i do not know why.

does the magnetic moment of an atom come only from the spins of the electrons?

2. Jan 9, 2014

### Staff: Mentor

Or their orbitals.
In both cases, it comes from moving electric charges.
I guess moving magnetic charges could produce an electric dipole moment, but there are no magnetic charges (or if they exist, we didn't find them yet, so they are certainly not in regular atoms).

3. Jan 10, 2014

### DrDu

In hydrogen, the 2s and 2p orbitals are degenerate, so there exist eigenstates of the hamiltonian which carry a permanent electric dipole moment.

4. Jan 10, 2014

### Meir Achuz

Most atoms do not have permanent electric dipole moments because that requires a mixed parity state.
The degeneracy of the 2s and 2p orbitals of hydrogen allow that to have an electric dipole moment,
but this get a bit complicated by the Lamb shift.

5. Jan 10, 2014

### DrDu

The absence of states with a permanent electric dipole moment is a consequence of inversion symmetry (parity):
Under inversion an eigenstate transforms into $\pm$ itself, so that no electric dipole moment is possible.
However, parity is not an exact symmetry of nature. Hence people make some precision experiments trying to measure a small dipole moment of some atoms.