Why is there no electric dipole term in the Hyperfine Hamiltonian? I have been looking a several discussions of hyperfine structure. (For example: here and in the explanation for eq. 2.1 here
It seems that the hyperfine interaction can be explained almost entirely in terms of the magnetic dipole interaction between electron and nucleus and the electric quadrupole interaction between electron and nucleus. 
Why is there no electric dipole term?
I sense that this arises from some semi-obvious property of the electron or nucleus, but I can't seem to figure out what this property might be.
 A: Neither the nucleus nor the electrons form electric dipoles of any kind - the electron is a point charge, a monopole; the nucleus contains only one type of charge, the positive protons (and the electrically neutral neutrons). There is no scope for electric dipole interactions between the nucleus and electrons.
The nucleus can still have an electric quadrupole moment, based on the proton distribution (see here). The magnetic dipole moment of course comes from the spin of the nucleons, and the electron.
It may be added that it is possible for a nucleus to have a fleetingly small momentary dipole moment, based on the arrangement of the positively charged up and negatively charged down quarks that make up the nucleons. This will probably produce a very (very) weak effect compared to the magnitude of hyperfine splitting, considering the fact that no permanent dipole moments have been observed. There's a little bit more about these internal dipole moments here.
A: Nuclei have a very small electric dipole moment. However, they can have a significant quadrupole moment, which influences hyperfine structure. You can refer to the Wikipedia article to get a quick understanding of the latter.
