I mean, I know that it's because they're neutral and therefore have a charge of zero, but since atoms have positively charged nuclei and negatively charged electrons, why can't we directly influence the constituents (not the whole) to subject the atom, or at least part of it, to an electric force?

I understand that charges sorta "group up," like 23 protons and 23 electrons with 28 neutrons in a single atom come together like 23(1) + 23(-1) + 28(0) = 0, but what is the extent of this "grouping"? Not exactly sure on the appropriate term for it.


Why can't we influence neutral atoms with electric fields if they have charged constituents?

We can influence the charged constituents with electric fields. It is just often difficult to see the results of that influence because the constituents are also bound by the strong nuclear force (for nuclei in the nucleus of an atom) or by the electromagnetic force (for the nucleus and its electrons).

This difficulty is similar to the difficulty of how it is hard to influence a neutral atom by electric fields. However, a high enough energy field will kick out an electron from an atom. The energy in that case often has to be on the order of x-ray energy. Similarly, you can kick apart a nucleus; you just have to use even high energy electromagnetic radiation, e.g., gamma rays and higher.

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  • $\begingroup$ I meant influence the charged constituents while they are still part of a neutral atom. Or is that what you covered in the first two sentences? $\endgroup$ – M. V. Jul 27 '18 at 4:45
  • $\begingroup$ Yes, you can influence any electrically charged constituents using electric fields. But sometimes it is hard to get them to actually move if they are already "bound" in a "bound state" (such as an electron and nucleus in an atom. Or a group of protons and neutrons in a nucleus. Or a group of quarks in a neutron. Etc.) $\endgroup$ – hft Jul 27 '18 at 4:48
  • $\begingroup$ Ah, thank you. So the problem in influencing them merely lies in how the electrically charged constituents are bound by forces then? $\endgroup$ – M. V. Jul 27 '18 at 4:51
  • $\begingroup$ Correct. This can also be understood classically by thinking about an equal amount of positive and negative charge bound by a classical spring. Applying an electric field will create a "dipole moment" by extending the spring until the spring force equals the force due to the electric field, but there will be no overall motion of the center of mass due to the electric field since the overall system is neutral. $\endgroup$ – hft Jul 27 '18 at 4:56
  • $\begingroup$ I think, in theory, an unbounded dipole in a non-uniform electric field should move along a field line, since, once the dipole rotates and orients itself along a field line, the magnitude of forces acting on its positive and negative sides won't be the same and won't completely cancel each other. $\endgroup$ – V.F. Jul 27 '18 at 17:48

why can't we directly influence the constituents (not the whole) to subject the atom, or at least part of it, to an electric force?

I am not sure where you heard this statement, but it is wrong. Electrons inside neutral atoms respond to electric as well as magnetic part of light. Since you mention force, trapping of neutral atoms with an optical tweezer is a common experimental technique.

EDIT: Not just trapping by light, but laser cooling is common as well.

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  • $\begingroup$ Optical tweezers? Okay, thanks. I thought that electrons would only be subject to electromagnetic forces, like an electric or magnetic field, when it was no longer bound to atoms. $\endgroup$ – M. V. Jul 27 '18 at 4:53
  • $\begingroup$ Even when the electrons are bound, electrons can make transitions to excited states due to electromagnetic forces. $\endgroup$ – wcc Jul 27 '18 at 4:55
  • $\begingroup$ By subjecting it to electromagnetic forces, I guess I wasn't detailed enough, sorry. I meant accelerate the bound electrons using an electric field or magnetic field, like in the Lorentz Force equation for electromagnetism, not really excitations of the electron's energy state. $\endgroup$ – M. V. Jul 27 '18 at 4:57
  • $\begingroup$ In a classical picture, accelerating electrons in a potential climb up the potential. Quantum mechanically, they climb up the ladder of excited states through transitions. You can bridge the two pictures by saying that force causes displacement, and quantum-mechanically, displacement can be thought of as a superposition of creation and annihilation operators of energy quantum. $\endgroup$ – wcc Jul 27 '18 at 5:08

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