Why are protons, rather than electrons, the nucleus in atoms?

Question

I have been wondering why only electrons revolve around protons instead of protons other way around. They have electrostatic force and I think mass factor has nothing to do here. Then why?

Answer 1

NB: I interpreted the question to essentially mean, why do protons rather than electrons reside in nuclei?

Electrons repel each other with a Coulomb force that grows very large when they are close together. Protons also repel each other in the same way, but the difference is that protons are also attracted to each other and to neutrons by the even stronger strong nuclear force (since protons are made up of quarks that feel the strong force), which acts over short range ($\sim 10^{-15}$m) and thus can be bound into dense nuclei (when you mix them with neutrons).

Electrons are point-like particles and not made up of quarks. They do not interact via the strong nuclear force and cannot be bound into dense nuclei.

Note that you can do physics in any coordinate system you choose. If you had an "electron nucleus" surrounded by a cloud of protons, you would probably still choose to define the nucleus as the centre. But electrons don't aggregate into nuclei for the reasons I explained.

Answer 2

Considering a classical model of two particles, they both actually revolve around the center of mass of the system. Same thing applies to the motion of the Earth-Sun system. If one of the two objects is a lot more massive than the other then the center of mass is very close to the massive object, even inside the volume occupied by the massive object. Then the motion of the lighter one is almost like revolving around the massive one. But for electrons in atoms, the "revolving around" is not a good description. The electrons are not moving around classical trajectories and some of them may have a non-zero probability to be inside the nucleus. Hovewer the configuration of the atom, even when described by quantum mechanics, depends on the ratio of the masses of the components. So the mass factor is essential.

Answer 3

The title of the question has had too many edits by various people, so I want to clearly answer the content:

why only electrons revolve around protons instead of protons other way around. They have electrostatic force and i think mass factor has nothing to do here.

The mass factor is important, because in systems bound with any type of force , either classical or quantum mechanical (and the atom is a quantum entity) have to obey the law of conservation of momentum . Momentum is the vector $p$ = $mv$, where $m$is the mass and $v$ the vector velocity, so that is how the mass comes in.

For example lets take the atom of hydrogen: The dimensions of the hydrogen atom are one angstrom=100.000fermi , the dimension of the proton is approximately one fermi. The orbital of the electron occupies a region 100.000 times larger than the proton dimension.

To see how momentum conservation affects the bound state of electrons in an atom , for hydrogen: the electron is ~0.5MeV, the proton ~1000Mev. If one measures the electron velocity in the hydrogen atom and thus measures the momentum, the proton momentum has to be equal and opposite . Because of the very large mass difference, the velocity will be very small. This means that a plot of the orbital of the electron in the center of mass system covers a large area, whereas the orbital of the proton will be located within its volume.

So it is because of the large difference between the masses of electrons and nuclei that one assumes that the center of mass is at the nucleus and the electrons have orbitals around it.

(In the comments the analogy of the orbit of the sun around the barycenter of the planetary system has been given, where the barycenter is often within the volume of the sun).

Answer 4

With a hydrogen-1 atom, the proton will have an orbital centered on the center of mass of the atom, but as a proton has a mass about 2000 times the mass of an electron, its motion will be much less. For larger atoms, the proton and neutrons in the nucleus are constantly exchanging virtual particles. While the nucleus is often modeled as having "protons" and "neutrons", to a great extent the nucleus is just a soup of quarks that can separate into proton-globs and neutron-globs of quarks given the right conditions. We can't separate out a "proton" wavefunction from this soup in the same way that we can separate out an electron wavefunction. The nucleus has one overall wavefunction that encompasses all the quarks that make it up, and the "protons" and "neutrons" are more fluctuations in this wavefunction than separate wavefunctions.

Answer 5

Essentially the core point is $m_{proton} \approx 1836\, m_{electron}$. If you take the quantum two-body problem i.e. the Schrodinger equation for two particles with a coulombian interaction and you do the approximation $m_{proton} >> m_{electron}$ you will get the discretized energy levels which belong to the hydrogen atom.

See here for a quick review.

In addition the quantum two body problem formulation solves the problem of the stability of a hydrogen atom because, in classical mechanics, you have energy losses due to an accelerated charged particle.