The behavior of electrons in atoms is described by quantum mechanics, specifically the Schrödinger equation, which gives rise to a set of allowed energy levels or "orbitals" for the electrons. These orbitals define the probability distribution of finding an electron in a specific region around the nucleus.
In an atom, the nucleus contains protons and neutrons, which are collectively known as nucleons. Nucleons are not subject to the same restrictions as electrons because they are composite particles made up of smaller particles called quarks. Quarks are governed by a different set of fundamental interactions known as the strong nuclear force, which binds them together inside the nucleus.
The strong nuclear force is different from the electromagnetic force that governs the behavior of electrons. It has a shorter range and is attractive over very short distances but becomes repulsive at extremely close ranges. This repulsive force between nucleons prevents them from being arbitrarily close to each other and imposes a certain structure on the nucleus.
However, within the nucleus, protons and neutrons can move relatively freely. They can exchange places, interact with each other, and occupy different energy levels within the nuclear potential. This is because the strong nuclear force acts between all nucleons and is not restricted to specific energy levels or orbitals like the electromagnetic force.
In summary, the restrictions on electron orbits in an atom arise from the wave-like behavior of electrons governed by quantum mechanics. On the other hand, nucleons inside the nucleus are subject to the strong nuclear force, which allows them to move freely within the confines of the nucleus without being confined to specific orbits.