Neutrons turn spontaneously into protons, which is called beta decay, and which happens in any nucleus with too many neutrons. This includes the free neutrons, which decay into protons in minutes.
Neutrons and protons differ in their composition, a neutron being made of 2 d quarks + 1 u quark, while a proton is made of 1 d quark + 2 u quarks, much in the same way as a nucleus of tritium differs from a nucleus of helium 3, the former being made of 2 neutrons + 1 proton, while the latter is made of 1 neutron + 2 protons.
For the strong interactions, nucleons (i.e. protons and neutrons) and nuclei are analogous to what atoms and molecules are for the electromagnetic interaction.
The strong interaction attempts to neutralize the hadronic charge (a.k.a. color charge), while the electromagnetic interaction attempts to neutralize the electric charge.
To a first approximation, the hadronic charge is neutralized in nucleons and the electric charge is neutralized in atoms.
However, because of the movement of the quarks inside of a nucleon and of the electrons inside an atom, the neutralization of the charge is imperfect and there remain some residual forces of attraction, respectively strong and electromagnetic, which bind the nucleons into nuclei and the atoms into molecules. Because they are just residual forces, the binding forces between nucleons in a nucleus are much weaker than those between quarks in a nucleon, similarly to how the binding forces between atoms in a molecule are much weaker than those that bind most of the electrons to the nucleus in an atom.