Free protons are considered stable particles because they are the lightest and most stable baryon (a type of subatomic particle made up of three quarks) and are the fundamental building blocks of atomic nuclei. Protons are composed of two up quarks and one down quark, and the interactions between these quarks result in a stable particle with a positive charge.
On the other hand, free neutrons are unstable when they exist outside the atomic nucleus. Neutrons are also composed of three quarks—two down quarks and one up quark. The key difference is that the neutron is slightly heavier than the proton. This small difference in mass makes a significant impact on the neutron's stability.
Neutrons undergo a process called beta decay, where a down quark within the neutron changes into an up quark, converting the neutron into a proton. During this process, a negatively charged electron and an electron antineutrino are emitted to conserve charge and energy. This decay process allows the neutron to stabilize and transform into a proton, which is more energetically favorable.
The instability of free neutrons is due to the fact that the mass of a neutron is slightly higher than the mass of a proton. As a result, isolated neutrons outside the atomic nucleus have a tendency to decay into protons to achieve a more stable configuration.
Inside atomic nuclei, however, neutrons can be stable due to the presence of the strong nuclear force, which binds the protons and neutrons together. The strong force overcomes the instability of individual neutrons, allowing them to exist in a stable state within the nucleus alongside protons.