The strong nuclear force, which is mediated by particles called gluons, is indeed responsible for both quark confinement and holding the atomic nucleus together. While it may seem paradoxical that quarks are tightly confined within nucleons (such as protons and neutrons), whereas nucleons themselves can separate and form the nucleus, this apparent contradiction can be resolved by considering the different aspects of the strong force.
Quark Confinement: Quark confinement refers to the phenomenon that individual quarks cannot be observed in isolation. When quarks are pulled apart from each other, the energy stored in the field between them increases. As the distance between the quarks grows, the energy stored in the field becomes large enough that it is more favorable to create new quark-antiquark pairs from the vacuum rather than separating the original quarks further. This results in the formation of new hadrons, like mesons, which consist of a quark and an antiquark bound together. As a result, quarks are always confined within color-neutral hadrons.
Holding the Nucleus Together: In contrast to quark confinement, the strong nuclear force also acts between the nucleons (protons and neutrons) within an atomic nucleus. This force binds nucleons together to form stable nuclei. The gluons mediate this force between the quarks within nucleons, allowing the exchange of gluons between quarks in different nucleons.
However, the range of the strong force is limited, and beyond a certain distance, called the range of the strong force, its influence becomes significantly weaker. This means that when nucleons are separated by distances greater than the range of the strong force, the attractive forces between them diminish, and other forces, such as electromagnetic repulsion, become dominant. As a result, nucleons can be separated from each other, allowing for the formation of a loosely bound nucleus.
In summary, the strong force is responsible for both quark confinement within nucleons and holding the atomic nucleus together. Quark confinement arises from the increasing energy of the strong force field between isolated quarks, leading to the creation of additional quark-antiquark pairs. On the other hand, the strong force between nucleons within the nucleus binds them together, but its influence weakens at larger distances, allowing for the separation of nucleons and the formation of a loosely bound nucleus.