The deuteron is a bound state of a proton and a neutron, which are held together by a strong nuclear force. The nuclear force is attractive and acts between nucleons (protons and neutrons) inside the nucleus, and it is responsible for holding the nucleus together.
The deuteron is an example of a loosely bound extended structure because it is not a point particle, but rather has a finite size and a finite lifetime. This is due to the fact that the nuclear force has a finite range and becomes weaker as the nucleons move farther apart.
The nuclear force is very strong at short distances, but it decreases rapidly with increasing distance. As a result, the binding energy of the deuteron is relatively small, and the deuteron is only weakly bound compared to other atomic systems. This means that the deuteron can be easily disrupted by collisions or interactions with other particles.
Moreover, the proton and the neutron inside the deuteron are not point particles but have a finite size and a spatial distribution. The spatial distribution of the proton and the neutron inside the deuteron is spread out, which means that the deuteron has an extended structure.
In summary, the deuteron is a loosely bound extended structure because it is held together by the nuclear force, which has a finite range and becomes weaker as the nucleons move farther apart. Additionally, the proton and the neutron inside the deuteron have a finite size and a spatial distribution, which gives the deuteron an extended structure.