The protons and neutrons are held together by the strong attractive forces inside the nucleus. These forces are called as nuclear forces.
(i) Nuclear forces are short-ranged. They exist in small region (of diameter 10–15 m = 1 fm). The nuclear force between two nucleons decrease rapidly as the separation between them increases and becomes negligible at separation more than 10 fm.
(ii) Nuclear force are much stronger than electromagnetic force or gravitational attractive forces.
(iii) Nuclear force are independent of charge. The nuclear force between two proton is same as that between two neutrons or between a neutron and proton. This is known as charge independent character of nuclear forces.

In a typical nuclear reaction
(i) In nuclear reactions, sum of masses before reaction is greater than the sum of masses after the reaction. The difference in masses appears in form of energy following the Law of inter-conversion of mass & energy. The energy released in a nuclear reaction is called as Q Value of a reaction and is given as follows.
If difference in mass before and after the reaction is m amu
(m = mass of reactants minus mass of products)
then Q value = delta m (931) MeV
(ii) Law of conservation of momentum is also followed.
(iii) Total number of protons and neutrons should also remain same on both sides of a nuclear reaction.

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