The positive charge and mass of an atom are densely concentrated at the centre of the atom, which is called nucleus. The overall dimension of a nucleus is much smaller than that of an atom. The positive charge in the nucleus is that of the protons. Proton-neutron hypothesis led to the discovery of a new particle called neutron. Neutron is a neutral particle carrying no charge having mass nearly equal to the mass of proton. Mass of nucleus is equal to the sum of masses of neutrons and protons. Einstein’s mass - energy equivalence theory state that mass is another form of energy and mass-energy can be converted into other forms of energy. Application of mass - energy equivalence relation is observed in nuclear forces. Nuclear forces are the strong forces of attraction, which hold nucleons together in the nucleus of an atom, in spite of strong electrostatic forces of repulsion between protons. An unstable nucleus leads to radioactivity. Radioactivity is process of decaying of an unstable nucleus to gain stability. The law of radioactive disintegration states the number of atoms disintegrated per second at any instant is directly proportional to the number of radioactive atoms actually present in the sample at that instant. The time during which number of atoms left undecayed in the decaying substance becomes half the total number of atoms present initially in the sample is called half life of the radioactive substance. Radioactive decay is the phenomenon of spontaneous emission of active radiations from the radioactive substance during its decay. The three types of radiations emitted by a radioactive substance while decaying are alpha, beta and gamma radiations. A radioactive nucleus whether on disintegration or fusion emits a large amount of energy termed as nuclear energy. When a heavy nucleus breaks down into two or more lighter nuclei, it is termed as nuclear fission. Nuclear fission is controllable and is used to generate electricity by constructing nuclear reactors. When two atomic nuclei fuse together to form a heavier nucleus, it is known as nuclear fusion. Nuclear fusion is an uncontrollable process till now. The enormous energy released in an atom bomb comes from uncontrollable nuclear fusion.

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