•    At the Centre of the atom, positive charge and mass are densely concentrated which is known as the nucleus of an atom.
•    Nucleus is much smaller than an atom. The positive charge in the nucleus is due to the presence of protons.
•    By using proton-neutron hypothesis james chadwick led to the discovery of particle called neutron.
•    Neutrons are electrically neutral carrying no charge having mass nearly equal to that of a proton.
•    Mass of nucleus is equal to the sum of masses of neutrons and protons.
•    Einstein through his mass - energy equivalence theory stated 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 and attractive in nature, which hold nucleons together in the nucleus of an atom, in spite of strong electrostatic forces of repulsion between protons.
•    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.  
•    Half life is the time during which number of atoms left undecayed in the decaying substance becomes half to the total number of atoms present initially in the sample of the radioactive substance.
•    The phenomenon of Radioactive decay is 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.
•    On disintegration or fusion a radioactive nucleus emits a large amount of energy which is known as nuclear energy.
•    When a heavy nucleus breaks down into two or more lighter nuclei, the phenomenon is known 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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