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The nucleus of an atom consists of positively charged protons and uncharged neutrons. According to Coulomb's law, protons must repel each other with a very large force, because they are close to each other and hence the nucleus must be broken into pieces. But this does not happen. It means that, there is some other force in the nucleus which overcomes the electrostatic repulsion between positively charged protons and binds the protons and neutrons inside the nucleus. This force is called nuclear force.
(i) Nuclear force is charge independent. It is the same for all the three types of pairs of nucleons (n-n), (p-p) and (n-p). This shows that nuclear force is not electrostatic in nature.
(ii) Nuclear force is the strongest known force in nature.
(iii) Nuclear force is not a gravitational force. Nuclear force is about 1040 times stronger than the gravitational force.
(iv) Nuclear force is a short range force. It is very strong between two nucleons which are less than 10−15 m apart and is almost negligible at a distance greater than this. On the other hand electrostatic, magnetic and gravitational forces are long range forces that can be felt easily.
Yukawa suggested that the nuclear force existing between any two nucleons may be due to the continuous exchange of particles called mesons, just as photons, the exchange particle in electromagnetic interactions.
However, the present view is that the nuclear force that binds the protons and neutrons is not a fundamental force of nature but it is secondary.
The phenomenon of radioactivity was discovered by Henri Becquerel in 1896. He found that a photographic plate wrapped in a black paper was affected by certain penetrating radiations emitted by uranium salt. Rutherford showed later that the radiations from the salt were capable of ionising a gas. The current produced due to the ions was taken as a measure of activity of the compound.
A few years later Madame Marie Curie and her husband Piere Curie discovered the highly radioactive elements radium and polonium. The activity of the material has been shown to be the result of the three different kinds of radiations, α, β and γ.
The phenomenon of spontaneous emission of highly penetrating radiations such as α, β and γ rays by heavy elements having atomic number greater than 82 is called radioactivity and the substances which emit these radiations are called radioactive elements.
The radioactive phenomenon is spontaneous and is unaffected by any external agent like temperature, pressure, electric and magnetic fields etc.
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