14(T)%20-%20Nucleus - 14 NUCLEUS Page 1 14.1 Nucleus...

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14 - NUCLEUS Page 1 14.1 Nucleus - General Information A nucleus consists of electrically neutral neutrons and positively charged protons. The nucleus of hydrogen has only one proton and no neutrons. The charge of proton = the charge of electron = 1.6 × 10 - 19 C. Mass of proton, m p = 1.673 × 10 - 27 kg, Mass of neutron, m n = 1.675 × 10 - 27 kg. Protons and neutrons are commonly known as nucleons. The nucleus of any element is symbolically represented by Z X A or X A Z . where, X is the chemical symbol of the element. Z is the atomic number of the element which represents the number of protons in its nucleus and shows the position of the element in the periodic table. In a neutral atom, the number of electrons is also Z. A is called the mass number of the element which represents the number of nucleons ( protons + neutrons ) inside the nucleus. N = A - Z represents the number of neutrons. e.g., the nucleus of carbon is represented by 6 C 12 and that of uranium by 92 U 238 . Thus, nucleus of carbon has 6 protons in 12 nucleons and 12 - 6 = 6 neutrons and the nucleus of uranium has 92 protons in 238 nucleons and 238 - 92 = 146 neutrons. same different examples Isotopes Z A and N Carbon - 6 C 12 , 6 C 13 and 6 C 14 Uranium - 92 U 233 , 92 U 235 and 92 U 238 Isobars A Z and N Pb 214 and Bi 214 Isotones N = A - Z A and Z 36 Kr 86 , 37 Rb 87 Isomers A, Z, N ( all ) different radioactive properties Pair of isomers of 35 Br 80 14.2 Nuclear Forces Despite Coulombian force of repulsion between the protons in the nucleus, nucleus does not break up. This is because of strong nuclear force of attraction between ( i ) protons and protons, ( ii ) neutrons and neutrons and ( iii ) protons and neutrons which is more than the Coulombian force of repulsion. As far as this force is concerned, there is no difference between protons and neutrons. Hence they are commonly known as nucleons. This strong force is a short range force which exists between the neighbouring nucleons but is negligible between the nucleons far away from each other as in the large nuclei.
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14 - NUCLEUS Page 2 The figure shows the qualitative graph of their potential energy U ( r ) corresponding to the forces acting between two nucleons versus distance ( r ) between them. Here, U ( r ) = - g 2 r e R r - for r > 1 fm, where R and g are constants and g is called the strength parameter. It can be seen from the graph that such forces act only unto 2 to 3 fm. For r < 1 fm, the forces are repulsive. This region of the nucleus is called its core. 14.3 Nuclear Stability The figure shows graph of Z versus N, called Nuclide chart, for some stable nuclei. In the nuclei of lighter elements, the number of protons ( Z ) and neutrons ( N ) are almost equal but in case of heavy elements, the number of neutrons is comparatively more. Stable nucleus lies on or very close to the stability line. Initially
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14(T)%20-%20Nucleus - 14 NUCLEUS Page 1 14.1 Nucleus...

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