The mass of a nucleus is less than the sum of the masses of (A-Z) number of neutrons and Z number of protons in the nucleus. The energy equivalent to the corresponding mass difference is known as the binding energy of the nucleus. A heavy nucleus of mass M can break into two light nuclei of masses m1 and m2 only if (m1 + m2) < M. Also, two light nuclei of masses m3 and m4 can undergo complete fusion and form a heavy nucleus of mass M’ only if (m3 + m4) > M'. The masses of some neutral atoms are given in the table below :
| 11H | 1.007825u | 21H | 2.014102u | 31H | 3.016050u | 42He | 4.002603u |
| 63Li | 6.015123u | 73Li | 7.016004u | 7030Zn | 69.925325u | 8234Se | 81.916709u |
| 15264Gd | 151.919803u | 20682Pb | 205.974455u | 20983Bi | 208.980388u | 21084Po | 209.982876u |
(1u = 932 MeV/c2)
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The correct statement is
The kinetic energy (in keV) of the alpha particle, when the nucleus at rest undergoes alpha decay, is
Q = (mPo – mpb – m) c2
= 209.982876
205.974455
4.008421
– 4.002603
= 0.005818 × 932 MeV
= 5422 keV
Almost 100 keV go to nucleus
= 5319 keV