Atoms & Nuclei - icbsexiiphysics
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1 You Can’t Miss It Previous Years Questions with Expected Answer/value Points Syllabus:- Atoms & Nuclei 6 Marks Alpha-particle scattering experiment; Rutherford’s model of atom; Bohr model, energy levels, hydrogen spectrum. Composition and size of nucleus, atomic masses, isotopes, isobars; isotones. Radioactivityalpha, beta and gamma particles/rays and their properties; radioactive decay law. Mass-energy relation, mass defect; binding energy per nucleon and its variation with mass number; nuclear fission, nuclear reactor, nuclear fusion.. --------------------------------------------------------------------------------------------------------------------Q1. What is the ratio of radii of the orbits corresponding to first excited state and ground state in a hydrogen atom? 1 A1. Q2. A heavy nucleus X of mass number 240 and binding energy per nucleon 7.6 MeV is split into two fragments Y and Z of mass numbers 110 and 130. The binding energy of nucleons in Y and Z is 8.5 MeV per nucleon. Calculate the energy Q released per fission in MeV. 2. A2. Q3. (i) Define ‘activity’ of a radioactive material and write its S.I. unit. (ii) Plot a graph showing variation of activity of a given radioactive sample with time. (iii) The sequence of stepwise decay of a radioactive nucleus is If the atomic number and mass number of D2 are 71 and 176 respectively, what are their corresponding values for D ? 3 A3. (i) The total decay rate (of a sample) at the given instant, i.e., the number of radio nuclides disintegrating per unit time is called the activity of that sample. ½ The SI unit for activity is becquerel (Bq) .½ 2 (iii) 72 and 180 ½+½ Q4. Define ionisation energy. What is its value for a hydrogen atom ? 1 A4. Minimum energy required to free an electron from the ground state. Its value for hydrogen atom is 13.6 eV ½+½ Q5. Draw a plot of potential energy of a pair of nucleons as a function of their separation. Write two important conclusions which you can draw regarding the nature of nuclear forces. 2 A5 Two important conclusions: (i) The nuclear force between two nucleons falls rapidly to zero at distances more than a few femtometres; (ii) The nuclear force is attractive for r > r0. (iii) The nuclear force is repulsive for r < r0. (iv) The nuclear force is a strong force. (Any two) ½+½ OR Draw a plot of the binding energy per nucleon as a function of mass number for a large number of nuclei, 2 < A < 240. How do you explain the constancy of binding energy per nucleon in the range 30 < A < 170 using the property that nuclear force is short-ranged ? 2 (ii) The constancy of the binding energy in the range 30 < A < 170 is a consequence of the fact that the nuclear force is short-ranged. If a nucleon can have a maximum of p neighbours within the range of nuclear force, its binding energy would be proportional to p. If we increase A by adding nucleons they will not change the binding energy of a nucleon inside. Since most of the 3 nucleons in a large nucleus reside inside it and not on the surface, the change in binding energy per nucleon would be small. Hence the binding energy per nucleon is a constant.[saturation property of nuclear force. ] 1 32 Q6 (a) Write symbolically the – decay process of 15 P (b) Derive an expression for the average life of a radionuclide. Give its relationship with the halflife. 3 A6. Q7. The energy levels of a hypothetical atom are shown below. Which of the shown transitions will result in the emission of a photon of wavelength 275 nm ? 3 Which of these transitions correspond to emission of radiation of (i) maximum and (ii) minimum wavelength? A7. If a photon ofwavelength = 275nmis to be emitted. then energy of photon is given by 4 Q8. State the law of radioactive decay. 3 Plot a graph showing the number (N) of undecayed nuclei as a function of time (t) for a given radioactive sample having half life T½. Depict in the plot the number of undecayed nuclei at (i) t = 3 T½ and (ii) t = 5 T½. A8. The number of nuclei undergoing decay per unit time, at any instant, is proportional to the total number of nuclei in the sample at that instant. 1 Alternatively [Note: The student is tomark the two point only for t= 3T½ and t = 5T½] 5 Q9. Write any two characteristic properties of nuclear force. 1 Short ranged, strong, attractive, charge independent, spin dependent, does not obey inverse square law, saturated, non central (Any two) ½+½ Q10. Using the curve for the binding energy per nucleon as a function of mass number A, state clearly how the release in energy in the processes of nuclear fission and nuclear fusion can be explained. 2 A10. The above curve shows that (i) When a heavy nucleus breaks into two medium sized nuclei (in nuclear fission) the BE/ nucleon increases resulting in the release of energy. ½ (ii) When two small nuclei combine to forma relatively bigger nucleus in nuclear fusion BE/ nucleon increases, resulting in the release of energy. ½ Q11. (a) Using de Broglie's hypothesis, explain with the help of a suitable diagram, Bohr's second postulate of quantization of energy levels in a hydrogen atom. (b) The ground state energy of hydrogen atom is - 13.6 eV. What are the kinetic and potential energies of the electron in this state ? 3 6 Q12 Q13 7 Q Q A15. Q16 Q16. 8 9 Q17. 3 Q18
