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1228, , PAYSICS ig |, , (GUIDELINES TO NCERT EXERCISES SS, , . iven at, 12.1. Choose the correct alternative from the clues given, , the end of each statement :, (a) The size of the atom in Thomson's model is ...atomic size in Rutherford’s model., , (much greater than, no different from,, much less than)., , the, , electrons are in stable, , b) In the ground state of ......, aa. y electrons always, , equilibrium, while in, experience a net force., (Thomson's model, Rutherford’s model)., (c) A classical atom based on...... is doomed to collapse., (Thomson's model, Rutherford’s model), (a) An atom has a nearly continuous mass distribution, but has a highly non-uniform mass, distribution in ......, (Thomson's model, Rutherford’s model)., (e) The positively charged part of the atom possesses, most of the mass of the atom in ......, (Rutherford’s model, both the models)., , Ans. (a) no different from, , (b) Thomson’s model, Rutherford’s model, , (c) Rutherford’s model, , (4) Thomson's model, Rutherford’s model, , (e) both the models, , 12.2. Suppose you are given a chance to repeat the, alpha-particle scattering experiment using a thin sheet of solid, hydrogen in place of the gold foil. (Hydrogen is a solid at, temperatures below 14 K). What results do you expect ?, , Ans. Hydrogen nuclei (or protons) are much lighter, than a-particles. So a-particles are not scattered by solid, hydrogen. They pass through solid hydrogen almost, undeflected from their paths., , 12.3. What is the shortest wavelength present in the, Paschen series of spectral lines ?, , Ans. For shortest wavelength of Paschen series, n, = 3,, Da, , or AWS, , = R” 1.097x107, = 8.2041 x10 m = 8204.1 A, , 12.4. A difference of 2.3 eV separates two energy levels in an, atom. What is the frequency of radiation emitted when the atom, makes a transition from the upper level to the lower level ? , electron in q hydrog, (b) Calculate the, , Ans. Here E=2.3eV =2.3x1.6x 10°), As E=hv, , .. Frequency,, 2.3x1.6 x10719, , 6.6x 10-4, =5.6 x10 Hz, , a, h, , 12.5. The ground state energy of. hydrogen atom is, , - acti - 3 136 ey,, What are the kinetic and potential energies of the electron in his, state ?, , Ans. Total energy, E=-—13.6eV, K.E. = - E=-—(- 13.6) =13.6 eV, P.E. =-2K.E.=-2x13.6=-27.2 ey,, , 12.6. A hydrogen atom initially in the ground level, a photon, which excites it to the n= 4 level, Determine the, wavelength and frequency of photon. [CBSE OD 14g), Ans. Energy of an electron in the nth orbit of H-atom,, , , , , , , , , , , , , , , , , , , , , ened, 5, Energy in the ground (n= 1) level,, 13.6, , EB OE, , Energy in the fourth (1 = 4) level,, 13.6, , Ey =~ “aie toe, AE=E,-E, = - 0.85 —(- 13.6) =, = 12.75 x 1.6 x10), , As AE=hy =H€, r, -. Wavelength,, aig, , Frequen Vi oe, Bh 2% 0.975 x1, = 3.077 x10, , 12.7 (a) Using the Bohr’s mode