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5.ATOMIC STRUCTURE eLearn.Punjab

(c) Evaluate mass of electron from the above two experiments.
Q6 (a) Discuss Chadwick’s experiment for the discovery of neutron. Compare the properties of electron,
proton and neutron.

(b) Rutherford’s atomic model is based on the scattering of a-particles from a thin gold foil. Discuss
it and explain the conclusions.
Q7. (a) Give the postulates of Bohr’s atomic model. Which postulate tells us that orbits are stationary and
energy is quantized?
(b) Derive the equation for the radius of nth orbit of hydrogen atom using Bohr’s model.
(c) How does the above equation tell you that

(i) radius is directly proportional to the square of the number of orbit.
(ii) radius is inversely proportional to the number of protons in the nucleus.
(d) How do you come to know that the velocities of electrons in higher orbits, are less than those in
lower orbits of hydrogen atom?
(e) Justify that the distance gaps between different orbits go on increasing from the lower to the
higher orbits.
Q8 Derive the formula for calculating the energy of an electron in nth orbit using Bohr’s model. Keeping in
view this formula explain the following:
(a) The potential energy of the bounded electron is negative.
(b) Total energy of the bounded electron is also negative.
(c) Energy of an electron is inversely proportional to n2, but energy of higher orbits are
always greater than those of the lower orbits.
(d) The energy difference between adjacent levels goes on decreasing sharply.
Q9. (a) Derive the following equations for hydrogen atom, which are related to the
(i) energy difference between two levels, n1 and n2.
(ii) frequency of photon emitted when an electron jumps from n2to n1.
(iii) wave number of the photon when the electron jumps from n2 to n1.
(b) Justify that Bohr’s equation for the wave number can explain the spectral lines of Lyman, Balmer
and Paschen series.
Q10. (a) What is spectrum. Differentiate between continuous spectrum and line spectrum.
(b) Compare line emission and line absorption spectra.
(c) What is the origin of line spectrum?
Q11. (a) Hydrogen atom and He+ are mono-electronic system, but the size of He+ is much smaller than H+,
why?
(b) Do you think that the size of Li+2 is even smaller than He+? Justify with calculations.

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