NCERT Physics Class 12 Exemplar Ch 12 Atoms Part 1 (For CBSE, ICSE, IAS, NET, NRA 2023)

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Very Short Answer

14 The mass of a H-atom is less than the sum of the masses of a proton and electron. Why is this?


  • Einstein՚s mass-energy equivalence gives . Thus the mass of a H-atom is where is the binding energy

15 Imagine removing one electron from and . Their energy levels, as worked out on the basis of Bohr model will be very close. Explain why.


  • Because both the nuclei are very heavy as compared to electron mass.

16 When an electron falls from a higher energy to a lower energy level, the difference in the energies appears in the form of electromagnetic radiation. Why cannot it be emitted as other forms of energy?


  • Because electrons interact only electromagnetically

17 Would the Bohr formula for the H-atom remain unchanged if proton had a charge and electron a charge , where . Give reasons for your answer.


Yes, since the Bohr formula involves only the product of the charges.

18 Consider two different hydrogen atoms. The electron in each atom is in an excited state. Is it possible for the electrons to have different energies but the same orbital angular momentum according to the Bohr model?


  • No, because acceding to Bohr model, and elections having different energies belong to different levels having different values of n.
  • So, their angular moment will be different, as