Chemistry Class 11 NCERT Solutions: Chapter 2 Structure of Atom Part 5

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Q: 12 Electrons are emitted with zero velocity from a metal surface when it is exposed to radiation of wavelength 6800 Calculate threshold frequency and work function ( ) of the metal.


Threshold wavelength of radian

Threshold frequency of the metal

Thus, the threshold frequency ( ) of the metal is

Hence, work function ( ) of the metal

Q: 13. What is the wavelength of light emitted when the electron in a hydrogen atom undergoes transition from an energy level with energy level


The transition will give rise to a spectral line of the balmer series. The energy involved in the transition is given by the relation,

Substituting the values in the given expression of E:

The negative sign indicates the energy of emission.

Wavelength of light emitted

Substituting the values in the given expression of

Q: 14. How much energy is required to ionize a H atom if the electron occupies n = 5 orbit? Compare your answer with the ionization enthalpy of H atom (energy required to remove the electron from n = 1 orbit).


The expression of energy is given by,


Z = atomic number of the atom

n = principal quantum number

For ionization from

Hence, the energy required for ionization from Energy required for

Hence, less energy is required to ionize an electron in the orbital of hydrogen atom as compared to that in the ground state.

Q: 15. What is the maximum number of emission lines when the excited electron of an H atom in drops to the ground state?


When the excited electron of an H atom in n = 6 drops to the ground state, the following transitions are possible:

Hence, a total number of (5+4+3+2+1) 15 lines will be obtained in the emission spectrum.

The number of spectral lines produced when an electron in the level drops down to the ground state is given by


Number of spectral lines =15