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

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

Image of Threshold Frequency and Work function for Structure of Atom

Image of Threshold Frequency and Work Function

Image of Threshold Frequency and Work function for Structure of Atom

Answer:

Threshold wavelength of radian Equation

Threshold frequency Equation of the metal

Equation

Thus, the threshold frequency ( Equation ) of the metal is Equation

Hence, work function ( Equation ) of the metal Equation

Equation

Equation

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

Answer:

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

Equation

Substituting the values in the given expression of E:

Equation

Equation

Equation

Equation

The negative sign indicates the energy of emission.

Wavelength of light emitted Equation

Equation

Substituting the values in the given expression of Equation

Equation

Equation

Equation

Equation

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).

Answer:

The expression of energy is given by,

Equation

Where,

Z = atomic number of the atom

n = principal quantum number

For ionization from Equation

Equation

Equation

Equation Equation

Equation

Equation

Hence, the energy required for ionization from Equation Energy required for Equation

Equation

Equation

Equation

Equation

Hence, less energy is required to ionize an electron in the Equation 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 Equation drops to the ground state?

Answer:

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

Image of excited electron for Structure of Atom

Image of Excited Electron

Image of excited electron for Structure of Atom

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 Equation level drops down to the ground state is given by Equation

Given,

Equation

Number of spectral lines Equation =15

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