Class 11 Chemistry Structure of Atom Emission And Absorbtion Spectra

EMISSION AND ABSORBTION SPECTRA

Atomic spectra: Absorption and Emission spectra.

This is the phenomenon that gave serious blow to Rutherford model.It can be defined as: ‘splitting of light’ into various colours or colour bands is called spectra.”

When we see VIBGYOR the Violet colour merges into another and likewise other color merges into another.

    Class_11_Chemistry_Structure_Of_Atom_Atomic_Spectra                                 

So, in that case:

  • Where one-colour merges into another and there is no definite boundaries, that spectra is called continuous spectra.
  • If we study it for atoms the spectra obtained is discontinuous spectra.
  • The study of spectra is called spectroscopy

Atomic spectra are of two types

  • Emission spectra
  • Absorbtion spectra

Emission spectra: It is obtained when emitted light is analyzed.The light is emitted when an atom is heated or electric current is passed.These are also referred as finger prints of atoms.

The apparatus is set as shown in figure:

         Class_11_Chemistry_Structure_Of_Atom_Emission_Spectra

When we analyze with spectrometer the spectra obtained is discontinuous. When white light is passed through the sample it absorbs few radiations and then transmitted light is analyzed.

Emission spectra of hydrogen

  • The hydrogen gas ,at low pressure is taken in the tube.
  • Then light emitted by it was examined
  • It was noticed that spectra obtained consist of large number of closely spaced lines which falls in different regions as shown:

        Class_11_Chemistry_Structure_Of_Atom_All_3__Spectra 

Rydberg gave the formula to determine the wavelength of these lines. That is:

Class_11_Chemistry_Structure_Of_Atom_Rydberg_Formula

Limiting line: the limiting line in H spectrum was when n2=infinity

Problem:

Question 1: Calculate the wave length of the spectral line obtained in the spectrum of Li2+ ion when the transition takes place between two levels whose sum is 4 and the difference is 2?

Answer 1:-

Class_11_Chemistry_Structure_Of_Atom_Answer_8

 

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