Class 11 Chemistry Structure of Atom | Explanation of Hydrogen Spectra |

__EXPLANATION OF HYDROGEN SPECTRA __

According to Bohr,

- When energy is supplied to atoms of hydrogen, the electron from lower energy gets excited to higher energy level.
- The excited state being unstable, it jumps back to its original state that is ground state.
- Some electrons move to their ground state in one jump, some in multiple jumps. Each jump corresponds to line in a spectrum.
- As we know the gas in tube consists of many hydrogen atoms.
- Therefore, each electron on getting energy gets excited.
- On returning to the ground state, they either move in single jump or multiple jump.
- This is the reason that we get so many lines in different regions in hydrogen spectrum.

The wavelength emitted by them can be calculated as:

__Limitations of Bohr model of an atom:__

- Bohr model failed to explain the line spectra of multi-electron atoms.
- He couldn’t explain splitting of line in the magnetic field (Zeeman effect) and in electric field (stark effect).
- He failed to explain three-dimensional model of an atom.
- He was unable to explain the shapes of molecule.
- He couldn’t explain the dual nature of matter and Heisenberg uncertainty principle.

This led to the discovery of Quantum mechanics. Let us first understand the dual nature of matter and Heisenberg uncertainty principle that couldn’t be explained by Bohr model.

__DUAL NATURE OF PARTICLE: DEBROGLIE AND HEISENBERG UNCERTAINTY PRINCIPLE __

As we have studied that light has dual character .That is particle as well as wave character. In the same way, it was proved that all material particles too have wave like character.

It was explained by De-Broglie: According to him when any particle is in motion it emits wave.”

The wavelength associated with it is given by:

__Significance of De- Broglie relation__

In real life, it does not have any significance. This is because in real life we come across only macroscopic objects which have significant mass. So, the wavelength emitted by them is almost nil. The wavelength is significant only for microscopic bodies, but in real life we do not come across microscopic objects.

**Derivation of Bohr postulate of angular momentum from de Broglie relation**

According to Bohr model, the electron move around the nucleus in circular orbit. According to De Broglie, the wave can be in phase or can be out of phase. For the wave to be in phase the circumference should be integral multiple of wavelength that is:

**Hence it also derives the postulate of bohr’s model.**

**Problem**** : Calculate the mass of a photon with wavelength 3.6A**

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