ABohr’s Atomic model
After the discovery of electron, protons and neutrons, attempt were made to give a physical picture of an atom. Rutherford’s models of an atom influenced by the solar system suffered the following set backs: ( Atomic )
(i) Newtonian law’s of motion can not be applicable to the macroscopic and charge particles.
(ii) According to Clark’s Maxwell whenever a charged particle like as an electron is subiected to acceleration, it emits radiation and losses energy :
Bohr argued therefore that if an electrons moves in an orbit it should be subjected to acceleration due to continuously change in its direction of motion. The electrons therefore, continuously emit radiations and loss energy. As the result, of this the electrons should move in a spiral path and ultimately collide with the nucleus. In other words, the atom should collapse. However, this does not happen.
(iii) it fails to explain atomic spectra of elements.
Neils Bohr is a Danish Physicist who’s in 1913 put forward a new model of atom combining the ideas of the J. J. Thomson, Rutherford and Plank’s quantum theory. He was also awarded Nobel prize for his distinguished work in 1922 .
Bohr’s postulates :
1- The electron revolves in circular orbits around the nucleus only in certain allowed energy states called stationary states or energy levels (as the earth revolves round the sun)
2. The electron did not radiate energy regularly during the motion and therefore it did not slower down. Thus, the motion of electron is restricted in such a manner that angular momentum (mvr) is quantized and is
where ‘m’ is the mass of electron, ‘v’ its tangential velocity, ‘ r’ the radius, ‘h’ the Plank’s constant and ‘n’ is an integer having values 1,2,3, for the first, second, third etc of Bohr’s orbits.
- 3. When electron jumps from one stationary state to another f.e. during electronic transition the energy of ‘e’ is being absorbed or emitted. This energy changed occurs in a fixed amount called as quanta.