Bohr’s Atomic Model
Bohr retained the essential features of the Rutherford model of the atom. However, in order to account for the stability of the atom he introduced the concept of the stationary orbits. The Bohr postulates are,
(1) An atom consists of positively charged nucleus responsible for almost the entire mass of the atom (This assumption is retention of Rutherford model).
(2) The electrons revolve around the nucleus in certain permitted circular orbits of definite radii.
(3) The permitted orbits are those for which the angular momentum of an electron is an intergral multiple of h/2π where h is the Planck’s constant. If m is the mass and v is the velocity of the electron in a permitted orbit of radius r then L = mvr = nh/2π; n =1, 2, 3, ……∞
Where L is the orbital angular momentum and n is the number of orbit. The integer n is called the principal quantum number. This equation is known as the Bohr quantization postulate.
(4) When electrons move in permitted discrete orbits they do not radiate or lose energy. Such orbits are called stationary or non-radiating orbits. In this manner, Bohr overcame Rutherford’s difficulty to account for the stability of the atom. Greater the distance of energy level from the nucleus, the more is the energy associated with it. The different energy levels were numbered as 1,2,3,4.. and called as K,L,M,N, …. etc.
(5) Ordinarily an electron continues to move in a particular stationary state or orbit. Such a state of atom is called ground state. When energy is given to the electron it jumps to any higher energy level and is said to be in the excited state. When the electron jumps from higher to lower energy state, the energy is radiated.