BOHR'S ATOMIC THEORY
• Bohr recognized the relationship between the nature of the series of spectral lines and the arrangement of electrons in the atom.
• Bohr applied Planck's quantum theory to the electrons revolve around the nucleus. He retained the basic concept of Rutherford's model of atom that electrons revolve round the positively charged nucleus.
• Bohr proposed his theory to explain the structure of atom.
The important postulates of his theory are:
• Electrons revolve around the nucleus with definite velocities in concentric circular orbits. These orbits are called stationary orbits as the energy of the electron remains constant. As long as the electron revolves in the same circular orbit it neither radiates nor absorbs energy.
• The angular momentum of the electron is quantised. The electronic motion is restricted to those orbits where the angular momentum of an electron is an integral multiple of h/2πor mvr = nh/2π. This is called Bohr's quantum condition or quantisation of angular momentum.
• Energy of the electron changes only when it moves from one orbit to another orbit.
• Energy is absorbed when an electron jumps from a lower orbit to a higher outer orbit.
• If electron is in 1s orbit, it can only absorb but cannot emit energy.
• Energy is released when an electron jumps from higher orbit to a lower orbit.
•The released or absorbed energy is equal to the difference between the energies of the two orbits.
•If E2 is the energy of the electron in the outer orbit (n2) and E1 is the energy of the electron in the inner orbit (n1), then E2 – E1 = E = hυ.
•Where n is called principal quantum number and it represents the main energy level.
•It takes all positive and integral values 1, 2, 3, 4… etc.
•With the help of these postulates Bohr derived the expression for the radius of the circular orbit, energy of the electron in a circular orbit and velocity of the electron in a circular orbit.
•Bohr's theory could satisfactorily explain the formation of different series of lines in hydrogen spectrum.
•The wavelengths and the frequencies of the lines determined experimentally are in excellent agreement with those calculated by using Bohr's equation.
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