Bohr’s Atomic Theory:
In 1913, Niels Bohr proposed a new model of the atom which explained all the difficulties faced by Rutherford and also the emission spectrum of hydrogen. Bohr’s theory was based on Planck’s quantum theory and was built on the following postulates:
Postulates of Bohr’s Theory:
1) Electrons travel around the nucleus in specific permitted circular orbits and in no others.
2) While in these specific orbits, an electron does not radiate or lose energy.
3) An electron can move from one energy level to another by quantum or photon jumps only.
∆E = E high – E low = hν
Where h is Planck’s constant and ν the frequency of a photon emitted or absorbed energy.
4) The angular momentum (mvr) of an electron orbiting around the nucleus is an integral multiple of Planck’s constant divided by 2π.
Angular momentum (mvr) = n h/2π
Where,
m = mass of electron
v = velocity of the electron
r = radius of the orbit
n = 1, 2, 3, etc.
h = Planck’s constant.
Limitations of Bohr’s Atomic Theory:
1) Bohr’s theory has successfully explained the observed spectra for H-atom and hydrogen like ions (e.g., He+, Li2+, Be3+ etc.), it cannot explain the spectral series for the atoms having more than one electrons i.e. multi-electron atom.
2) Bohr assumes that an electron in an atom is located at a definite distance from the nucleus and is revolving round it with definite velocity i.e., associated with a fixed value of momentum. This is against the Heisenberg’s Uncertainty principle according to which it is impossible to determine simultaneously with certainty the position and the momentum of a particle.
3) No explanation for Zeeman Effect (If a substance which gives a line emission spectrum is placed in a magnetic field, the lines of the spectrum get split up into a number of closely spaced lines, called Zeeman Effect).
4) No explanation for Stark Effect (If a substance which gives a line emission spectrum is placed in an external electromagnetic field, its lines get split up into a number of closely spaced lines, called Stark effect).