Bohr’s Atomic Model

Neil Bohr in 1913 AD formulated an atomic model based on Planck’s quantum theory of radiation and some classical concept of physics. The atomic model has following postulates or assumption:

Ø  Electron revolves around the nucleus in a selected circular orbital path called orbit.

Ø  Electron doesn’t lose or gain energy when revolves around the nucleus such a state of electron is called stationary state. Stationary energy of electron is constant but its motion change with time. This postulate justify about stability of atom.

Ø  The centrifugal force of electron is balanced by electrostatic force of attraction between nucleus and electron i.e.

          where  m = mass of electron

            r = radius of orbit

              Z = atomic number

              v = velocity of electron

              E₀ = permitivity of medim

Ø  Electron can revolve only in those orbits in which angular momentum of revolvingelectron is an integral multiple of h/2π i.e.

                 where, h = Planck’s constant = 6.62 x 10^-34 JS
                               n = an integer called principle quantum number = 1, 2, 3, ………

Ø  The absorption or emission of energy can occur on by transition of electron from lower to higher energy level or vice-versa. Energy is absorbed when an electron jump from lower state to higher state and energy is emit when an electron jump from higher to lower energy state. The difference of energy is ΔE and is equal to one quanta of energy i.e.

                                                        E_higer – E_lower = ΔE = hf

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Drawback of Bohr’s Model

ü  Bohr’s model fail to explain spectrum of multi-electron system

ü  It doesn’t explain the splitting of spectral lines under electric field (Stark effect) and magnetic field (Zeeman effect)

ü  It fails to explain line spectrum of hydrogen

ü  This theory is not accordance with Heisenberg’s uncertainty principle