Structure of the atom : Bohr’s Model of an Atom

 

Bohr’s Model                                                                                                                                                                           

Bohr model of atom was proposed by Neil Bohr in 1915. It came into existence with the modification of Rutherford’s model of atom. Rutherford’s model introduced nuclear model of atom, in which he explained that a nucleus (positively charged) is surrounded by negatively charged electrons.

Bohr modified this atomic structure model by explaining that electrons move in fixed orbits (shells) and not anywhere in between and he also explained that each orbit (shell) has a fixed energy level. Rutherford basically explained nucleus of an atom and Bohr modified that model into electrons and their energy levels.

Postulates of Bohr’s Model:

  • Electrons revolve around Nucleus in fixed circular path called Orbits. 
  • Electrons revolve in their orbits without radiating energy. In a particular orbit, the energy of an electron is constant. This is why orbits are called stationary orbits or stationary shells.
  • Orbits are also known as energy levels.
  • These orbits or shells are represented by the letters K, L, M, N,… or the numbers n=1, 2, 3, 4,….
  • When an electron goes from lower orbit to higher orbit ; it absorbs energy. This absorbed energy is exact difference of Energy of two orbits. The presence of electron in higher energy orbit is called Excited State.
  • Electron remains in higher orbit (called excited state) for small interval (10-8 seconds) ; then comes back to initial lower orbit by emitting the energy which was absorbed.

Limitations of Bohr’s Model of an Atom

  • Bohr’s model of an atom failed to explain Zeeman Effect (effect of magnetic field on the spectra of atoms).
  • It also failed to explain the Stark effect (effect of electric field on the spectra of atoms).
  • This Model is applicable for single electron atom or ions, like H, He+,Li2+

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Structure of the atom : Discovery of Neutrons