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describe the Bohr model of an atom and what are its merits and demerits.

Niels Bohr proposed the Bohr Model of the Atom in 1915. Because the Bohr Model is a modification of the earlier Rutherford Model, some people call Bohr's Model the Rutherford-Bohr Model. The modern model of the atom is based on quantum mechanics. The Bohr Model contains some errors, but it is important because it describes most of the accepted features of atomic theory without all of the high-level math of the modern version. Unlike earlier models, the Bohr Model explains the Rydberg formula for the spectral emission lines of atomic hydrogen.

The Bohr Model is a planetary model in which the negatively-charged electrons orbit a small, positively-charged nucleus similar to the planets orbiting the Sun (except that the orbits are not planar). The gravitational force of the solar system is mathematically akin to the Coulomb (electrical) force between the positively-charged nucleus and the negatively-charged electrons.

Main Points of the Bohr Model

  • Electrons orbit the nucleus in orbits that have a set size and energy.
  • The energy of the orbit is related to its size. The lowest energy is found in the smallest orbit.
  • Radiation is absorbed or emitted when an electron moves from one orbit to another.

Bohr Model of Hydrogen

The simplest example of the Bohr Model is for the hydrogen atom (Z = 1) or for a hydrogen-like ion (Z > 1), in which a negatively-charged electron orbits a small positively-charged nucleus. Electromagnetic energy will be absorbed or emitted if an electron moves from one orbit to another. Only certain electron orbits are permitted. The radius of the possible orbits increases as n2, where n is the principal quantum number. The 3 → 2 transition produces the first line of the Balmer series. For hydrogen (Z = 1) this produces a photon having wavelength 656 nm (red light).

Problems with the Bohr Model

  • It violates the Heisenberg Uncertainty Principle because it considers electrons to have both a known radius and orbit.
  • The Bohr Model provides an incorrect value for the ground state orbital angular momentum.
  • It makes poor predictions regarding the spectra of larger atoms.
  • It does not predict the relative intensities of spectral lines.
  • The Bohr Model does not explain fine structure and hyperfine structure in spectral lines.
  • It does not explain the Zeeman Effect.
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 Neils Bohr gave atomic model in 1913.The main points of this model are:

1) An atom consists of a central positive part called the nucleus which contains all the protons and neutrons.

2) Electrons which are present outside the nucleus revolve around the nucleus in definite circular paths called orbits or shells.

3) Each orbit has a fixed energy and the orbitals are represented by the symbols K, L,M, N etc or by the numbers 1,2,3 …

4) Each orbit can accommodate not more than a fixed number of electrons.

5) Electrons moving in the same orbit donot lose or gain energy.

6) Electrons lose energy on jumping from a higher orbit to lower one and vice-versa.

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since,hydrogen atom has only one proton and one electron.
electron revolves around nucleus. So, it req. centripetal F. which is provided by- coulomb''s F.
                                            F=1/4pi E0.q1.q2/r2.
DRAWBACKS---
does not give any identification about arrangement nd distribution of electrons in an atom
could not explain the spectra of complex atoms having more than one electron
could not account for the wave nature of electrons
 
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thanx #
 
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please refer the ncert textbook.
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Bohr’s Atomic Model 

Thomson’s atomic model and Rutherford’s atomic model failed to answer many questions related to the energy of an atom and its stability. In the year 1913, Niels Bohr proposed an atomic structure model, describing an atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the positively charged nucleus as planets around the sun in our solar system, with attraction provided by electrostatic forces, popularly known as Bohr’s atomic model.It was basically an improved version of Rutherford’s atomic model overcoming its limitations. On most of the points, he is in agreement with him, like concepts of nucleus and electrons orbiting it. Salient features of Bohr’s atomic model are:

  • Electrons revolve around the nucleus in stable orbits without emission of radiant energy. Each orbit has a definite energy and is called energy shell or energy level.
  • An orbit or energy level is designated as K, L, M, N shells. When the electron is in the lowest energy level, it is said to be in the ground state.
  • An electron emits or absorbs energy when it jumps from one orbit or energy level to another. When it jumps from higher energy level to lower energy level it emits energy while it absorbs energy when it jumps from lower energy level to higher energy level.
  • The energy absorbed or emitted is equal to the difference between the energies of the two energy levels (E1, E2) and is determined by Plank’s equation.

 

                            ΔE = E2-E1 = hv

Where,

ΔE = energy absorbed or emitted
h= Plank’s constant
v= frequency of electromagnetic radiation emitted or absorbed

  • Angular momentum of an electron revolving in energy shells is given by:

 

                             mevr = nh2π

Where,
n= number of corresponding energy shell; 1, 2, 3 …..
me= mass of the electron
v= velocity
r=radius
h= Plank’s constant

Limitations of Bohr Atomic Model Theory:

  • It violates the Heisenberg Uncertainty Principle. The Bohr atomic model theory considers electrons to have both a known radius and orbit i.e. known position and momentum at the same time, which is impossible according to Heisenberg.
  • The Bohr atomic model theory made correct predictions for smaller sized atoms like hydrogen, but poor spectral predictions are obtained when larger atoms are considered.
  • It failed to explain the Zeeman effect when the spectral line is split into several components in the presence of a magnetic field.
  • It failed to explain the Stark effect when the spectral line gets split up into fine lines in the presence of electric field.
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