Question

Drawbacks of Bohr atomic model.

Answer 1

Hint: Bohr’s atomic model is described in his three postulates, in which he explains the stability of electrons in the orbits and also explains the emission of energy by electrons. From our perspective to solve this question, first, we will take a look at Bohr’s atomic model and then at its drawbacks.

Complete step by step answer:
Before Bohr’s atomic model there were several attempts made to give an accurate atomic model to determine the atomic structure and the behavior and arrangement of electrons in the atom.
Bohr’s model is the advanced version of the Rutherford model. Although the Rutherford model tells us about some important features of atom-like:
1) Every atom has a tiny center core, called the atomic nucleus in which the entire positive charge and the entire mass is concentrated.
2) The size of the nucleus is of ${{10}^{-15}}m$ and the size of the atom is ${{10}^{-10}}m$ .
3) The atomic nucleus is surrounded by a certain number of electrons and the atom in whole is electrically neutral.
4) Electrons revolve around the nucleus in circular orbits like planets revolve around the sun.

There are some unanswered questions from Rutherford’s model like:-
a) If electrons revolve around the nucleus then it will lose energy continuously and eventually will fall into the nucleus.
b) Why the atom exists is a continuous spectrum, but what we observe is the line spectrum.
To answer these questions Bohr proposed his atomic model, in which he describes the behavior of electrons in three postulates.
1. Every atom consists of a central core called the nucleus, in which the entire positive charge and almost all the mass of the atom are concentrated, and the electron revolves around this nucleus in circular orbits. The centripetal force is balanced out by the electrostatic force of attraction.
  $\dfrac{m{{v}^{2}}}{r}=\dfrac{kZ{{e}^{2}}}{{{r}^{2}}}$
2. An electron can revolve only in certain distinct non radiating orbits, called stationary orbits, for which the total angular momentum of the revolving electron is an integral multiple of $\dfrac{h}{2\pi }$ , h is planck's constant.
  $mvr=\dfrac{nh}{2\pi }$
   n = 1,2,3….
3. The emission/absorption of energy occurs only when an electron jumps from one of its specified non radiating orbits to another. Emission takes place when an electron jumps from outer to inner orbit and absorption takes place when an electron jumps inner to outer orbit.
  $hv={{E}_{2}}-{{E}_{1}}$
Limitation of Bohr’s model:
  1) This theory only applies to the simplest atom like hydrogen with Z = 1. The theory fails for Z greater than 1 (where Z is atomic number)
  2) Theory does not explain why orbits of the electron are taken as circular, while elliptical orbits are also possible.
  3) Does not explain the fine structure of spectral lines.
  4) Does not say anything about the wave properties of the electron.
  5) Does not say about relative intensities of spectral lines.

Note:
This is advised to students that don’t memorize the Bohr model and its drawbacks. And be curious how the drawbacks of the Bohr model were solved.
For example, Bohr was unable to say anything about the wave properties of an electron, after him, this was de Broglie who proposed that particle also shows wave nature in his very famous equation \[\lambda =\dfrac{h}{mv}\] and there is the symmetry between wave and particle.