General Chemistry

Bohr’s Atomic Theory

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Who was Niels Bohr?

Niels David Henrik Bohr was a physicist and Nobel Prize winner from Denmark who lived and researched from 1885 to 1962. During his lifetime, he worked with some influential researchers such as Heisenberg and Rutherford. One of their research results in the development of a model that makes statements about the structure of atoms. This model is based on the findings of Ernest Rutherford, with whom Bohr had worked for some time. He had found out that atoms do not consist of massive “spheres”, as previously assumed, but are composed of an atomic nucleus and an atomic shell. The core consists of positively charged protons and neutrally charged neutrons. In the shell, negatively charged electrons move around this atomic nucleus. Some tests carried out by Rutherford showed that there is nothing between the core and the shell, i.e. free space.

Bohr’s Atomic Model represents only an intermediate stage in the development of the model from the point of view of the quantum mechanical atomic model recognized today. Notably, the retention of descriptive electron orbits around the nucleus has to be criticized from the point of view of quantum mechanics. In some cases, BOHR’s ​​postulates are not justified either, Roman Sexl speaks of a “legal solution “to the problems of the RUTHERFORD atomic model by the Dane Neil Bohr”.

Problems of the RUTHERFORD atomic model

RUTHERFORD’s atomic model cannot explain the stability of the atoms. From the classical point of view, the circling electrons execute an accelerated movement and accelerated charges emit electromagnetic energy. The consequence of this would be a crash of the electrons into the nucleus.

RUTHERFORD’s atomic model cannot explain the quantum emission and absorption of energy by the atoms. Because of this experimentally proven fact (e.g. BALMER series; reversal of the Na line; FRANCK-HERTZ experiment), one must assume discrete energy states in the atom. However, since in RUTHERFORD’s atomic model all possible radii of the electron orbits and thus all-electron speeds were allowed, the total energy (potential energy + kinetic energy) of the electron cannot assume any discrete values.

BOHR’s postulates

BOHR solved the problem in 1913 by introducing postulates (casually: “by decree”) by transferring to the atom the quantization introduced by PLANCK for the black body and by EINSTEIN for the photon. However, it should be noted at this point that his third postulate (quantum condition) is no longer tenable from today’s perspective. It also showed that his theory was only successful for hydrogen and closely related systems.

1. Postulate (discrete energy levels):

The energy of an electron in an atom can only assume discrete values ​​En.

2. Postulate (Light emission):

The frequency of the emitted electromagnetic radiation results from the energy difference between the initial and the final state.

h∨=Em-En                        

Where m Higher energy level & n Lower energy level.

This second postulate appears to us to be nothing other than the application of the law of energy to the process of light emission. In BOHR’s ​​time, however, this statement was spectacular, since according to the classical view, the emitted radiation was always the same as the frequency of the orbiting electron. After this postulate seemed to be confirmed, EINSTEIN said: “This is one of the greatest inventions”.

Third, postulate (quantum condition):

The electrons only circulate on certain discrete orbits. No energy is radiated on these paths. The orbits must meet the following quantum condition:

me⋅rn⋅vn= n ⋅ h/2 ⋅ π

Successes of Bohr’s Atomic Model

  • The three series formulas for hydrogen empirically found up to 1913 (BALMER, LYMAN and PASCHEN series) could be explained.
  • Further series in the infrared range can be predicted.
  • The RYDBERG constant R. and the ionization energy of hydrogen are based on known natural constants.
  • The atomic radius results in the correct order of magnitude.

Defects of Bohr’s Atomic Model

Bohr’s theory can successfully explain the origin of the spectrum of H-atom and ions like He+, Li+2 and Be+3 etc. These are all one-electron systems. However, this theory is not able to explain the origin of the spectrum of a multi-electrons system like He, Li and Be etc. When the spectrum of hydrogen gas is observed using a light resolving power spectrometer, the individual spectral lines are replaced by several very fine lines, i.e., original lines are seen divided into other lines. The Ha-line in Balmer series is found to consist of five component lines. This is called fine structure or multiple structures. The appearance of several lines in a single line suggests that only one quantum number is not sufficient to explain the origin of various spectral lines. Bohr suggested circular orbits of electrons around the nucleus of the hydrogen atom. Still, researches have shown that the motion of the electron is not in a single plane, but takes place in three-dimensional space. The atomic model is not flat. When the excited atoms of hydrogen, which give an emission of line spectrum, are placed in a magnetic field, its spectral lines further split up into closely spaced lines. This type of splitting of spectral lines is called ‘Zeeman Effect’. Therefore, if the source, which is producing the Na-spectrum, is placed in a weak magnetic field, it causes the splitting of two lines of Na into component lines. Similarly, when the excited H-atom is placed in an electrical field, then similar splitting of the spectral line takes place, which is called ‘Stark Effect’. Bohr’s theory does not explain either the Zeeman Effect or Stark Effect.

Summerfield’s Modifications of Bohr’s Atomic Model

In 1915, Summerfield suggested that the moving electron might describe, in addition to the circular orbits, elliptical orbits as well, with the nucleus situated at one of the foci of the ellipse. The elliptical paths of the moving electron go on changing their position in space, and the nucleus is buried by the electronic cloud from all the sides.

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