Bohr’s Atomic Model

Bohr’s Atomic Model . Quantized model of the atom proposed in 1913 by the Danish physicist Niels Bohr , to explain how electrons can have stable orbits around the nucleus and why the atoms presented characteristic emission spectra (two problems that were ignored in the previous Rutherford model). Furthermore, Bohr’s model incorporated ideas taken from the photoelectric effect, explained by Albert Einstein in 1905 .



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  • 1 History
  • 2 Bohr’s Postulates
  • 3 See also
  • 4 Source


Bohr was based on the atom of hydrogen for the model named after him. Bohr was trying to make an atomic model capable of explaining the stability of matter and the discrete emission and absorption spectra observed in gases. He described the hydrogen atom with a proton in the nucleus, and spinning an electron around it. Bohr’s atomic model was conceptually based on Rutherford’s atomic model and the emerging ideas on quantization that had emerged a few years earlier with the investigations of Max Planck and Albert Einstein . Due to its simplicity the Bohr model is still frequently used as a simplification of the structure of matter.

In this model the electrons rotate in circular orbits around the nucleus, occupying the lowest possible energy orbit, or the closest possible orbit to the nucleus. Classical electromagnetism predicted that a charged particle moving in a circular fashion would emit energy so that the electrons should collapse on the nucleus in short moments of time. To overcome this problem, Bohr assumed that the electrons could only move in specific orbits, each characterized by its energy level. Each orbit can then be identified by an integer n that takes values ​​from 1 onwards. This number “n” is called the Principal Quantum Number.

Bohr further assumed that the angular momentum of each electron was quantized and could only vary in whole fractions of the Planck constant. Based on the main quantum number, he calculated the distances to the nucleus of each of the allowed orbits in the hydrogen atom.

These levels were originally classified by letters beginning with “K” and ending with “Q”. Later electronic levels ordered by numbers. Each orbit has electrons with different levels of energy obtained, which must then be released, and for this reason the electron jumps from one orbit to another until it reaches one that has the appropriate space and level, depending on the energy it has, to free itself. without problem and again return to its original orbit.

However, it did not explain the fine-structure spectrum that could be explained some years later thanks to Sommerfeld’s atomic model. Historically the development of the Bohr atomic model together with the wave-corpuscle duality would allow Erwin Schrödinger to discover the fundamental equation of quantum mechanics

Bohr’s Postulates

In 1913, Niels Bohr developed his famous atomic model according to four fundamental postulates:

  1. Electrons orbit the nucleus of the atom in discrete and quantized energy levels, that is, not all orbits are allowed, only a finite number of them.
    2. Electrons can jump from one electronic level to another without going through intermediate states.
    3. The jump of an electron from one quantum level to another implies the emission or absorption of a single quantum of light (photon) whose energy corresponds to the difference in energy between the two orbits.
    4. The allowed orbits have discrete or quantized values ​​of the orbital angular momentum.

The fourth hypothesis assumes that the minimum value of n is 1. This value corresponds to a minimum radius of the electron orbit of 0.0529 nm. This distance is called the Bohr radius. An electron at this fundamental level cannot descend to lower levels by emitting energy.

This set of hypotheses can be shown to correspond to the hypothesis that the stable electrons orbiting an atom are described by standing wave functions. An atomic model is a representation that describes the parts that an atom has and how they are arranged to form a whole. Based on Planck’s constant, he was  able to quantify the orbits by observing the lines of the spectrum.


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