Bohr Model: Meaning Definition

Danish physicist Niels Bohr proposed atom’s structures, especially those similar to hydrogen. The Bohr model describes the properties of the atomic electrons in terms of set values. Radiation is emitted by atoms when the electrons jump from one stationary orbit to another. There is a positively charged heavy core within an atom known as the nucleus. The cloud of electrons revolves around the nucleus. The discrete wavelengths in the emission spectrum of hydrogen can be found by limiting the orbiting electron series. Light can only be radiated from a hydrogen atom when an electron transition happens  between the different energy levels.

Neil Bohr proposed the Bohr model of the atom in 1915

Neil Bohr proposed the Bohr model for hydrogen-like atoms in the year 1915. Rutherford’s experiment established the planetary model of the atom. Rutherford atom model predicts all the atoms to be similar. The Bohr model was introduced at the first Solvay. The need for the quantum theory in describing atom models was understood at that conference.

Postulates of Bohr model

• The electrons can revolve around only certain energy levels. While revolving in these orbits, the electrons do not release energy. These are stationary orbits that are stable in nature.

• The acceleration of electrons in these stationary orbits does not cause radiation and energy loss. In the second postulate, we get the idea of the angular momentum of a revolving electron in a stationary orbit. 

That is (mvr = nh/2π) where ‘n’ is the energy level and ‘h’ is Planck’s constant.

• When an electron transition happens between two orbits, it continuously releases or gains some amount of energy.

Drawbacks 

Bohr’s model consisted of several limitations, and thus it was replaced by the quantum mechanics model.

The model stood firm in explaining spectra of lighter atoms similar to hydrogen. It could not explain the spectral lines for heavier atoms.

Spectral evidence for quantisation (Hydrogen spectrum’s explanation based on Bohr atomic model)

• The change in the orbit of an electron is due to the phenomenon of absorption or emission of the light ray. These light rays are recorded on the photographic plate as a series of lines, referred to as an optical spectrum. This optical spectrum consists of several series of lines, which are also called Balmar, Lyman, Brackett, Paschen, Pfund and Humphrey. The name of the scientist who named these spectral series discovered them.

Failure of Bohr Model

• Bohr’s theory successfully predicted and accounted for the energies of line spectra of 

Hydrogen. However, the Bohr Model failed to explain the line spectra of the atom, which has more than one electron.

• This theory falls short to explain the presence of multiple spectral lines. 

• This theory falls short to explain the Zeeman effect (related to a magnetic field) and the Start effect (related to an electric field).

• This theory falls short to explain the concept of the dual nature of matter, later by De Broglie’s concept. 

• This theory failed to describe the uncertainty principle.

• The Bohr model could not conclude the concept of quantisation of energy.

Einstein’s photoelectric effect

The Bohr formula assumes that some energy will be radiated during a quantum jump, like Einstein’s photoelectric effect. Bohr did not consider the existence of photons in this theory.

Electron energy levels

The Bohr model is well applicable for two charged point systems. A one-electron system like hydrogen atom (lithium-ion) is good for the Bohr model. The Bohr model is more primitive than the valence cell model. 

De Broglie received the Nobel prize for his study of the wave-like behaviour of matter in 1927. Initially, de Broglie thought that natural waves having a direct physical interpretation were associated with particles. 

A wave formalises the wave aspect of the matter function defined by the Schrödinger equation. The wave function is a purely mathematical entity. It is a complex function. It has a probabilistic interpretation without the support of fundamental physical elements. This wave function can explain the appearance and structural behaviour of the wave nature of any matter.

Bohr- Kramers-slater theory

This model’s theory was proven wrong as the extension of the Bohr Model. The conservation of energy and momentum is violated in this theory. In the de Broglie theory, the electron is compared with a wave.

Bohr hydrogen model

The Bohr model is well applicable for hydrogen-like atoms. Hydrogen only has 1 electron in its structure. So all the postulates of the Bohr model hold good in the case of hydrogen. 

Conclusion

The Bohr model is made for 1 electron species which are hydrogen-like atoms. The Bohr model has three postulates all total. We get the angular momentum concept of a revolving electron inside an atom around a nucleus from one of the Bohr postulates.