Proposed by Neils Bohr in 1913, his atomic theory revolves around the structure of the hydrogen atom. In the quantum hypothesis,  electrons can occupy certain energy states around the nucleus. Electrons move around a core which is the nucleus, in closed orbits. The theory assumes that energy is released as radiation when electrons leap to a lower-energy orbit. Bohr’s atomic theory explained why atomic spectrums produce light of fixed frequencies on excitation.

Origin of Bohr’s Atomic Theory

Bohr took the initiative by analysing the structure of an atom using the old-styled mechanics. Still, he noticed that for a viable atomic model, he would have to incorporate some principles of the still-in-infancy Quantum Mechanics.

Bohr saw that the quantisation of energy as put forward by Max Planck had a fundamental constant, called the Planck’s constant, that had dimensions of angular momentum.

On that basis, Bohr opined that as energy was quantised, so can angular momentum be. He then used a whole number, the quantum number ‘n’, to mark the orbits.

Furthermore, Bohr described how electrons radiated energy when they jumped from a higher energy orbit to a lower energy orbit and absorbed energy when an electron transferred from a lower-energy state to a higher one.

The energy emitted would have been the emission spectra, while the one absorbed in the absorption spectra.

This was the first that the spectrum of an atom was given a theoretical basis. This was also the first time that an Atomic model was used to explain an experimental fact. 

Bohr’s Atomic Theory is a pioneering advancement for both of these reasons. Bohr’s atomic theory gives Bohr’s model for describing an atom. The key features are described in the next section.

Bohr’s Model

The basic feature of Bohr’s atomic theory is a positively charged nucleus around which the negatively charged electron orbits in a fixed path.

Many individuals also assert it as the Rutherford-Bohr Model because of the inspiration taken from the Rutherford Model. 

Here are some salient features of Bohr’s Atomic Theory: 

•  The framework consisting of a little nucleus encompassed by circling electrons is similar to that of planets around the Sun. The primary distinguishing factor is that electrons can occupy only certain well-defined orbits.
• Bohr’s model depends on quantum mechanics.
• Bohr argued that the maximum number of electrons that can be accommodated for an orbit would be 2n2, where ‘n’ is the order of the orbit.
• Bohr further modified the shell model to explain why atoms with a certain number of electrons are more stable than others even though they had more protons and electrons.
• Bohr’s model additionally made sense why the noble gases were unreactive.
• Bohr’s model also explained why atoms on the left half of the Periodic table were reducing agents. At the same time, the right side of the table depicted the oxidising behaviour.

Postulates

Neil Bohr came up with some postulates that explain the theory in-depth. Moreover, the postulates demonstrated insight into understanding the rules and principles.

The postulates were mainly applicable to the molecules having a single electron as he majorly emphasised such molecules.

Bohr’s model of the hydrogen molecule depends on these hypotheses: 

• An electron moves around the core, which is the nucleus.
• The electrons revolve around a fixed and definite orbit, as explained by the quantisation of angular momentum.
• In these allowed orbits, electrons do not emit any energy (as they should have by Electromagnetic theory)
• Energy is radiated or absorbed only when electrons change orbits.
• The shells have different energy levels indicated as K, L, M, N.

Limitations

The Bohr Model could successfully be applied only for a Hydrogen atom.

The Bohr model of hydrogen is a semi-old style model. The theory consolidates the classical idea of electron circles with the new idea of quantisation. 

The striking outcome of this model provoked numerous physicists to look for clarification for why such a model should work by any means and look for a comprehension of the material science behind the hypotheses of the early quantum hypothesis. 

With Bohr’s atomic theory advancement, we also face limitations that the theory fails to explain. Some limitations are: 

• Bohr’s atomic theory goes against de Broglie’s theory that electrons reveal wave nature. But Bohr considered electrons as particles.
• Bohr’s atomic theory only works for molecules that consist of a single electron. For example – Hydrogen, lithium, etc.
• Bohr’s model couldn’t make sense of the spectra of multi-electronic particles.
• A particle has three dimensions, but Bohr’s model was specified for two dimensions.
• The model failed to explain the differences which were depicted by the spectrometer.
• Bohr’s atomic theory did not make any inferences regarding the splitting of spectral lines (Zeeman effect).
• According to Heisenberg’s Uncertainty principle, it is impossible to gauge the position and force of electrons simultaneously. But, Bohr stated that these two parameters (force and position) could be calculated simultaneously. 

Conclusion

Bohr’s atomic theory is an integral part of Chemistry and quantum mechanics, which has led to new ideas and an introduction to atomic structure.According to the Bohr Model of an atom, electrons can only move within the atomic structure model’s fixed orbitals (shells). In addition, the energy level of each orbit is constant. Essentially, Rutherford defined the atom’s nucleus, and Bohr transformed that model into electrons and their respective energy levels.

According to Bohr’s model, electrons revolve around a small, positively charged nucleus in a series of orbits. For Bohr, distance from the core significantly affected electron energy, while proximity to the nucleus had a much smaller effect.