Limitations of Bohr’s Model

In 1913, a Danish physicist named Niels Henrik David Bohr formulated an atomic structure model. His proposal followed the failure of Rutherford’s and Thomson’s atomic models, as they could not resolve any questions related to an atom’s stability and energy. 

Niels Bohr described an atom as a small nucleus with a positive charge, surrounded by electrons. Around the positively charged nucleus, the electrons travel in a circular path. It is similar to how the planets travel in a circular route around the sun in our solar system. This model of an atom is the Bohr’s atomic model.  

Bohr’s Atomic Model

In 1913, Niels Bohr proposed Bohr’s atomic model. It was a modification of Rutherford’s atomic model. Bohr’s model of an atom is also known as the Rutherford-Bohr Model.

According to Rutherford’s model, a nucleus carries a positive charge and negatively charged electrons surround it. Bohr modified this model by adding that the electrons travel in fixed circular orbits around the nucleus. No electron travels between these fixed orbits. Each orbital or shell has a definite energy.

What is Bohr’s Atomic Model?

Bohr’s model of an atom is similar to the planetary model. In Bohr’s model, electrons move in fixed circular orbits around a positively charged nucleus. There is fixed energy associated with each orbit. Each circular orbit has a fixed distance from the nucleus. 

Bohr’s model of an atom explains the electron’s properties in terms of allowed values. Bohr’s model can explain the absorption and emission of radiation when an electron makes a transition between different energy levels. 

In 1922, Bohr received the Nobel Prize in physics for his work.

Why Did Bohr Modify Rutherford’s Model?

Niels Bohr modified Rutherford’s atomic model and provided the Bohr’s Atomic Model due to the following limitations of Rutherford’s model:

• The electrons in Rutherford’s model travelled in simple circular paths (not fixed orbits). An accelerating electron emits radiation, so the electron must decay and fall into the nucleus. Rutherford’s model did not address this problem.

• It failed to explain the position of the electrons.

• It could not talk about the constituents of the nucleus. Rutherford’s model could only tell that the nucleus had a positive charge.

Limitations of Bohr’s Model

Bohr’s atomic model explains many features of the atomic theory, but it has its limitations. The problems with the Bohr model are listed below:

• The electrons move around the nucleus in definite circular orbits. These orbits are also called ‘shells’ or ‘energy levels.’

• Each orbit is stationary and has a definite energy. These circular orbits or shells with definite energy are called ‘orbital shells.’

• Bohr labelled these orbits or energy levels by the quantum number ‘n.’ The first shell, closest to the nucleus, is designated by the alphabet K. The second shell is denoted by L, the third by M, and the fourth by N and so on. 

The K, L, M, N… can accommodate a maximum of 2, 8, 18, 32… electrons, respectively. The lowest energy level is called the ground state.

• Electrons do not radiate energy while revolving in these shells of fixed energy. There is energy absorption/radiation only when the electrons transit from one energy level to another. 

• When an electron jumps from a higher energy level to a lower energy level, it loses some energy. But when an electron moves from a lower energy level to a higher energy level, it gains some energy (or photons). Energy is emitted or absorbed in discrete quantities known as ‘quanta.’

• It contradicts Heisenberg’s Uncertainty Principle in assuming that electrons have both a known orbit and radius. 

• The model fails to predict the spectra of larger atoms and provides insufficient data for the spectra of smaller ones.

• It fails to explain the Zeeman Effect, wherein a magnetic field causes the spectrum to disintegrate.

• The model does not explain the Stark Effect, wherein an electric field breaks the spectrum.

Other Drawback/Demerits of Bohr’s Atomic Model

Here are some additional demerits of Bohr’s atomic model.

• Bohr’s model is only valid for the electrons that consist of one atom. For example, Li+2, Na+1, H, etc. 

• It fails to explain the shapes of molecules.  

• Bohr’s model does not explain the spectral intensity. 

• It considers the nucleus stationary even though it rotates on its axis. 

• The model could not clarify the minute structure in the spectrum line.

• It failed to explain the spectrum of some atoms with doublets. 

The Orbits and Their Representation

Bohr’s model defined the following orbits:

• K represents the energy level or the 1st orbit. It can hold up to two electrons. 

• The L shell is the energy level or the 2nd orbit that has up to eight electrons. 

• The energy level or the 3rd orbit is M, and it has a capacity of up to 18 electrons. 

• N denotes the energy level or the 4th orbit. Its maximum electron limit is up to 32. 

It continues for all the orbits and energy levels in the same manner.

Conclusion

The atomic structure model formulated by Niels Bohr was an improved edition, overcoming the limitation of Rutherford’s atomic model. The model agreed with some assumptions of the Rutherford model. These hypotheses include electrons orbiting the nucleus and the concept of the nucleus. 

It made a wrong prediction regarding the spectra of the large atoms. The model could not explain the Zeeman and Stark effects. It also did not shed light on the relativistic intensities of spectral lines.