Bohr's Model of an Atom

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Bohr's Model of an Atom, proposed by Niels Bohr in 1913, revolutionized the understanding of atomic structure and addressed the limitations of Rutherford's model. It was based on the principles of quantized energy levels and angular momentum.

The important postulates are:

01. Electrons Move in Specific Orbits: The electrons revolve round the nucleus only in certain selected circular paths called orbits. These orbits are associated with definite energies and are called energy shells or energy levels or quantum levels. These are numbered as 1, 2, 3, 4 etc. (starting from the nucleus) or designated as K, L, M, N..., etc.

02. Radiation Emission and Absorption: As long as an electron remains in a particular orbit, it does not lose or gain energy. This means that energy of an electron in a particular path remains constant. Therefore, these orbits are also called stationary states. Electrons can move between different orbits by either absorbing or emitting photons of electromagnetic radiation. When an electron transitions from a higher energy level to a lower one, it emits a photon of specific energy corresponding to the difference in energy between the two levels. Conversely, when an electron absorbs a photon, it moves to a higher energy level.

03. Angular Momentum Quantization: Only those orbits are permitted in which angular momentum of the electron is a whole number multiple of h/2π, where 'h' is Planck's constant. An electron moving in a circular orbit has an angular momentum equal to mur where m is the mass of the electron and u, the angular momentum, the angular momentum, mvr is a whole number multiple of h/2π, i.e.,

mvr = nh/2π            where, n = 1, 2, 3, ...

In other words, angular velocity of electrons in an atom is quantised.

04. Quantized Energy Levels: Electrons in Bohr's model can only exist in certain energy levels or orbits, and each orbit has a specific energy associated with it. Electrons can move between these energy levels by absorbing or emitting energy in discrete packets called photons.

Limitation of Bohr's Model of an Atom

• According to Bohr, the radiation results when an electron jumps from one energy orbit to another energy orbit, but how this radiation occurs is not explained by Bohr.

• Bohr's theory had explained the existence of various lines in H spectrum, but it predicted that only a series of lines exist. At that time this was exactly what had been observed. However, as better instruments and techniques were developed, it was realized that the spectral line that had been thought to be a single line was actually a collection of several lines very close together (known as fine spectrum). Thus, the appearance of the several lines implies that there are several energy levels, which are close together for each quantum number n. This would require the existence of new quantum numbers. 

• Bohr's theory has successfully explained the observed spectra for hydrogen atom and hydrogen like ions (eg., He, Li, Be, etc.), it can not explain the spectral series for the atoms having a large number of electrons.

• There was no satisfactory justification for the assumption that the electron can rotate only in those orbits in which the angular momentum of the electron (mvr) is a whole number multiple of h/2π, i.e., he could not give any explanation for using the principle of quantisation of angular momentum and it was introduced by him arbitrarily.

• Bohr assumes that an electron in an atom is located at a definite distance from the nucleus and is revolving round it with definite velocity, i.e., it is associated with a fixed value of momentum. This is against the Heisenberg's Uncertainty Principle according to which it is impossible to determine simultaneously with certainty the position and the momentum of a particle.

• No explanation for Zeeman effect If a substance which gives a line emission spectrum, is placed in a magnetic field, the lines of the spectrum get split up into a number of closely spaced lines. This phenomenon is known as Zeeman effect.

• No explanation of the Stark effect If a substance which gives a line emission spectrum is placed in an external electric field, its lines get spilt into a number of closely spaced lines. This phenomenon is known as Stark effect. Bohr's theory is not able to explain this observation as well.




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