Short Answer

Bohr’s atomic model is a structure of the atom proposed by Niels Bohr in 1913. According to this model, electrons revolve around the nucleus in fixed circular paths called orbits or shells. Each orbit has a definite energy, so electrons do not lose energy while moving in these paths.

Bohr also explained that electrons can jump between energy levels by absorbing or releasing a fixed amount of energy. When an electron moves to a higher orbit, it absorbs energy, and when it falls back to a lower orbit, it emits energy as light. This model helped explain hydrogen’s spectrum and became an important step in atomic theory.

Detailed Explanation :

Bohr’s atomic model

Bohr’s atomic model is one of the most important milestones in the history of atomic physics. Before Bohr, J. J. Thomson and Ernest Rutherford gave earlier models of the atom. Thomson described the atom as a “plum pudding” with electrons scattered inside a positive sphere. Later, Rutherford discovered the nucleus and proposed that electrons revolve around it. However, Rutherford’s model had a serious problem: according to classical physics, electrons moving in circular paths should continuously lose energy and spiral into the nucleus, making the atom unstable.

Niels Bohr solved this problem by introducing a new model based on quantum ideas. In 1913, he proposed that electrons revolve around the nucleus only in specific allowed orbits, and while doing so, they do not radiate energy. These orbits are called stationary states or energy levels. Bohr’s ideas successfully explained atomic stability and the line spectrum of hydrogen.

Main postulates of Bohr’s atomic model

Bohr’s model is based on a few important postulates or assumptions:

1. Electrons revolve in fixed orbits:
   Bohr said that electrons move around the nucleus in certain stable, circular paths. These paths are called orbits or shells. Each orbit is associated with a specific energy, and electrons in an orbit do not lose energy while moving.
2. Quantisation of angular momentum:
   Bohr proposed that the angular momentum of an electron in an orbit is quantised. It is given by:
   mvr = nh/2π,
   where nis a whole number (1, 2, 3…), called the principal quantum number. This quantisation explains why electrons cannot exist in between orbits.
3. No radiation in stable orbits:
   Electrons in fixed orbits do not emit energy. This solved Rutherford’s problem of atomic collapse.
4. Absorption and emission of energy:
   Electrons can jump from one orbit to another.

• When an electron moves to a higher orbit, it absorbs energy equal to the difference between the two levels.
• When it falls to a lower level, it emits energy in the form of light (photon).

The energy of the emitted or absorbed photon is given by:
E = hν = E₂ − E₁

These postulates helped explain why atoms emit only certain wavelengths of light instead of a continuous spectrum.

Explanation of hydrogen spectrum

One of the biggest successes of Bohr’s atomic model was its ability to explain the line spectrum of hydrogen. Hydrogen emits light of specific colours when its electrons jump between energy levels. These colours appear as bright lines in the spectrum.

Bohr showed that the wavelengths of these lines correspond to the energy difference between orbits. For example, when an electron falls from a higher level (like n = 3) to n = 2, it produces a visible light line. When it falls to n = 1, it produces ultraviolet lines. This matched the experimental results perfectly.

The success of this explanation was strong evidence in favour of Bohr’s model.

Achievements of Bohr’s atomic model

Bohr’s model was extremely important for several reasons:

1. Explained atomic stability:
   Electrons in fixed orbits do not lose energy, so atoms remain stable.
2. Explained hydrogen spectrum:
   The model accurately predicted the wavelengths of lines in the hydrogen spectrum.
3. Introduced quantum ideas:
   Bohr used Planck’s quantum theory to explain atomic structure, leading to the beginning of quantum mechanics.
4. Gave the idea of energy levels:
   Bohr introduced the concept of shells or energy levels, which later became central to understanding chemical behaviour.
5. Supported later atomic models:
   Although later models improved upon Bohr’s ideas, his model laid the foundation for future developments.

Limitations of Bohr’s atomic model

Even though Bohr’s model was successful, it had limitations:

• It could not explain atoms with more than one electron.
• It could not explain fine structure and spectral line details.
• It did not describe the wave nature of electrons.
• It did not match the results of more complex experiments.

Later, quantum mechanical models replaced Bohr’s model by using Schrödinger’s wave functions and orbitals. These models give a more accurate picture of electron behaviour.

However, Bohr’s model is still taught because it provides a simple and useful introduction to atomic structure.

Conclusion

Bohr’s atomic model states that electrons revolve around the nucleus in fixed energy levels and do not lose energy while doing so. Electrons can move to higher or lower orbits by absorbing or emitting energy. This model explained atomic stability and the hydrogen spectrum and played a major role in the development of modern atomic theory. Although later replaced by quantum mechanics, Bohr’s model remains an important step in understanding atomic structure.