Describe Bohr’s model of the atom.

Short Answer

Bohr’s model of the atom explains that electrons revolve around the nucleus in fixed circular paths called orbits or shells. Each orbit has a definite energy level, and electrons cannot stay between these shells. The model also states that electrons can move from one orbit to another by absorbing or releasing energy.

This model successfully explained the stability of atoms and the line spectra of hydrogen. Although later replaced by modern quantum theory, Bohr’s model remains an important step in understanding atomic structure and electron arrangement.

Detailed Explanation :

Bohr’s Model of the Atom

Bohr’s model, proposed by Niels Bohr in 1913, is one of the most important atomic models in the history of chemistry. It improved upon Rutherford’s nuclear model by explaining how electrons are arranged around the nucleus and why they do not fall into the nucleus despite the attractive force. Bohr combined ideas from classical physics and quantum theory to create a more accurate and stable picture of the atom.

Bohr’s model marked a major turning point in atomic physics because it introduced the concept of quantized energy levels, which means electrons exist only in specific orbits and not anywhere else. This model successfully explained the atomic spectrum of hydrogen and provided a strong foundation for modern atomic theory.

Main Postulates of Bohr’s Model

Bohr proposed several key ideas to describe the structure and behaviour of electrons inside an atom:

  1. Electrons revolve in fixed orbits

Electrons move around the nucleus in fixed circular paths called orbits, shells, or energy levels. These orbits are stable and do not allow the electron to spiral into the nucleus.

Bohr denoted the orbits as K, L, M, N… or n = 1, 2, 3, 4…

Each orbit has a fixed radius and energy.

  1. Electrons do not lose energy while revolving

According to classical physics, moving charged particles should lose energy. But Bohr said that electrons in fixed orbits do not radiate energy. They remain stable in their shells without collapsing into the nucleus.

This explained the stability of atoms.

  1. Each orbit has a definite energy level

Electrons in different shells have different amounts of energy.

  • Electrons in inner shells have lower energy.
  • Electrons in outer shells have higher energy.

Because electrons cannot stay between shells, these energy levels are quantized.

  1. Electrons absorb or release energy when they jump between orbits

Electrons can move from one energy level to another only if they absorb or release a specific amount of energy.

  • If an electron absorbs energy, it jumps to a higher orbit (excited state).
  • If an electron releases energy, it falls back to a lower orbit (ground state).

The energy released or absorbed appears as spectral lines in the emission or absorption spectrum.

This helped explain the line spectrum of hydrogen.

  1. Energy difference determines spectral lines

The colour or wavelength of light emitted depends on the difference in energy between the two orbits. This concept is important in spectroscopy.

For hydrogen, each line in the spectrum corresponds to an electron transition.

Structure of Orbits in Bohr’s Model

Bohr labelled the shells as:

  • K-shell → n = 1
  • L-shell → n = 2
  • M-shell → n = 3
  • N-shell → n = 4

The maximum number of electrons that each shell can hold is given by the formula 2n².

For example:

  • K-shell: 2 × 1² = 2 electrons
  • L-shell: 2 × 2² = 8 electrons
  • M-shell: 2 × 3² = 18 electrons

This helped explain the arrangement of electrons in different atoms.

Successes of Bohr’s Model

Bohr’s model was a major advancement in atomic theory because it explained several things:

  • Stability of atoms
  • Electron arrangement into shells
  • Emission and absorption spectra
  • Hydrogen spectrum correctly
  • Quantum nature of energy levels

These successes made the model widely accepted for many years.

Limitations of Bohr’s Model

Although highly successful, Bohr’s model had some limitations:

  • It worked well only for hydrogen-like atoms (single electron).
  • It could not explain spectra of complex atoms.
  • Bohr assumed fixed orbits, but electrons behave like waves (as shown in quantum theory).
  • Did not explain electron spin or sub-shells.
  • Could not describe chemical bonding accurately.

These limitations led to the development of the modern quantum mechanical model.

Importance of Bohr’s Model in Modern Chemistry

Even though it is not fully accurate today, Bohr’s model is important because it:

  • Introduced the idea of energy levels
  • Helped develop quantum mechanics
  • Made atomic structure easier to visualize
  • Explained spectral lines for the first time
  • Formed a bridge between classical and modern atomic theory

It remains an essential part of chemistry education.

Conclusion

Bohr’s model of the atom describes electrons revolving in fixed shells around a central nucleus. Each orbit has a specific energy level, and electrons move between orbits by absorbing or releasing energy. Although it had limitations, Bohr’s model successfully explained the hydrogen spectrum and introduced the concept of quantized energy levels. It remains a crucial foundation for modern atomic theory and helps students understand the structure of atoms.