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Bohr's Model Of An Atom

Bohr's Model Of An Atom

Shivani PooniaUpdated on 02 Jul 2025, 05:50 PM IST

Danish physicist Niels Bohr in the early years of the twentieth century endeavour to develop the progressive atom theory that not only revolutionized the outlook on atomic form but also formed the new quantum theories of modern times. Bohr’s version succeeded in overcoming the limitation of classical physics regarding the stability of atoms and the inherent line atomic spectra. At the center of Bohr’s model are several postulates suggested, in addition to the traditional tenets of classical mechanics.

This Story also Contains

  1. Bohr's Model and its Postulates
  2. Solved Examples Based On (Bohr's Postulate)-
  3. Conclusion
Bohr's Model Of An Atom
Bohr's Model Of An Atom

In this article, we will cover the concept of Bohr's model and its postulates. This concept falls under the broader category of Atomic structure, which is a crucial chapter in Class 11 chemistry. It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination (JEE Main), National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE, and more.

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Bohr's Model and its Postulates

Bohr’s model was proposed to be an improvement of the planetary model and was for the same reason developed; it included the electrons circling in orbits around the nucleus and in these orbits, they were only allowed to exist in certain fixed and discrete energy levels as opposed to the planetary model which predicted that electrons would move in free and smooth paths like planets around a star. These levels are quantized in the sense that they have been allocated specific values and an electron can only transfer from one level to another in quanta of energy equal to the difference between the two levels.

Bohr’s model also gave some explanation for the phenomenon that is known as stationary states, whereby the electrons in their stable orbits do not radiate as perceived by classical theories. This model was able to thoroughly account for the spectrum of hydrogen and also paved the way for something that turned out to be quantum mechanics. Even though there are significant weaknesses in Bohr’s model like the inability to explain the phenomena of multielectron atoms, this model is considered one of the important steps in the development of atomic theory as it gives a proper understanding of the electron behavior in an atom.

1. The electron in the hydrogen atom can move around the nucleus in a circular path of fixed radius and energy. These paths are called orbits, stationary states, or allowed energy states and are arranged concentrically around the nucleus. The force of attraction between the nucleus and an electron provides the centripetal force required by the electron to carry out the circular motion.

2. The energy of an electron in the orbit does not change with time. However, the electron will move from a lower stationary state to a higher stationary state when the required amount of energy is absorbed by the electron or energy is emitted when an electron moves from a higher stationary state to a lower stationary state

3. Energy can be absorbed or emitted when an electron transitions between two different orbits, and the frequency of the photon involved can be calculated using the formula:

|E1−E2|=hν

4. The angular momentum of an electron is quantized. In a given stationary state, it can be expressed as

L= mvr= nh/2$\pi$, n = orbit number

So only those energy states (or orbits) are allowed in which the above equation holds for the angular momentum.

Note: Bohr's model is only valid for Hydrogen species or electronic species that contain only a single electron.

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Solved Examples Based On (Bohr's Postulate)-

Example 1: Which of the following statements is incorrect for Bohr's model of an atom?

1) (correct) It is valid for a multi-electronic species

2) The angular momentum of an electron is quantized

3) The centripetal force of attraction required for circular motion is provided by the electrostatic force of attraction between the electron and the nucleus

4) Orbits have fixed energy and are referred to as stationary states

Solution:

Bohr's model is valid for only uni-electronic species.

All other given statements given in the options are correct.

Hence, the answer is the option (1).

Example 2:

Which of the following is incorrect for Bohr's model of an atom?
1) (correct) It is valid for multi-electronic species.
2) The angular momentum of an electron is quantized.
3) The centripetal force of attraction required for circular motion is provided by the electrostatic force of attraction between the electron and the nucleus.
4) Orbits have fixed energy and are referred to as stationary energy.

Solution

Bohr's model is valid for electronic species only.

Hence, the answer is the option (1).

Commonly Asked Questions

Q: What is the significance of the Bohr magneton in atomic physics?
A:
The Bohr magneton is a fundamental unit of the electron's magnetic moment in Bohr's model. It's important in understanding the magnetic properties of atoms and their interactions with external magnetic fields.
Q: How does Bohr's model explain the Balmer series in hydrogen's spectrum?
A:
Bohr's model explains the Balmer series as transitions of electrons from higher energy levels to the second energy level (n=2) in hydrogen atoms. The model accurately predicts the wavelengths of these spectral lines, providing strong support for its validity.
Q: How does Bohr's model contribute to our understanding of chemical reactions?
A:
While Bohr's model is primarily an atomic model, its concept of electron shells and energy levels provides a basic framework for understanding chemical reactions. It helps explain why certain elements are more reactive than others and forms the basis for more advanced theories of chemical bonding and reactivity.

Conclusion

The version that was presented by Bohr was able to explain the spectral traces of hydrogen. Bohr’s model of the atom can be viewed as a scientifically critical stage in the paradigm shift from the old paradigm of classical physics to the new approaches of the modern quantum mechanics paradigm. While Bohr’s model had some problems, such as its inability to deliv­er a qualitative description of multielectron atoms and the quantization of angular momentum, it was useful in develop­ing an early atomic model that helped accurately describe atomic spectra.

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Frequently Asked Questions (FAQs)

Q: What is the connection between Bohr's model and the development of laser technology?
A:
Bohr's model, by explaining how atoms emit light through electron transitions between energy levels, laid the groundwork for understanding stimulated emission – the key principle behind lasers. The model's description of excited states and energy level transitions is crucial for grasping how lasers function.
Q: How does Bohr's model relate to the concept of quantum confinement?
A:
While Bohr's model predates the concept of quantum confinement, it introduces the idea of electrons being confined to specific energy levels within an atom. This notion of confinement and discrete energy states is fundamental to understanding quantum confinement in modern nanoscale systems.
Q: How does Bohr's model contribute to our understanding of atomic absorption?
A:
Bohr's model explains atomic absorption by showing that atoms can absorb photons with specific energies, causing electrons to jump to higher energy levels. This process is the reverse of emission and helps explain why atoms absorb light at the same frequencies they emit it.
Q: What is the relationship between Bohr's model and the concept of quantum numbers?
A:
Bohr's model introduces the principal quantum number (n) to describe electron energy levels. This laid the groundwork for the development of additional quantum numbers (l, m, s) in more advanced models, which are necessary to fully describe an electron's state in an atom.
Q: How does Bohr's model explain the concept of metastable states?
A:
While Bohr's model doesn't directly address metastable states, it provides a framework for understanding them. Metastable states can be thought of as excited states with relatively long lifetimes, which can be visualized using Bohr's energy level diagrams.
Q: What is the significance of the Bohr-Kramers-Slater (BKS) theory?
A:
The BKS theory was an attempt by Bohr and colleagues to reconcile quantum theory with classical electromagnetism. Although ultimately unsuccessful, it played a crucial role in the development of quantum mechanics by highlighting the need for a more comprehensive theory of atomic behavior.
Q: How does Bohr's model explain the concept of quantized angular momentum?
A:
Bohr's model introduces the quantization of angular momentum by proposing that electrons can only exist in orbits where their angular momentum is an integer multiple of ħ (h-bar, or h/2π). This was a crucial step towards understanding the quantized nature of atomic properties.
Q: How does Bohr's model contribute to our understanding of chemical bonding?
A:
Bohr's model contributes to our understanding of chemical bonding by introducing the concept of electron shells and energy levels. This forms the basis for explaining valence electrons and how atoms combine to form molecules, even though more advanced models are needed for a complete understanding.
Q: How does Bohr's model explain the photoelectric effect?
A:
While Einstein explained the photoelectric effect, Bohr's model supports this explanation by showing that electrons in atoms exist in discrete energy levels. This quantization of energy aligns with the observation that light behaves as discrete packets (photons) in the photoelectric effect.
Q: What is the connection between Bohr's model and spectroscopy?
A:
Bohr's model provides a theoretical foundation for spectroscopy by explaining the discrete nature of atomic spectra. It shows how the unique spectral lines for each element result from electrons transitioning between specific energy levels, enabling element identification through spectral analysis.
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