Structure of Atom

Bohr’s Model of Atom

Quantum mechanics, Quantum physics, the theory of relativity, etc are the modern subjects that interests, astound, and confuse almost everybody. These topics form the basis of modern physics. However, the very first-time quantum theory was incorporated in Bohr’s Model of an atom or Bohr atomic model. Later this model became the predecessor of complete quantum mechanical models.

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The physicist Niels Bohr said, “Anyone who is not shocked by quantum theory has not understood it.”  He also said, “We must be clear that when it comes to atoms, language can only be used as in poetry.” So what exactly is this  Bohr atomic model? Let us find out!

Bohr atomic model and the models after that explain the properties of atomic electrons on the basis of certain allowed possible values.  The model explained how an atom absorb or emit radiation when electrons on subatomic level jump between the allowed and stationary states. German-born physicists James Franck and Gustav Hertz obtained the experimental evidence of the presence of these states.

Bohr Atomic Model

 

A Danish physicist named Neil Bohr in 1913 proposed the Bohr atomic model. He modified the problems and limitations associated with Rutherford’s model of an atom.  Earlier in Rutherford Model, Rutherford explained in an atom a nucleus is positively charged and is surrounded by electrons (negatively charged particles).

Learn about Rutherford’s Atomic Model here in detail.

The electrons move around in a predictable path called orbits. Bohr modified Rutherford’s model where he explained that electrons move around in fixed orbital shells. Furthermore, he explained that each orbital shell has fixed energy levels. Therefore, Rutherford basically explained a nucleus of an atom whereas Bohr took the model one step ahead. He explained about electrons and the different energy levels associated with it.

According to Bohr Atomic model, a small positively charged nucleus is surrounded by revolving negatively charged electrons in fixed orbits. He concluded that electron will have more energy if it is located away from the nucleus whereas the electrons will have less energy if it located near the nucleus.

Bohr atomic model

Bohr’s Model of an Atom (Source Credit: Britannica)

Postulates of  the Bohr Atomic Model

  • Electrons revolve around the nucleus in a fixed circular path termed “orbits” or “shells” or “energy level.”
  • The orbits are termed as “stationary orbit.”
  • Every circular orbit will have a certain amount of fixed energy and these circular orbits were termed orbital shells. The electrons will not radiate energy as long as they continue to revolve around the nucleus in the fixed orbital shells.
  • The different energy levels are denoted by integers such as n=1 or n=2 or n=3 and so on. These are called as quantum numbers. The range of quantum number may vary and begin from the lowest energy level (nucleus side n=1) to highest energy level. Learn the concept of an Atomic number here.
  • The different energy levels or orbits are represented in two ways such as 1, 2, 3, 4…  or K, L, M, N….. shells.  The lowest energy level of the electron is called the ground state. Learn the concept of Valency here in detail.
  • The change in energy occurs when the electrons jump from one energy level to other. In an atom, the electrons move from lower to higher energy level by acquiring the required energy. However, when an electron loses energy it moves from higher to lower energy level.

Therefore,

  • 1st orbit (energy level) is represented as K shell and it can hold up to 2 electrons.
  • 2nd orbit (energy level) is represented as L shell and it can hold up to 8 electrons.
  • 3rd orbit (energy level) is represented as M shell and it can contain up to 18 electrons.
  • 4th orbit (energy level) is represented as N Shell and it can contain maximum 32 electrons.

The orbits continue to increase in a similar manner.

Watch and Learn more about Modern Atomic Theory

Distribution of Electrons in Orbits or Shells:

Electronic distribution of various orbits or energy levels can be calculated by the formula 2n2. Here, ‘n’ denotes the number of orbits.

  • The number of electrons in K shell (1st orbit) can be calculated by 2n2= 2 x 12 = 2. Thus, maximum number of electrons in 1st orbit = 2
  • Similarly, The number of electrons in L shell (2nd orbit)= 2 x 22 = 8. Thus, maximum number of electrons in 2nd orbit = 8

We can determine the maximum number of electrons in a similar way.

Read about Thomson’s Model of an Atom, the very first model of an Atom by J.J. Thomsons.

Limitations of Bohr’s Model of an Atom:

Bohr atomic model had few limitations. They are:

  • Failure to explain Zeeman Effect (how atomic spectra are affected by magnetic fields).
  • It contradicts Heisenberg Uncertainty Principle.
  • Unable to explain how to determine the spectra of larger atoms.

What are Isotopes? Learn the concept of Isotopes and Isobars.

Solved Examples for You

Example 1: Calculate the maximum number of electrons an o shell can hold.

Solution: We know that O shell means 5th shell.
Therefore, n=5. Applying the formula 2n= 2 x 52 = 50
Thus, the maximum number of electrons O shell can hold is 50.

Example 2: What happens when an electron changes its orbit from outer to inner energy? Energy remains constant

  1. absorbed
  2. no change
  3. released

Solution: The answer is 4. Energy is released when an electron jumps from higher to lower energy level.

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