Development of Bohr’s Atomic Model

Key Concepts

• Atomic model
• Bohr’s model of atom
• Shell and subshells

Introduction: 

Atomic models: 

Observations and experiments mentioned by some scientists suggested that Dalton’s indivisible atom is composed of sub-atomic particles carrying positive charge and negative charge. The major problems before the scientists could recognize after the discovery of sub-atomic particles were:  

1. Stability of atom. 

2. Comparing the elements behavior with both the physical and chemical properties. 

3. Formation of molecules combining with identical atoms. 

Atomic models were proposed to explain about the charged particles in an atom.  

Two of these models, one is proposed by J.J. Thomson and the other proposed by Ernest Rutherford Bohr’s model explains how electrons revolve.  

Explanation: 

After the Gold foil experiment, Rutherford proposed planetary model or nuclear model of atom, just like solar system, as sun is at the center of the solar system in the same way nucleus lies in the center of the atom and electrons  

To overcome the defects and drawback of Rutherford atomic model and the origin of lines of hydrogen spectrum was given by Bohr’s model. 

Bohr’s model of atom:  

Niels Bohr (1885–1962), a Danish physicist, he is the student and working under Rutherford, studied the regions surrounding the nucleus, which were now known to contain negatively charged electrons. 

In the year 1913, Neils Bohr was the first to explain quantitatively the features of Hydrogen atomic spectrum. Later he successfully explained the hydrogen spectrum and proposed the atomic model. 

Bohr’s postulates of atomic model: 

1. The electrons in the hydrogen atom move around the nucleus in a circular path or path with some fixed energies and radius. These paths are called orbits. 

2. He proposed that electrons surround the nucleus in specific energy “levels” or “shells.”

3. Each orbit is associated with definite amount of energy. So, these are also called energy levels or energy states. 

4. The energy of electron only changes when the electron moves from one shell to the another. 

5. Electron jumps from lower energy state to the higher energy state, when energy is observed. 

6. When energy is supplied to atom; the electron gets excited from the lower energy to the higher energy. As when the excited state becomes unstable, electron jumps back again to its original position. This we can finally say that, electron gets energy when it is excited. 

7.  Electron jumps from higher energy state to lower energy state, when energy is lost.

8. These are designed as 1, 2, 3, 4, ……. or K, L, M, N, …….  from the side of the nucleus. 

9. Here each shell is represented with ‘n’, here ‘n’ defines principal quantum number. 

10. Each shell or orbit, consists of subshells.    

11. The last shell or the outermost shell of the atom is called valence shell 

First shell which is close to the nucleus is n=1(K-shell) has one subshell (s) where ‘s’ can accommodate 2e^– 

Second shell, where n=2(L-shell) has 2-subshells (s, p), whereas ‘s’ can accommodate 2e^– and ‘p’ can accommodate 6e^– 

Third shell, where n=3(M-shell) has 3-subshells (s, p, d), where ‘s’ can accommodate 2e^– and ‘p’ can accommodate 6e^– and ‘d’ can accommodate 10e^– 

Fourth shell, where n=4(N-shell) has 4-subshells (s, p, d, f), where ‘s’ can accommodate 2e^– and ‘p’ can accommodate 6e^– and ‘d’ can accommodate 10e^– and ‘f ‘can accommodate 14e^– 

Advantages of Bohr’s model: 

1. It explains the stability of atom. 

2. It explains the origin of lines of hydrogen spectrum. 

3. It explains the energy level of atoms 

Limitations of Bohr’s model: 

1. He failed to explain the splitting of electrons with electromagnetic field. 

2. He failed to explain the shape of molecule. 

3. He could not explain the heavier atoms 

4. Failed to explain the dimensional structure or model of atom. 

5. He was not able to explain the dual nature of matter. 

Summary:

1. Niels Bohr (1885-1962), a Danish physicist, he is the student and working under Rutherford, studied the regions surrounding the nucleus, which were now known to contain negatively charged electrons.
2. The electrons in the hydrogen atom move around the nucleus in a circular path or path with some fixed energies and radius. These paths are called orbits.
3. These are designed as 1, 2, 3, 4, or K, L, M, N, from the side of the nucleus.
4. Each shell or orbit, consists of subshells.
5. He failed to explain the splitting of electrons with electromagnetic field.
6. He failed to explain the shape of molecule.