Shortcomings of Bohr’s Atomic Model

Bohr’s atomic model explains that the negatively charged electrons orbit around the positively charged protons in the nucleus. The energy levels determine how the electrons move. Bohr assumed that the electrons travel in orbits at a certain radius. It was later proved wrong by Werner Heisenberg in his uncertainty principle. Bohr’s model also does not align with Zeeman and Stark’s effects.

Facts about Bohr’s atomic model

• This atomic model was given by a physicist, Niels Bohr, in 1913.
• Bohr’s model is derived using the classical laws of physics and quantum theory of radiation.
• It gives a new perspective on atomic structure.
• It explains the stability of an atom.
• It states that the electrons travel/orbit around the nucleus.
• The orbits are quantum shells. The first shell is represented as 1s2; the second shell is designated as 2s2, 2p6.
• Electrons can jump from one orbit to another. They emit energy when they leave an orbit and absorb energy when they enter an orbit.
• Bohr’s model explains the quantum theories and valence shell theories. 
• It also explains the formation of spectrum lines in hydrogen.

Issues with Bohr’s model

• Bohr’s model does explain the formation of spectrum lines in other atoms; it is limited to hydrogen only. Because different elements are larger atoms, they contain more than one electron. They do not undergo electron correlation effects.
• Bohr’s theory explains spherical orbits, not elliptical orbits.
• Bohr’s model cannot explain the Zeeman effect and the Stark effect.
• It cannot explain the intensity of spectral lines.
• This model disagrees with the later discoveries that showed an electron could be a wave and a particle.
• It is unable to explain the 3D model of an atom.
• It cannot describe the shapes of molecules.

Uses of Bohr’s atomic model

• Bohr’s atomic model explains how an atom becomes stable. An electron does not lose its energy unless it leaves its orbit. 
• It also explains the line spectrum of hydrogen formed when energy is passed through electrons (heat or electricity). 
• Electrons get excited and jump from their orbits to higher energy levels. When the energy is used up, they bounce back to lower energy levels, as they are no more excited. 
• When electrons jump, energy is emitted in the form of a photon of light. Each photon draws a wavelength that appears as a line in the spectrum.

Steps to draw Bohr’s model

1. Draw the nucleus.
2. Note down the number of neutrons and protons in the nucleus.
3. Draw the first energy level and the electrons in that shell.

Here is an example of the hydrogen atom. It has one electron, one proton, and 0 neutrons.

Advantages of Bohr’s model

• Bohr’s atomic formula helps explain the Balmer formula.
• It shows that the difference between orbits is due to their light energies. 
• Bohr’s model explains the value of Proportionality R’s, which is a constant.
• Bohr’s model can be used for single-electron systems other than hydrogen. Some examples are given below:

1. He+: Helium has a charge of 1+, which means it can accept one electron. The atomic number of helium is 2. Thus, it needs two electrons to become a stable atom.  
2. Li 2+: Lithium has a charge of 2+, which means it needs two electrons to become stable. The outer shell of Lithium contains a single electron. The atomic number of Lithium is 3.

Isotopes

Isotopes are atoms that have different numbers of neutrons but the same amount of protons. Due to this, their mass number is also different. Each isotope has its unique properties. The difference between two isotopes of the same element can be that one can cure a terminal disease while the other can be the reason for it. 

Some isotopes are radioactive, which means they decay very fast. There are also stable isotopes that do not decay at all or decay very slowly.  

Some common isotopes and their uses

• Phosphorus 32 (32P): This isotope of phosphorus is used for detecting cancer of the eyes or skin. 
• Iron (59Fe): Iron isotope is used in anaemia diagnosis. 
• Cobalt-60 (60Co): This isotope of cobalt is used in the gamma irradiation of tumours.
• Iodine (131I): This isotope of iodine is used in diagnosing and treating thyroid infections.
• Gold (198Au): This isotope of gold is used to diagnose liver diseases.

Conclusion 

• Bohr’s model shows spectrums of atoms that have only a single electron.
• Bohr’s model shows that electrons revolve around the nucleus at fixed energy levels. Orbits away from the nucleus are at higher energy levels than those near the nucleus. 
• When electrons jump to lower energy levels, they emit energy in the form of light. 
• The charge of electrons in Bohr’s formula is in the 4th power of the shell. The energy of an electron is related to the size of its orbit. 
• Smaller the orbit, the lower the energy of an electron. Similarly, the larger the orbit, the higher the energy of an electron.