All about Bohr’s model of an atom

Danish physicist Neils Bohr introduced the Bohr’s model of an atom in 1915. This discovery was the key to the journey towards understanding atoms. According to Bohr’s model, also called the Rutherford-Bohr model, an atom comprises a small positively charged nucleus orbited by negatively charged electrons. The modern Bohr’s model is built on quantum mechanics. There is also some error in Bohr’s model, but it is still considered necessary as it describes many features of atomic theory without complex-level maths. Rydberg’s formula is explained by Bohr’s model of an atom. Bohr’s model is a modification of Rutherford’s model. In atoms, electrons surround the nucleus. Orbits have energy shells or energy levels. Bohr’s model applies to species that contain only one electron. 

Introduction to Bohr’s model 

Bohr’s model is a modification of the Rutherford model of atoms. According to the Rutherford model, the nucleus is a positively charged boundary by negatively charged electrons. There is a path in which electrons move around them, which are called orbits. Bohr described that in stable orbits, electrons move around them. He also described that the orbits or shells consist of fixed energy levels. The association of different energy levels between them and electrons was explained by Bohr. In Bohr’s atomic model, a positively charged nucleus is surrounded by rotating negatively charged electrons in stable orbits. He concluded that if an electron is placed far away from the nucleus then it has more energy, but if it places near to the nucleus then it will have less energy. 

Postulates of Bohr’s atomic model

• Orbits or shells or energy levels are present, which means electrons rotate around the nucleus in a stable circular path. 
• The orbits are also known as stationary orbits.
• Every orbit will have a definite amount of stable energy and these orbits are called orbital shells. If in a stable orbital shell, electrons continuously rotate around the nucleus then they do not emit energy. 
• Integers (such as n=z1, n=2, n=3) were used to donate different energy levels. And these integers are also called quantum numbers. Quantum numbers can vary from the smallest energy level to the highest energy level.
• Energy levels orbits are considered in two different ways—such as 1,2,3,4 or K, L, M, N shells. Electrons’ lowest energy level is well known around electrons moving from one energy level to another energy level then a change takes place. When an electron moves from a higher energy level to a lower energy level that means it loses energy. 

More about orbits

• The first orbit or energy level is also known as the K shell and it can  hold 2 electrons.
• The second orbit or energy level is also known as the L shell and it can hold up to 8 electrons.
• The third orbit or energy level is also known as the M shell and it can hold up to 18 electrons.
• The fourth orbit or energy level is also known as the N shell and it consists of a maximum number of 32 electrons. 

Limitations of Bohr’s model

• Bohr’s model can not detect the Heisenberg uncertainty principle.
• It does not make any prognosis of large-sized atoms.
• It can not explain the Zeeman effect (magnetic field effect on atoms spectra).
• It can not explain the stark effect (electric field effect on atoms spectra).
• Spectra acquired from larger atoms can not be explained by Bohr’s model of an atom.

Conclusion

After the discovery of hydrogen spectra and the photoelectric effect, Danish physicist Neils Bohr introduced a new atomic model in 1915. He described that electrons do not emit energy, if, in a stable orbital shell, electrons continuously rotate around the nucleus. They can exist in a constant state called a stationary state. Bohr’s work primarily depends on hydrogen spectra emission. Bohr also got the Nobel Prize in 1922 for his work on the model of an atom. He describes that electrons can move in different orbits. The Bohr atomic model is the modification of the Rutherford model and after modification, he described that the orbital shell consists of stable energy levels. In the Bohr model, a positively charged nucleus is surrounded by rotating negatively charged electrons in stable orbits. Orbits can also be termed stationary orbits. Energy levels or orbits can be represented in two different ways i.e. 1,2,3,4 or K, L, M, N shells. This model also has some limitations. It can not explain the Heisenberg uncertainty principle, Zeeman effect and Stark effect and it also does not explain spectra acquired from larger atoms.