Bohr’s Atomic Model

In 1915, Niels Bohr proposed Bohr’s Atomic Model. This model was a modification of Rutherford’s atomic model. Bohr’s model of an atom is also known as the Rutherford-Bohr Model.

According to Rutherford’s model, a nucleus is positively charged and is surrounded by negatively charged electrons. Bohr modified this model by adding that the electrons travel in fixed circular orbits around the nucleus. No electron travels between these fixed orbits. Each orbital or shell has a definite energy.

What is Bohr’s Atomic Model?

Bohr’s model of an atom is similar to the planetary model. In Bohr’s model, electrons move in fixed circular orbits around a positively charged nucleus. The energy associated with each orbit is fixed. Each circular orbit has a fixed distance from the nucleus. 

Bohr’s model of an atom explains the electron’s properties in terms of allowed values. Bohr’s model can explain the absorption and emission of radiation when an electron makes a transition between different energy levels. In 1922, Bohr was awarded the Nobel Prize in physics for his work.

Why Did Bohr Modify Rutherford’s Model?

Niels Bohr modified the Rutherford’s atomic model and provided the world with Bohr’s Atomic Model due to the following limitations of Rutherford’s model:

• The electrons in Rutherford’s model travelled in simple circular paths (not fixed orbits). An accelerating electron emits radiation, so the electron must decay and fall into the nucleus. Rutherford’s model did not address this problem
• It failed to explain the position of the electrons
• It could not talk about the constituents of the nucleus. Rutherford’s model could only tell that the nucleus was positively charged

Postulates of Bohr’s Atomic Model

• The electrons move around the nucleus in definite circular orbits. These orbits are also called ‘shells’ or ‘energy levels.’
• Each orbit is stationary and has a definite energy. These circular orbits or shells with definite energy are called ‘orbital shells.’
• Bohr labelled these orbits or energy levels by the quantum number ‘n.’ The first shell, closest to the nucleus, is designated by the alphabet K. The second shell is designated by L, the third by M, and the fourth by N and so on. The K, L, M, N… can accommodate a maximum of 2, 8, 18, 32… electrons respectively. The lowest energy level is called the ground state
• Electrons do not radiate energy while revolving in these shells of fixed energy. The energy is radiated or absorbed only when the electrons transit from one energy level to another

When an electron jumps from a higher energy level to a lower energy level, it loses some of its energy. But when an electron moves from a lower energy level to a higher energy level, it gains some energy (or photons). Energy is emitted or absorbed in discrete quantities known as ‘quanta.’

Distribution of Electrons in Orbits

According to Bohr’s Atomic model, the number of electrons that an orbit or shell can hold can be calculated using the formula: 2n2. Note- ‘n’ is the number of the orbit or shell. For K-shell n=1, for L-shell n=2, for M-shell n=3, and so on.

The K-shell can hold a maximum of 2 electrons, the L-shell can hold a maximum of 8 electrons, and so on.

Quantization of Energy

Quantities with certain specific values are known as quantized. In Bohr’s model, each shell has a fixed value of energy. An electron in a shell can have a certain value of energy associated with that particular shell. Thus, the energy of an electron is said to be quantized.

When an electron makes a transition between energy levels, the emitted or absorbed energy is in a discrete quantity known as ‘quanta’ or ‘photon.’

Bohr’s model of an atom was the first model to incorporate the quantization of energy. It gave results similar to those obtained experimentally for hydrogen atoms. However, Bohr’s model had certain limitations.

Limitations of the Bohr’s Atomic Model

Bohr’s atomic model explains many features of the atomic theory, but it has its limitations. The problems with the Bohr model are listed below:

• It contradicts Heisenberg’s Uncertainty Principle. The consideration of the Bohr’s atomic model that electrons have both a known orbit and radius violates Heisenberg’s Uncertainty Principle
• It fails to predict the spectra of larger atoms and provides insufficient data for the spectra of smaller atoms
• It fails to explain the Zeeman Effect. Zeeman Effect is the splitting of spectral lines into several components in the presence of a magnetic field

• It does not explain the Stark Effect. The Stark effect is the splitting of spectral lines into various components in the presence of an electric field. The Bohr’s model of an atom fails to clarify this effect
• It fails to explain the shapes of molecules.

Bohr’s model can explain why electrons do not fall into the nucleus. Niels Bohr postulated that electrons revolve in fixed energy levels. The energy levels have discrete energies associated with them. Bohr’s model was the first one to incorporate quantum theory. 

The model was later modified by Sommerfeld due to its limitations. Sommerfeld’s model assumed the orbits of electrons to be elliptical rather than circular. Sommerfeld’s model could explain spectral effects, but it had limitations too. 

All the atomic models were replaced by W. Pauli’s model, which was based on quantum physics. In 1926, Erwin Schrodinger improved W. Pauli’s model.