Drawbacks of Bohr's Atomic Model

In this chapter, we will understand the drawbacks of Bohr’s atomic model in detail. Niels Bohr’s atomic model or theory successfully explained in a very systematic way the spectra of hydrogen atom and hydrogen like ions such as He⁺, Li²⁺, Be³⁺, etc.

He was also able to successfully calculate the radii and energies of the permissible orbits in the hydrogen atom. Therefore, the theory was largely accepted, and he was awarded the Nobel Prize in recognition of his work.

After a few years later, however, several drawbacks were observed in Bohr’s theory. Let’s understand these drawbacks.

Drawbacks of Bohr’s Atomic Model

The main limitations or drawbacks of Bohr’s atomic model or theory are as follows:

No explanation of fine structure: Bohr’s theory was successfully explained various spectral lines in the hydrogen spectrum. But, when hydrogen spectrum was observed using a high resolving power spectroscope, it is found that the individual spectral lines are not really single, but it is a collection of several very fine lines which are very close to one another. For example, the hydrogen-alpha (H_α) line in the Balmer series of hydrogen spectrum is found consisting of five component lines. Such a spectrum is called fine structure or multiple structure, which is made up of closely spaced spectral lines. Bohr’s theory could not explain this fine structure of spectral lines in the hydrogen spectrum.
No explanation for the spectra of multi-electron atoms: Bohr’s theory could successfully explain the origin of the spectrum of hydrogen atom and of hydrogen-like ions such as He⁺, Li²⁺, Be³⁺, etc. in which there is only one electron in the atoms or ions. But, he could not explain the spectra and energy levels of atoms containing more than one electron.
Inability to explain the three-dimensional model of atom: According to Bohr’s atomic model, electron moves in circular orbits around the nucleus in one planner. But, modern researches have proved that the motion of electron is not limited to a single plane but takes place in three-dimensional space. In other words, the atomic model is three-dimensional, not flat, as suggested by Bohr.
Inability to explain splitting of lines in the magnetic field: It has been observed that when an excited atom which produces a line emission spectrum is placed in a strong magnetic field, its spectral lines are further split up into a number of closely spaced lines. This type of splitting of spectral lines in the magnetic field is called Zeeman effect.
Inability to explain splitting of lines in the electric field: Another identical splitting of spectral lines into a number of thinner lines is also observed when the excited atom is put in an electric field. This types of splitting of spectral lines in an electric field is called Stark effect. Bohr’s theory was no explanation for this splitting of spectral lines.
Inability to explain the dual nature of electron: Bohr had assumed that an electron behaves as a material particle of small mass moving around the nucleus in fixed circular orbits. But the French physicist, de Broglie in 1923, suggested that an electron has a dual nature, i.e. an electron behaves both as a particle as well as a wave nature (like light).
Inability to explain Heisenberg’s uncertainty principle: Bohr has assumed that an electron, which is a material particle, revolves around the nucleus in circular orbits situated at a fixed distance from the nucleus and with a definite velocity. But, according to the Heisenberg’s uncertainty principle, the exact position and exact momentum of a small moving particle like electron cannot be determined simultaneously with absolute accuracy. This was probably the biggest drawback or objection of Bohr’s atomic model.
Bohr’s atomic model could not explain the geometry and shapes of molecules.

These are the main drawbacks of Niels Bohr’s atomic model that we have discussed in detail.

Limitations of Bohr’s Model of Atom in Brief

Niels Bohr’s model of atom:

could not explain the spectrum of atoms other than hydrogen atom.
could not explain the ability of atoms to form molecules by chemical bonds.
was unable to explain the splitting of spectral lines in the presence of magnetic field (called Zeeman effect).
was unable to explain the splitting of spectral lines in the presence of electric field (called Stark effect).