Causes that Led to the Failures of Bohr's Atomic Model

Atomic theory was the best way to approach the atom until it was overthrown by quantum mechanics. Bohr’s atomic theory is still widely used today, but many different models were tried that failed to make quantum mechanics work to be a better explanation of atoms. Some of those models included Niels Bohr’s revised atomic model, Erwin Schrödinger’s equation, Max Planck’s constant h, and Rutherford’s experiments. A basic understanding of the atomic model is necessary to understand the failure of Bohr’s atomic model. Bohr’s atomic model was based on an earlier model proposed by John Dalton in the early 19th century.

What is Bohr’s Atomic Model?

The atomic model of Niels Bohr was proposed in 1913. It was based on Dalton’s work, which provided the most important step to Bohr’s model because it allowed Bohr a feasible way to analyze atoms based on the smallest possible unit of an atom. The atomic model is a framework that determines all characteristics and interactions of the electron by its distance from the nucleus. The nucleus acts as a potential barrier for an electron so that it can only transfer its orbital motion around the atom in one direction.

Postulates of Bohr’s Atomic Model

In order to keep the atom stable, Bohr made the following postulates:

The electrons are restricted to specific orbits or energy states.
Each energy state can only hold a specific amount of energy; once an electron has gained or lost the amount of energy dictated by its current state, it needs to move onto a higher or lower level—an electron cannot stay in one state for long.
The electrons can only jump from one level to another in a discrete amount of time.
A single energy state can only be occupied by one electron at a time.
The probability that an electron will emit or absorb a photon (i.e., to move from one state to another) is related directly to Planck’s constant and the energy difference between the two-state levels involved in the transition.
The electron can only absorb radiation of certain energy and only emit radiation of a higher energy level. The amount of time that the electron is in each state determines the probability that it will stay in that state or jump to another level.
The transition between levels takes place with an extremely sharp wavefront called a wave-packet.

Success and Failure of Bohr’s Atomic Model

The success of Bohr’s Atomic Model

Bohr’s atomic model explained why the Balmer-Rydberg formula worked so well with spectral lines. He was able to apply this knowledge to every other line in the hydrogen spectrum and later extended it to all of the elements. It also predicted that atoms could only emit or absorb certain wavelengths of light, which was proven correct in 1923 by Robert Andrews Millikan.

Failure of Bohr’s Atomic Model:

Bohr’s model performed well until it was noted that the helium atom did not show Balmer-Rydberg lines, and others did not fit the model correctly. However, there were not many reasons behind the failure of Bohr’s atomic model. In 1925, Rutherford came up with the idea that electrons in an atom were knocked off or kicked out of the nucleus to create a cloud of negatively charged particles called electrons or a positron. This meant that no electron could be found in orbit around the nucleus, and electrons must be emitted from the nucleus as positrons. The occurrence of this type of decay can be predicted by the Schrödinger wave equation.

Factors behind Failure of Bohr’s Atomic Model:

Following are some factors that led to the failure of Bohr’s atomic model.

It had a hard time explaining why some spectral lines are not exactly 100% accurate.
It was unable to explain the different spectra lines given off by gasses of different atoms or molecules (Balmer-Rydberg formula).
It does not accurately describe the structure of atoms with more than one electron.
It is not able to describe different types of atoms, such as alkanes and halogens.
It does not accurately describe the energy of atomic transition
Even though electrons are restricted to specific orbits, Bohr’s model does not explain why atoms absorb and emit light in such a way that can be easily determined by ear or eye (phi quantities).
Finally, it does not explain the relationship between nuclear events and radiation.

Conclusion

It was Planck, not Bohr, who won the argument pertaining to the emission of radiation by an electron in orbit around a nucleus. However, Bohr’s model of the atom plays a large part in quantum theory today because it introduced the idea that electrons are restricted to specific orbits or energy states. The failures of Bohr’s atomic model can be attributed to the fact that it had a hard time explaining why some spectral lines are not exactly 100% accurate. It is a model which explains all of the characteristics and interactions of an electron by its distance from the nucleus.