Thomson’s and Rutherford’s atomic models failed to answer any atom’s energy and stability problems. Niels Bohr suggested an atomic structure model, commonly known as Bohr’s atomic model, explaining an atom as a tiny, positively charged nucleus surrounded by electrons which move in circular orbits all around positively charged nucleus such as planets around the sun in our solar system, with attraction supplied by electrostatic forces. It was essentially an upgraded version of Rutherford’s atomic model that overcame its shortcomings. On the majority of topics, he agrees with him, such as the notions of nucleus and electrons surrounding it.

The Path To Bohr’s Atomic Model

Previously, studies from radiation interactions with matter offered a plethora of information on the structure of atoms and molecules. Niels Bohr utilised these observations to elaborate on Rutherford’s concept. Two breakthroughs were crucial in the development of Bohr’s atomic model. These were: electromagnetic radiation’s dual nature suggests that it has both wave-like and particle-like properties; and only by assuming quantised electronic energy levels in atoms could experimental data about atomic spectra be described.

Bohr’s Atomic Model

To address the criticisms levelled about Rutherford’s atomic model, Neils Bohr proposed the following postulates concerning the atomic model:

  • Only specific unique orbits known as discrete orbits of electrons are permitted inside the atom.
  • Electrons do not emit energy when rotating in distinct orbits.

In Niels Bohr’s atomic theory, electrons with fixed sizes and energies travel in orbits around a positively charged nucleus, similar to how planets orbit the sun. To summarise Bohr’s atomic model, electron energy levels are focussed on the size of the orbits. Electrons in smaller orbits will have less energy as a result. In addition, atoms are unstable because electrons shift to lower orbits, causing radiation to be produced. As a result, an atom in the smallest orbit will be stable since the electron will not hop to a lower orbit. As a result, it was proposed that an electron might travel between these orbits by absorbing or emitting photons (energy).

Bohr’s Model of a Hydrogen Atom

Neils Bohr (1913) was the first to statistically explain the general properties of the structure and spectrum of the hydrogen atom. He applied Planck’s notion of energy quantification. We may use the model to justify much atomic structure and spectra points, though not current quantum mechanics. The following postulates underpin Bohr’s model of the hydrogen atom:

  • The electron in a hydrogen atom can travel in a circular route with a set radius and energy around the nucleus. Orbits, stationary states, and permitted energy states are all names for these routes. These orbits are arranged in a concentric circle around the nucleus.
  • The energy of an electron in orbit remains constant over time. However, when the electron or energy absorbs the requisite quantity of energy is released when the electron goes from a higher stationary state to a lower stationary state, the electron will move from a lower stationary state to a higher stationary state. The shift in energy does not occur continuously.
  • The frequency of radiation received or released when a transition occurs between two stationary states with ΔE energy differences.
  • An electron’s angular momentum is quantised. It may be defined as a function of electron mass, velocity, and the radius of the orbit where the electron is travelling in a particular stationary condition.

The Bohr model could not explain atoms with more than one electron, such as lithium and helium. This concept was only relevant to atoms with one electron. Bohr’s hypothesis explained only spherical orbits. Elliptical orbits remained unexplained.

Failures of Bohr’s Atomic Model

Bohr’s concept of the hydrogen atom was unquestionably superior to Rutherford’s nuclear model in that it could account for the stability and line spectra of hydrogen atoms and hydrogen-like ions. However, Bohr’s model was unable to take into consideration the following facts:

  • It violates the Heisenberg Uncertainty Principle. According to Heisenberg, electrons cannot have both a known radius and an orbit, i.e. known location and momentum simultaneously, according to the Bohr atomic model theory.
  • When smaller atoms like hydrogen are examined, the Bohr atomic model theory makes proper predictions, but poor spectral predictions are achieved when bigger atoms are included.
  • It was unable to explain the Zeeman effect, which occurs when a spectral line is fragmented into many components in the presence of a magnetic field.
  • It was unable to explain the Stark effect, which occurs when a spectral line is broken into fine lines in the presence of an electric field.

In other words, considering the arguments raised above, a better theory that can explain the essential aspects of the structure of complex atoms is required.

Conclusion

Bohr’s atomic model was an advancement of Thomson’s model, but there were some reasons for the failure of Bohr’s atomic model. Bohr’s atomic model can explain the arrangement of subatomic particles and their movements in orbits. It does not validate the precise intricacies in the hydrogen atom spectrum recorded by advanced spectroscopic methods. This model is also incapable of explaining the spectrum of atoms other than hydrogen, such as the helium atom, which has only two electrons. Furthermore, Bohr’s theory was incapable of explaining spectral line splitting in the existence of a magnetic field (Zeeman effect) or an electric field (Stark effect). Finally, it could not account for atoms’ capacity to create molecules through chemical bonding.