Know Bohr’s Atomic Model and its Postulates

Niels Bohr, a Danish physicist, published an atomic model that was extremely influential in quantum mechanics. He introduced the idea of an atom to describe Bohr’s model of the atom and never had any doubt that it would eventually be verified experimentally. His first postulate postulates the existence of electrons “in orbits’ ‘ around a positively charged nucleus. His second postulate states that there must be a limited number of electrons in each “orbital,” which he also called “shell.” His third postulate that electrons could exist in atoms and orbits explained the necessity of orbits for electrons in an atom. 

Bohr’s Theory

Bohr describes Bohr’s model of the atom by the ‘elements’ of a chemical element as an “electron” and an “atomic number” (A). An element has A electrons to balance its “missing hypercharge.” This was already known from chlorine (Cl), with the formula Cl, which means there are 8 electrons. The “electrons” in the atom are not fixed in their orbits; they can move freely around the nucleus. According to Bohr, this is what provides energy in the atom.

Describe Bohr’s Model of the Atom

Bohr describes Bohr’s model of the atom by the electron’s orbit as a “planetary” or “cloud-type” system because of its shape. The energy level at which an electron orbits around the nucleus is determined by the principal quantum number n, and this value is constant. It changes if a different principal quantum number n is used. The energy of an electron at a certain point in an orbit is determined by the square of its kinetic energy, the squared value of its velocity divided by the speed of light. The amount at which this exponent is 0 is called “fine structure constant,” and it plays an important role in chemistry.

Postulates of Bohr’s Atomic Model

Bohr’s Atomic Model First Postulate:

The size of an orbital is described by a number called “quantum number.” This quantum number has discrete values instead of classical physics (which is continuous).

Two electrons can never share an orbital, while with classical physics, they could.

Bohr’s Atomic Model Second Postulate:

Electrons must have the same number of “orthogonal” (equal) energy levels. The fixed nature of the orbits, which is the center postulate, is a progressive modification of Bohr’s original model. Bohr thought that the electrons could not be fixed in their orbits in his original model.

Bohr’s Atomic Model Third Postulate:

The electrons are quantized. We call an electron’s ” energy levels ” a continuum of possible orbits in classical physics, which we call an electron’s “energy levels.” In Bohr’s atomic model, the electrons have quantized energies. The energy level at which an electron will orbit is always determined by its quantum number n, and this value is fixed.

Bohr’s Atomic Model Fourth Postulate:

Electrons will always be found at a distance from the nucleus that is inversely proportional to their wavelength. Bohr’s atomic model fourth postulate:

As a result of this postulate, the “electron affinity” is proportional to the energy of an electron. Its wavelength is inversely proportional to its energy (as well as its photon energy).

Shell Model

This part of Bohr’s atomic model describes Bohr’s model of the atom. It was formulated to allow the orbitals to be divided into shells.

The energy of an orbital is determined by its principal quantum number “n” and by the shell in which it is located. If the energy of a specific orbital depends on the radius r, its radius becomes proportional to “k”.

Bohr’s atomic model describes electrons with discrete orbits, but shells or regions of different energies can be formed.

Pros and cons of Bohr’s Atomic Model:

The Bohr atom has demonstrated its efficacy in explaining the physical phenomena of atoms and molecules. However, there are a few defects in Bohr’s model, which at the same time constitute one of the main bases for developing a new model of the atom.

Bohr said that Planck’s quantum theory was not true and not suitable to describe atomic objects. He also said that relativity theory did not apply to atoms because it showed a difference between the energy and momentum of particles. He said that wave-particle duality was a myth. Because the photon is not a particle but an energy form, Bohr argued that it could not be the atom’s energy carrier (or momentum). However, Heisenberg had proven that photons are particles through his uncertainty principle, while Planck’s quantum theory did not predict any deviation from this principle.

Heisenberg’s uncertainty principle shows that we can not ignore the wave-like behaviour of the photon. For instance, when the photon illuminates an object, there is a change in the momentum of the photons, which means that they are waves. So Bohr’s theory is incorrect because he ignored the wave-particle duality of photons. 

Conclusion

Bohr’s atomic model, which describes the atomic model of the atom, and quantum mechanics have been criticized for many years. Many physicists are beginning to question Bohr’s model for various reasons. Some researchers think it is not complete and may be too simple or incomplete in some aspects. One example here is the question of the exclusion principle. Bohr’s model is based upon the assumption that electrons can not occupy the same quantum state, but more and more evidence shows that this assumption is not true even if an electron can occupy a quantum state.