Quantum Mechanical Model of Atom for NEET, JEE & CBSE

Quantum mechanical model of the atom

The first scientists who attempted to comprehend the atom represented it as something that was tidy and predictable. They even compared it to solar systems with tiny planets having regular orbits around the Sun. Theories like Rutherford's and Bohr's were incremental milestones, but eventually, experiments contradicted the idea of atoms acting in a very neat and mechanical way. In very small dimensions, particles act in a manner that is opposite to the daily experience. Quantum mechanics becomes very much involved at this point. The quantum mechanical model of the atom is not a mere coincidence; it is the outcome of deliberate experiments and mathematical reasoning, hence it is the most precise representation of atomic structure that we possess at present.

Why a New Model Was Needed

The Bohr model was sufficient for explaining the spectrum of hydrogen; however, it could not be used for multi-electron atoms. Furthermore, it presupposed that electrons revolved around the nucleus in fixed, circular paths, a hypothesis that was soon discredited by experimental work. The dilemmas that the electrons were viewed as behaving both as particles and as waves at the same time were the result of scientists’ investigations. Classical physics could not explain this dual behaviour. As a result, a completely new way of thinking was required, leading to the development of the quantum mechanical model.

Basic Ideas of the Quantum Mechanical Model

The quantum mechanical model does not describe the exact path of an electron. Instead, it focuses on the probability of finding an electron in a particular region around the nucleus. In this model:

Electrons do not move in fixed orbits.
The exact position and momentum of an electron cannot be known simultaneously.
The behaviour of electrons is described using wave equations.

Now this may seem strange at first, but in fact it expresses the way things really are at the level of atoms.

Dual Nature of Electrons

One of the most important ideas behind this model is that electrons have a dual nature. They behave as particles when they collide with other particles, but they behave as waves when they move. It was confirmed by experiments such as electron diffraction. It is for this wave-like character that electrons are expressed in terms of wave functions, rather than existing as simple curves in space.

Heisenberg’s Uncertainty Principle

The Uncertainty Principle, which was proposed by Heisenberg, is one of the major principles in quantum physics that has pointed out the impossibility of fully determining an electron’s position and momentum at the same time. One property becomes less known the more knowledge there is about the other. This limitation is not caused by the imprecision of the tools used for measurement, but rather it is a characteristic of the natural world. Hence, the concept of a fixed orbit goes out of the question in the quantum realm.

Schrödinger Wave Equation

The Schrödinger wave equation is the primary foundation of the quantum mechanical model. Nevertheless, despite the fact that the math is extremely intricate, it is still feasible to grasp the implications of the equation in plain language. The equation gives a wave function (ψ) for an electron, and the square of the wave function (ψ²) expresses the probability of finding the electron in a given volume of space. Therefore, instead of orbits, we use "orbitals" now, which indicate the areas around the nucleus where the electron is most likely to be found.

Atomic Orbitals

An orbital is not a path but a three-dimensional region around the nucleus. Different orbitals have different shapes and energies.

s-orbitals are spherical in shape.
p-orbitals are dumbbell-shaped.
d- and f-orbitals have more complex shapes.

Each orbital can accommodate a maximum of two electrons with opposite spins. The arrangement of electrons in these orbitals explains the chemical behaviour of elements.

Energy Quantisation

Another important feature of the quantum mechanical model is that electron energy is quantised. This implies that electrons can only possess certain energy levels and not just any level by chance. An electron, after absorbing energy, moves to a higher energy Orbital, and when it loses energy, it comes back to a lower one. This provides a much better explanation of atomic spectra compared to earlier models.

Importance of the Quantum Mechanical Model

The model accounted very well for atomic spectra, chemical bonding, periodic trends, and the stability of atoms. Also, it constituted the basis of modern chemistry and physics, like spectroscopy, semiconductor technology, and quantum chemistry. The model provided a basis for our complete understanding of atoms.

Conclusion

The quantum mechanical model of the atom is a radical new concept in science. It no longer considers electrons to be located in definite orbits. Still, it acknowledges the uncertainty and probability present in the atomic realm as the very nature of the atomic world. To ease our understanding of the situation, the mathematics of the quantum mechanical model is complicated, but the main point is that electrons are not in specific locations; they are just in regions where they are most likely to be found. The model does more than match the experimental results; it also gives us a grasp on the chemical properties of the substances at the most basic level.

F.A.Q.

1. Why do orbitals replace electron orbits?

Since electrons do not move through fixed pathways, they are probability-based regions of plotting called orbitals.

2. Is the quantum mechanical model better than Bohr’s model?

Yes. Because a neutral atom contains N number of protons, neutrons, and electrons, and since electron numbers are the same in an ion, atomic fractions ought to relate to the number of protons, neutrons, and electrons.