Quantum Week: The Entry of Rutherford and Bohr In Quantum Physics.

Today is the second day of our Quantum Week. In the first article (click here to read), we discussed 3 major events that gave a blow to the old classical physics and showed that a new theory is required to explain what was happening at the tiniest level. Today we will learn the work of two eminent scientists, who had a major contribution in explaining the structure of the atom: Ernst Rutherford and Niels Bohr.

In 1896, J.J. Thomson discovered the electron. It was a turning point in the history of mankind as it was the first elementary particle to be discovered. Based on his discovery, Thomson tried to explain the structure of the atom. So using best of his knowledge, he developed the Plum Pudding model of the atom. In this model, the atom is considered to be a positively charged sphere with electrons embedded in it, just like plums on a pudding.

Thomson's Plum Pudding Model of Atom

This model of the atom had some unseen flaws in it. They were only noticed after a major experiment by his own student, Ernst Rutherford in collaboration with Geiger and Marsden. The name of the experiment was the Gieger-Marsden Experiment or the Rutherford's alpha scattering experiment. In this, alpha particles were bombarded on a gold foil kept at some distance. It was observed that the alpha particles were deflected by the gold foil in different directions. A very few alpha particles were deflected at an angle close to 180 degrees (this is opposite to the incident direction). These few alpha particles startled Rutherford.

If Thomson was right, then there would have been no deflection of the alpha particles. This is because the electrons that are embedded in the positive sphere of atoms are 8000 times lighter than the alpha particles. This was so unexpected that Rutherford later said, "It was the most amazing moment of my life. It was as if I fired a 15 inch shell on a tissue paper and it came back and hit me."

Using his observations, Rutherford developed his new model of the atom.According to his interpretation, an atom was mostly empty space, with most of its mass being concentrated at its centre in the form of positive nucleus. The electrons revolved around the positively charged nucleus in certain predictable paths known as orbits at very high speeds, just like the planets revolving around the sun. Due to this similarity, his model was also named as the planetary model of atom.

No doubt, Rutherford's model of atom was based on experimental observations, still it failed to explain certain things. The first thing it could not explain was the stability of an atom. According to Maxwell, any charge that accelerates in the presence of an electric field will lose its energy, consequently emiting radiations. Since, in Rutherford's model of atom, electrons are accelerated due to electric field produced by the nucleus, so they should also lose energy, and should collapse into the nucleus tracing a spiral path. However, the atom was observed to be quiet stable, which means that Rutherford's model didn't go well with Maxwell's theory. This was its first limitation. The second thing that Rutherford's model could not explain was the arrangement of electrons in their specific orbits and didn't tell anything about their energies. Moreover, electrons were supposed to be continuously losing energy, if this was true, then the atom should have been observed to emit radiations of continuous frequencies, but this was not the case. On the contrary, the atoms were only observed to emit radiations of certain discrete frequencies.

Rutherford-Bohr Model of an Atom

Due to these shortcomings, again there was a need for a modified model of atom and this is when Niels Bohr came to rescue. Bohr proposed his model of atom in 1915. Since the Bohr model is a modification of Rutherford's model of atom, so it is also known as the Rutherford-Bohr model.

Bohr proposed that the electrons are moving in fixed orbits around the nucleus, having a fixed set of energies.This explained the stability of an atom. The energy of an orbit is related to its size, ie. the nearest orbit to nucleus being the smallest one has the minimum energy, with the farthest one having the maximum energy. Bohr stated that the angular momentum of electrons revolving around the nucleus is quantized. Moreover, according to his model of atom, the electrons jumped from orbits of lower energy to those of higher energy by absorbing energy of certain frequencies and they emited radiations of certain frequencies while jumping from higher to lower energy states. This explained the fact that atoms only emited EM radiations of certain fixed frequencies. Bohr's model was also helpful in explaining the spectral emission lines of atomic hydrogen. 

Rutherford (Left)
Niells Bohr ( Right)

Even though, not all the early atomic models were accurate and some discrepancies existed, still all of them formed the basis of the quantum mechanics and played a key role in the development of quantum mechanics in one way or the other. Time and again, scientists are developing new and improved theories on what the atom looks like, still the most commonly used model of the atom till date is the Rutherford-Bohr model, thus making Rutherford and Bohr, two of the most important pillars of quantum theory!

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