AZ Chemistry

Atomic Theory by Rutherford – Model, Experiment, and Limitations

Atomic theory by Rutherford is the next atomic theory after the J.J. Thomson theory. We know that after the discovery of Dalton, the scientist were able to develop some great theories and experiments to help us understand the chemical models of atom. After Thompson’s atomic theory, Ernest Rutherford, and many more the experiment is held by any other scientists. Now, we will learn about the atomic theory by Rutherford.

In 1911, he performed the experiment using the alpha particles. Rutherford believed that there were the positive charge nucleus in the center of atom and negative charge electron around the nucleus. The nucleus finding by Rutherford is really helpful to understand the chemistry study, especially the chemical models of atom. This article will cover the atomic theory by Rutherford including its experiment, theory, limitations and Rutherford’s life.

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The Prior Study of Atomic Model

In 18977, J.J Thomson successfully discovered the electron part in the atom. He did the experiment using the cathode ray. He represented the cathode rays as the negative charge. Based on this experiment, he released the concept of atom model as the plum pudding. The raisins represented the negative charge electron while the dough represented the positive charge of atom.

He believed that the mass of the atom was uniformly distributed. He also concluded some electron properties including electron’s moving path in straight line, electron’s kinetic energy and other useful points. However atomic theory by J.J. Thomson has some limitations. The plum pudding model proposed by the J.J. Thomson was lack of experiment proves. He could not describe his point about the uniform mass distribution of atom in the exact experiment.

Based on this fat, other scientists tried to conduct more experiment to test his atomic model. They tried to prove that the atomic model of plum pudding was actually not true in describing the real model of atom. One of the successor of atomic theory of JJ Thomson was Rutherford. He then did the great experiment which explained the flaws in the JJ Thomson’s atomic model.

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Atomic Experiment by Rutherford

We know previously that atomic theory by J.J. Thomson had some limitations and also failed to describe some experiment results about the atom model. Up to this fact, Ernest Rutherford performed the experiment to discover the true atom model. After conducting his experiment, he observed the result of his experiment and created the new model of atom which we know as atomic theory by Rutherford.

In his experiment, he did bombarding the thin gold sheet with the alpha particles and saw the directions of alpha particles after made interaction with the gold sheet. He got the alpha particles from the radioactive source and these particles attacked the gold foil with the thickness of only 100 nano meter. This experiment resulted that most of the alpha particles passed through the gold foil.

However, there were few alpha particles which are deflected back. To observe the cause of this deflection, he put the screen of fluorescent zinc sulfide around the gold foil. Any alpha particles which hit the fluorescent screen, it would make the light burst or scintillation. This kind of light burst can be seen with the microscope which was placed in the back of the screen. This screen itself could freely move so that it can detect any alpha particles which were deflected by the thin gold foil. Then, he made the report about his experiment which opposed the atomic model of J.J. Thomson.

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Atomic Model by Rutherford

Based on the experiment that he performed, Rutherford concluded some points regarding the structure of atom as follow:

The major part of the alpha particles which was struck to the gold foil would pass through it without deflections. It showed that the most part of atom is the empty space
There were some parts of alpha particles which were deflected by gold foil in small angles. It showed that there were positive charge in the atom and they were not in uniform distribution. The positive charge in atom is placed in the small volume.
Only small parts of alpha particles were deflected back or had the deflection angle of 180 degree. It also showed that the volume of positive charge in the atom is very small if we compared to the overall atom volume.

These points above certainly opposed the atomic theory of J.J. Thomson which described atom model as the plum pudding. Based on this result, Ernest Rutherford made the new atomic theory to explain the better model of atom. The atomic theory of Rutherford consists of following points:

The positive charge of atom or protons and the major mass of atom were located in the small volume. He named this area as the nucleus. Nucleus was located in the center of atom
Negative charge of atom or electrons were located around the atom’s nucleus. These atoms did move around the nucleus in the high speed and in circular path. The path where electrons revolved was called as electron orbits.
There as the strong electrostatic force formed by the electron’s negative charge and nucleus’s positive charge. These two charges attracted each other hence created the strong electrostatic force.

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Limitation of Atomic Theory by Rutherford

Even though the atomic theory by Rutherford were made based on the real experiment, it also failed to explain some other things, just like the previous theory by J.J. Thomson. Here are some points which Rutherford could not explain in his atomic model:

The atomic model by Rutherford doesn’t match with the Maxwell theory. Maxwell explains that the particles which move will surely emit the electromagnetic radiations. In this case, the electrons which move around the nucleus will also emit the electromagnetic radiation.
The radiation will transfer the energy from the electron’s move which will impact the path of electron’s motion or electron’s orbits. This radiation will make the electron orbit become smaller. As result, electrons will collapse in the nucleus. Based on the calculation, electrons will collapse in the nucleus in less than 8 seconds. The atomic theory by Rutherford was certainly could not explain the atom stability as it didn’t match with Maxwell’s principles.
Rutherford at that time was not able to describe the arrangement of the electrons in the atom. The atomic theory by Rutherford was not complete without the explanation about the electrons’ arrangement

After Rutherford’s atomic theory, his student, Neils Bohr conducted the experiment to find out the spectrum in hydrogen atoms. He could give better understanding about electron arrangement in atom using the Rutherford’s basic theory and Planck’s quantum theory.

However, this Rutherford’s atomic model is one of the greatest inventions in the atomic study and has become the great base for the further study. This atomic theory by Rutherford has successfully become the foundation to the quantum mechanics study. This also makes Rutherford as one of the most influential people in the chemistry and physics.

Rutherford Brief History

Ernest Rutherford is the British scientist who was born in the New Zealand at 30 August 1871. At his early study, he devoted his time to learn about the radioactive half life concept. He tried to prove that the radioactivity occurs in the nuclear transmutation of chemical element. He also discovered that radioactivity can be divided by the alpha and beta radiation.

He did this work at the McGill University in Canada. This theory led Rutherford to gin his Nobel Prize in Chemistry in 1908. He got the award for his discovery in the disintegration of elements and radioactive substances chemistry. He was the first Canadian and Oceanian who got that Nobel Prize.

In 1907, he moved to Victoria University of Manchester or University of Manchester in UK. In there, the proved that alpha radiation is the helium nucleus. He did this experiment with Thomas Royds. In 1911, he did another experiment using alpha particles to discover the atomic model as we described on the above section. In this experiment, he directed the undergraduate student named Ernest Marsden and he was also helped by the German physicist Hans Geiger.