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Rutherford's Gold Foil Experiment

The structure of atom was first given by the plum pudding model of J.J. Thomson before the experiment of Ernest Rutherford. The plum pudding model explained an atom as a positive charge body which contains small negatively charged particles which are called electrons. He also described that the negative charge in atom is balanced with the equal amount of positive charge to maintain the neutrality of atom. But there were some faults in this model of Thomson. He did not give the complete structure of atom which was then given by Rutherford in his Gold Foil Experiment in 1898 which was published in 1911.

He discovered the concept of nucleus in atom. His research is based on the experiment with alpha particles. Alpha particles are helium atom particles. He did bombardment of positive alpha particles on thin foil of gold approx. 8.6 x 10 -6 centimetres thick and took the observations on the screen of zinc sulphide which was behind the gold foil. He observed the deflection of these bombarded alpha particles on the photographic film. Let’s have a look on the Gold foil experiment of Rutherford.

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Rutherford Experiment

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In 1910, a physicist from New Zealand, Ernest Rutherford performed an experiment known as Rutherford’s gold foil experiment. This experiment was determined to find out the structure of an atom. By this time it was discovered by J.J. Thomson that electrons are present in an atom and that they are negatively charged. So it was assumed that since an atom is neutral and electrons present are negatively charged, there should be some positive charge inside it that makes it neutral. So Rutherford worked under the discoveries and assumption of J.J. Thomson. He accepted J.J. Thomson’s model of an atom which was plum pudding model.

According to the plum pudding model, there were negatively charged particles i.e. electrons embedded or suspended in a sphere of positive charge (electrons presented as plums inside the bowl of pudding)

Plum Pudding Model

Earnest Rutherford set up an apparatus and did an experiment that could confirm JJ Thomson’s model of an atom. But he ended up with some new facts in the structure of the atom. His experiment is as follows:

Construction of his experiment:
It has a radioactive source rich in positively charged heavy alpha particles inside a cube shaped thick lead box with a narrow opening.

Alpha Scattering Experiment

The alpha particles were confined to a narrow beam by passing then through a lead sheet through a slit. An extremely thin gold foil was bombarded with the narrow beam of fast moving alpha particles. On bombarding the alpha particles were scattered in different directions with different angles and were detected by florescent rotatable detector which has a microscope and a screen coated with zinc sulphide. The whole experimental setup was placed in an evacuated chamber to prevent scattering by the air molecules. These particles after striking on the screen caused scintillations. Before performing this experiment it was assumed by Rutherford that most of the alpha particles would pass through the gold foil with less deflection. He assumed this on the basis of theory proposed by JJ Thomson. This was assumed because the alpha particles are heavy and the negative charge in the "plum pudding model" is widely spread.

After performing his experiment he made observations:
  • Almost all the alpha particles did pass through the foil but
  • Some alpha particles were deflected off at different angles as observed at the screen of the detector.
  • Very few of the alpha particles (one or two) even bounced backwards after hitting the gold foil.
On the basis of these observations Rutherford made the following conclusions:
  • Since most of the alpha particles passed straight through the gold foil without any deflection, most of the space within the atoms is empty.
  • Since some of the alpha particles (which are big in size) were deflected by large angles or bounced backwards, they must have approached some positively charged region responsible for the deflection. This positively charged region is now called the nucleus.
  • As very few alpha particles undergone the deflection, it was concluded that the volume occupied by the central region ( nucleus ) is very small.
  • Since alpha particles which are relatively denser, were deflected by the central volume of charge, it shows that almost the complete mass of the atom must be within the central volume.

Rutherford's Atomic Model

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The Rutherford atomic model also known as planetary model is a model of the atom proposed by the physicist, Ernest Rutherford. Rutherford directed the famous experiment done by Geiger-Marsden in 1910. Upon analysis of his experiment in 1911, he suggested that the plum pudding model of J.J. Thomson of the atom had drawbacks. Rutherford's new model for the atom, based on the experimental results, contained the new features as follows:
  • High central charge was concentrated into a very small volume in comparison to the rest of the atom and
  • This central volume also contained most of the atomic mass of the atom. This region was named the "nucleus" of the atom in later years.
Rutherford's model did not make any new headway in explanation of the electron-structure of the atom. Rutherford's paper merely mentioned the earlier 1904 "Saturnian" atomic model of Hantaro Nagaoka in this regard, in which a number of small electrons circled the nucleus like the particles then speculated to make up the ring around Saturn. Rutherford's concentration of most of the atom's mass into a very small core, made some type of planetary model, as such a core would contain most of the atom's mass, similar to the Sun containing most of the solar system's mass. Rutherford's model was later improved and quantified by one of his students, Niels Bohr, with the known Bohr model of the atom.

"For concreteness, consider the passage of a high speed α particle through an atom having a N positive central charge, and surrounded by an equal charge of N electrons."

Rutherford was able to calculate that the radius of his gold central charge from purely energetic considerations of how far particles of known speed would be able to penetrate toward a central charge of 100e. He founded that it would need to be less (how much less could not be told) than 3.4 x 10-14m. This was in a gold atom known to be 10−10 metres or so in radius—a very surprising finding since it implied a strong central charge less than (1/3000)th of the diameter of the atom.

The Rutherford model served to concentrate a great deal of the atom's charge and mass to a very small core. But he could not contribute for any structure of the remaining electrons and remaining atomic mass. It mentioned the atomic model of Hantaro Nagaoka, which proposed that the electrons are arranged in one or more rings, with the specific metaphorical structure of the stable rings of Saturn.

Rutherford’s discovery has contributed a lot in the field of modern science. After Rutherford's theory, scientists started to consider that the atom is not a single particle ultimately, but it is made up of very smaller subatomic particles. Following research was done to figure out the exact atomic structure which led to Rutherford’s gold foil experiment. They discovered eventually that atoms have a positively-charged nucleus (with an exact atomic number of charges) in the center which has radius of about 1.2$\times$10−15$\times$[Atomic Mass Number]1/3 meters. Since electrons were found to be even smaller, this concluded that the atom consists of mostly empty space.

Afterwards, by using X-rays scientists found the expected number of electrons (equal to the atomic number) in an atom. When an X-ray passes through an atom, some of the rays is scattered and the rest passes through the atom. As the X-ray loses its intensity mainly due to scattering at electrons, the number of electrons contained in an atom can be accurately estimated by noting the rate of decrease in X-ray intensity.

Ernest Rutherford Atomic Theory

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Rutherford conducted the famous gold foil experiment and on the basis of that he proposed a new atomic theory after JJ Thomson. His theory is considered to be very important in the history of atomic theories as he was the first who discovered and proved the existence of central positive charge i.e. nucleus inside an atom. Later on the basis of his theory further improvements were made in the structure of an atom by Neil Bohr and other scientists.

Following are the main points of Rutherford’s theory:
  • Most of the part of an atom is empty.
  • Approximately all the mass of the atom is concentrated at the center of atom which is now called nucleus.
  • In the central region of atom, the positively charged particles are present.
  • The charge on the nucleus of an atom is positive and is equal to Z.e where Z is charge number, e is charge of proton.
  • The negatively charged particles i.e. electrons revolve around the central positive portion in different circular orbits.
  • Central region (nucleus) is very small in size if compared to the size of atom.

There were mainly two defects in Rutherford's atomic theory as follows:
  1. Being a charged particle, electron must emit energy when it is accelerated, according to classical electromagnetic theory. We know that around the nucleus, the motion of electron is an accelerated motion, hence it must radiate energy. But this does not happen in actual practice. Assume that if it occurs then due to continuous loss of energy orbit of electron must decrease continuously. As a result electron will fall into the nucleus eventually after some time. But this is against the practical situation and hence this shows that atom is unstable.
  2. If the electrons emit energy continuously, continuous spectrum should be formed. But in practical line spectrum is observed.
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