How Earnest Rutherford used his Gold foil experiment to discover the nucleus of an atom. Experimentation, conclusions, importance, drawbacks and advanced level problems for IIT JEE, Mains & Advanced, NEET, AIEEE on Rutherford’s model.
Sir Earnest Rutherford was a New Zealand-born British scientist who is known as Father of nuclear physics. He was famous for his gold foil alpha particle scattering experiment and discovery of nucleus of atom. But, most of us don’t know that his scientific success was one of the reasons for his early death. In this article, we will tell you that story behind his unfortunate death. But let us, first find out, why his experiments was a major breakthrough in science
what is atom?
The concept that the atom is the smallest unit of any substance was old and was known from ancient Greek and Indian culture. Actually, the term “atom” came from the Greek word “atomos”, which means “uncuttable”. For a long period of time, it was believed that atom is the smallest particle of any substance. This idea was mainly popularized by English Physicists, chemist John Dalton in the late 18th century. His ‘atomic theory’ was very popular among the scientists in 19th century. The indivisibility of atom was the central idea of this theory.
Discovery of Sub-atomic particles :
But his theory was put under scanner when in 1858 “Cathode Rays” was discovered. When gas is kept at low pressure inside a discharge tube in between two electrodes, on applying high voltage an invisible ray comes from the cathode or negatively charged electrode to the anode or positively charged electrode. When this experiment was performed in dark and with the labeling of phosphorescence substance on the opposite wall of the tube. After thorough experiments, scientists understood that the origin of the cathode ray is not cathode actually. it is the gas inside the tube which is getting ionized here.
In 1897, J.J. Thomson, a British physicist, and Noble Laureate proved experimentally that cathode rays are composed of negatively charged particles called ‘electron’. Since the origin of electrons is atom itself, it was realized there should be a positively charged particle, inside the atom. soon, the positively charged ray, the canal ras, was discovered. this particle was named as the “proton”.
These newly discovered electrons, protons are called sub-atomic particles.
Now we know about another very important sub-atomic particle, called “neutron”, which is electrically neutral. But since it was discovered very late, in 1932, it is not included in most of the discussion.
Atomic model: Thomson’s palm pudding model :
After the discovery of electron and protons, J J Thomson proposed an atomic model which consider the electroneutrality of atom, although contains positively charged proton, and negatively charged electron. According to his proposal, these electrons and protons are uniformly distributed on spherical surface of atom, in such a way that the overall positive charge is equal to the all negative charge. Since, it resembles the distribution of cherry on the surface of pudding, it is named palm pudding model.
But this model was disproved in 1904, by German physicist Hans Geiger, who was famous for his contribution to making Geiger-Muller counter, the instrument used to detect the radioactivity. Also, in 1909, Ernest Rutherford disproved Thomson’s atomic model by his famous gold foil experiment.
Alpha particle scattering experiment by Ernest Rutherford :
When both physicists and chemists were busy in understanding the structure of atom, in the light of newly discovered subatomic particles, another field, radioactivity was growing parallelly. Some elements degrade to other elements by releasing alpha and beta particles, was discovered by Henri Becquerel and pioneered by Polish scientist Marie Curie. Rutherford was a friend of Marie Curie, and another pioneer of same field, asked for some radioactive sample of Radium Bromide. Now, how Rutherford shipped this sample from Poland to UK is a mystery, but he managed to do that, and was curious about the effect of high-speed alpha particle on thin gold-foil plate.
He placed a thin gold-foil after a chamber of radioactive source, and a detector behind the gold foil. It was easy to calculate the number of alpha particles passing through the foil by using Geiger-Muller counter. But, what he observed was quite amusing.
Most of the alpha particles were passed through the foil and reach the detector. But, to their disbelief, the found some alpha particle behind the source. How it came behind the source? In his own words “It was as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you”.
He concluded from this experiment that, most of the part of an atom is vacant, and most of the mass is concentrated on the centre. Since the mass of electron is very less compared to proton, the mass of proton is considered when we talk about atomic mass. So, he proposed, proton is present at the centre of the atom called “Nucleus”. The electron surrounds this nucleus in a circular path. This is a model resembling our solar system. So, sometimes it is called Rutherford’s planetary model. And it is the picture of his atomic model, we see on most of the textbooks.
Since alpha particle is a positively charges particles, and has high velocity, it passes through the atom, but when it approach a close distance to the nucleus, which contain positively charged protons, it gets deflected from its path. When there is a head-on collision with nucleus, then alpha particle back-scattered.
This experiment changes the face of science. Now, scientists have a clear picture of the atom, and it enriched our understanding of nature and structure of universe.
Now, we will come back to the story of his death. After his success in the field of science, he became a “Knight” of England. But, ironically, this became one of the cause of death. One day, he was admitted to the hospital due to severe stomach pain, but no doctor was allowed to visit him. Since at this time there was a rule in England that if you are a Knight, the only doctor, who can treat you, should be a Knight himself. But th only doctor who was knight at this hospital was away from the city. So, he was left without any treatment for one and half day at hospital When that particulatr doctor came back, it was late for the treatment, and he died next day.
Well, his death was unfortunate. And later, his theory was also disproved, since it could not explain the stability of atom and was unable to explain the line spectra of atom. But his contribution to the structural elucidation of atom will be always remembered.
Why this is important for IIT JEE, NEET?
This is also an important topic for the various board and competitive exams like IIT JEE, Mains & Advanced, NEET, AIEE etc. Each year, you can expect at least two questions on atomic structure. In the corresponding video,you will find advanced-level problems on Rutherford atomic model and how to calculate the distance of closest approach of alpha particle to nucleus.
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