Ernest Rutherford and the discovery of the atomic nucleus
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Among the pioneers who initiated the atomic age, one of the most important is Ernest Rutherford.
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Birth and education
Ernest Rutherford was born on 30 August 1871 in Spring Grove in New Zealand. His father James Rutherford had immigrated from Scotland to New Zealand at
the very young age of 5. He was a farmer, but income was poor and work was hard. This played an important part in the life of Ernest.
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Rutherford's mother Martha Thomson had immigrated from England when she was a child. Since she was a teacher, she was better educated than her husband.
Rutherford was the fourth child among eight.
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Even when he was ten, he finished the book "Primer of Physics". He was regarded as the best student in the school. When he was fifteen he passed the scholarship examination needed for joining the college in the neighboring town Nelson, with very high marks (580/600). He won many prizes and scholarships in the college. Although he passed in all the subjects like English, History, French and Latin, with high marks, his outstanding ability was seen in Mathematics and Physics.
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In his 18th year, Rutherford won the scholarship for joining the Christ Church College of the New Zealand University at Canterbury. In 1892, he got his BA degree and in 1893, he got MA in Science and Mathematics. In 1894, he got BSc through a research essay.
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Cavendish Laboratory (1895-1899)
In 1895, he got scholarship for joining the world-famous Cavendish Laboratory in Cambridge, England. Rutherford was the first research student to join there.
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Cavendish Laboratory was established by James Clerk Maxwell. After that JJ Thomson became its Director. It became the best Physics Laboratory in the world.
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Rutherford followed in the footsteps of Maxwell. He succeeded in sending the electromagnetic waves over a distance of 88 metres. But the scientists in England
had kept physics research and technology as separate. They did not like a Cavendish scientist to do research in technology. So Rutherford had to leave that research.
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Rutherford's research on electromagnetic waves had gone further than Guglielmo Marconi's. Nevertheless Marconi's name only survived in the ability to send
electromagnetic waves over long distances!
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X-rays were discovered by Roentgen and radioactivity was discovered by Becquerel. These two discoveries shook the world of Science. Thomson searched for young scientists suitable to do research on these two new discoveries. Rutherford, who was alert and ambitious for success, was the first person who caught the eye of Thomson. So Rutherford started this research.
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In 1897, Thomson discovered the electron. This was the discovery of the first elementary particle. Rutherford discovered that the radioactivity from elements like Uranium are of two types and he named them as alpha and beta radioactivity. He found both the alpha and beta rays were deflected by electric or magnetic fields and hence both rays were made of charged particles. He also found that the alpha rays had 100 times less penetrating power than beta rays. Thomson had already determined the ratio of the electric charge to mass of the electron. Becquerel proved that the beta particles had the same ratio. So the beta particle was surmised to be the same as electron.
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McGill University (1899-1906)
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In 1899, Rutherford decided to shift to McGill University in Canada. He knew that it was not such a great place. His reason for leaving Cambridge was that Cambridge University did not treat him well. He thought they treated him badly since he was a colonial from a distant land. Further, he deserved to be made a Fellow of Cambridge University and hedid not get the Fellowship. With that, his salary also would have increased. In McGill,he got Professorship.
Another reason for his choice of McGill was that through the donation of a rich man, McGill was provided with an excellent laboratory.
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At McGill, Rutherford started research in radioactivity with increased energy. He took a chemist Fredrick Soddy as his collaborator and studied the radioactivity from Uranium and Thorium.
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Rutherford and Soddy discovered the true nature of radioactivity. They found that in radioactivity one element changed into another element! For many centuries, before Chemistry advanced, there was research on alchemy. The aim of that research was to convert base metals like lead into gold! Finally this was found to be impossible and it became an accepted principle in Chemistry that one element cannot be converted into another element.
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Now Rutherford and Soddy discovered that all of radioactivity is the conversion of one element into another. Thus a basic principle of Chemistry had to be given up!
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Rutherford and Soddy discovered that radioactivity decreases with time. For instance, the radioactivity of Thorium decreases to half in 30 seconds. Rutherford named this as half-life. Every radioactive elements has a specific half-life. Different radioactive elements have half-lives ranging from a second to millions of years.
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The number of radioactive nuclei decrease exponentially in time t :
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All the unstable radioactive nuclei as well as all the unstable elementary particles decay by this radioactive decay law discovered by Rutherford and Soddy.
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Rutherford found that apart from alpha and beta rays a third kind of rays also are emitted in radioactivity. These rays were not deflected by electric and magnetic fields. So they were uncharged. Rutherford called these gamma rays. William Henry Bragg showed that these were also electromagnetic waves like X-rays but with higher frequency.
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Radioactive element changes into another element; that also decays by radioactivity into yet another element. Thus, Uranium and Thorium change into other elements that emit alpha, beta and gamma rays through a chain reaction. The chain reaction stops with lead which is stable. Rutherford proved by experiment that there is lead wherever radioactive elements are found.
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It takes millions of years for the radioactive chain to end in lead. Therefore if we find the amount of lead in old rocks, we can determine the age of the rocks and the age of the Earth. This is an important discovery for research on the Earth. Thus Rutherford showed that the age of the Earth is above a billion years. This result agreed with the results found in Geology and with the age required for Darwin's theory of evolution. So, radioactivity became an important tool for Geology.
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Alpha particle was positively charged and had a mass of more than a thousand times that of the electron. It was found later that alpha particle was the nucleus of the Helium atom.
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Rutherford's discoveries made him very famous. He got invitations to join in many universities such as Yale, Columbia and Stanford. He was invited to become the Director of the Smithsonian Institution in Washington. But he preferred to return to Europe where research was at a higher level.
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In 1906, he was invited to join Manchester University. In England, Manchester was next best to Cavendish Laboratory. So he accepted it. At that time he was 35.
Manchester (1906-1917) and the discovery of the nucleus
Rutherford was awarded the Nobel Prize for Chemistry in 1908. It was for his discoveries about radioactive decays and the chemistry of radioactive elements. The irony was Rutherford's famous statement: "Physics alone is Science; the rest are stamp-collecting."
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Hans Geiger was the co-discoverer of the famous particle detector called Geiger-Muller counter. Rutherford and Geiger made many studies on radioactivity using this G.M counter.
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One of these is Rutherford's most important research. It was a scattering experiment in which alpha particles were made to collide on gold atoms in a thin foil and the idea was to determine at what angles the alpha particles were scattered by the gold atoms. Rutherford, Geiger and the research student Marsden pursued this experiment.
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In particular, Rutherford asked Marsden to see whether any alpha particle turned back by 180 degrees after collision with gold atom. Infact, they did ! From this Rutherford concluded that that there was a nucleus of positive charge at the centre of the atom and that most of the mass of the atom was in the nucleus and it is the nucleus that turns the colliding alpha particle backwards at 180 degrees. This discovery of the atomic nucleus happened in the year 1911.
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Before that, it was already known that the atom consists of the negatively charged electrons and some substance with positive charge equal to the total magnitude of the negative charge. This was JJ Thomson's discovery. Thomson thought that this positively charged stuff was spread throughout the atom while the point-like electrons were fixed at various positions in this stuff. This was called Thomson's plum-pudding model of the atom.
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If this were true, the spread-out positive charge could not have deflected the alpha particle to such a large angle as 180 degrees. That is why Rutherford had to conclude that all the positive charge was concentrated. In Rutherford's atom, the nucleus sits at the centre and the electrons go around, like the planets in the solar system.
Rutherford's model of the atom led to a puzzle. According to electrodynamic theory, the orbiting electrons will radiate electromagnetic waves and lose energy continuously. Therefore electrons will fall into the nucleus in about 10^(-10) seconds. So atoms will not be stable at all!
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Niels Bohr who came to Rutherford's laboratory was the one who solved this puzzle in 1913. He proposed that electrons will orbit at certain allowed radii only and in these allowed orbits electrons will not radiate. Only when the electron jumps from one orbit to another, it will radiate the difference of energy in the form of a quantum of electromagnetic wave. This is an important part of the quantum revolution. Thus the Rutherford-Bohr atom solved the
puzzle of the structure of the atom.
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Rutherford had to do war-related work during the first world-war (1914-1918). He returned to research in 1917. He found that when alpha and Nitrogen atom collide, Oxygen and Hydrogen result. This is the discovery of the first nuclear reaction. He concluded that all nuclei are made of Hydrogen nuclei. He named Hydrogen nucleus as proton. He surmised that nucleus might contain neutral particles also and named the neutral particle as neutron. James
Chadwick who was brought by Rutherford discovered the neutron through a nuclear reaction.
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It became clear all atomic nuclei are made of protons and neutrons.
Cavendish (1919-1939)
In 1919 Thomson retired from Cavendish and Rutherford became its Director. Rutherford spent rest of his life there and continued his research on nuclei. The Cavendish scientists collided neutrons on many nuclei and studied the resulting reactions. In 1934, they produced Tritium by colliding neutrons on Deuterium. Deuterium and Tritium are avatars of Hydrogen. Deuterium nucleus is made of one proton and one neutron while Tritium nucleus is made of one proton and two neutrons. Both of them are called isotopes of Hydrogen and in fact they are heavier form of Hydrogen.
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Rutherford had received knighthood in 1914 and became Baron Rutherford of Nelson in 1934. He died in 1939 and was buried in Westminster Abbey where famous scientists like Newton had been buried.
Rutherford's discovery of the nucleus transformed our knowledge of the atom and initiated a new branch of Physics called Nuclear Physics.
Reference
Ten physicists who transformed our understanding of reality, by R Evans and B Clegg, Robinson, Running Press, London, 2015.