The History of the Atom by Year 13 Students 2014

Leucippus and Democritus by Sarangan Kathirgamanathan 13V



Leucippus was a Greek philosopher who was said to be the founder of atomisation but very little is known about him. He is believed to be the originator of the theory that the universe consists of two elements which he called solid and empty. He thought that everything in the universe was made up of atoms and that these atoms were small particles suspended in the empty space. He believed that matter is actually made up of a number of atoms joined together. We do not have any evidence of his research but his pupil Democritus used Leucippus work in his research.



Democritus was born in Abdera, Greece in 460BC. He lived to be 90 years old, dying in the year 370BC. He studied natural philosophy in Thrace, Athens, and Abdera, Greece. Many consider him to be the “father of modern science”.

The theory of Democritus and Leucippus held that everything is composed of “atoms”, which are physically, but not geometrically, indivisible; that between atoms, there lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size. Of the mass of atoms, Democritus said “The more any indivisible exceeds, the heavier it is.” But his exact position on weight of atoms is disputed.

He suggested that the solidness of the material corresponded to the shape of the atoms involved. Thus, iron atoms are solid and strong with hooks that lock them into a solid; water atoms are smooth and slippery; salt atoms, because of their taste, are sharp and pointed; and air atoms are light and whirling, pervading all other materials. Using analogies from our sense experiences, he gave a picture or an image of an atom that distinguished them from each other by their shape, their size, and the arrangement of their parts. Moreover, connections were explained by material links in which single atoms were supplied with attachments: some with hooks and eyes others with balls and sockets. The Democritean atom is an inert solid (merely excluding other bodies from its volume) that interacts with other atoms mechanically.

Democritus believed that atoms cannot be destroyed, differ in size, shape and temperature, are always moving, and are invisible. He believed that there are an infinite number of atoms.

Democritus atomic theory

1. All matter consists of invisible particles called atoms.

2. Atoms are indestructible.

3. Atoms are solid but invisible.

4. Atoms are homogenous.

5. Atoms differ in size, shape, mass, position, and arrangement.

->Solids are made of small, pointy atoms.

->Liquids are made of large, round atoms.

->Oils are made of very fine, small atoms that can easily slip past each other.

Aristotle and Boscovich By Nicholas Savva 13I



Aristotle (322 BCE) was a Greek philosopher and scientist born in the Macedonian city of Stagirus.

He had many good ideas about science but his ideas about atomic theory were not and unfortunately they were accepted for over two thousand years. His predecessors, Democritus and Leucippus more correct ideas on atomic theory were forgotten.

He believed that there were only five elements: air which was light, earth which was cool and heavy, water which was wet, fire which was hot, and Aether which he viewed as a divine substance which made up the stars and planets. Aristotle believed that all matter was made up either of one of the elements of water air earth and fire or combinations of these four elements, with the exception of stars and planets which were made of aether. He thought that no matter how many times you split matter, it always goes into a smaller piece of that matter.

Aristotle’s theory of matter has been proven wrong. Many more elements have been discovered. Just Looking at the periodic table proves this. The only thing Aristotle discovered that carries on to the modern atomic theory is the fact that there are elements, which is implied in Aristotle’s theory. A negative point about his theories it that they were accepted until the seventeenth century until physicists (then termed natural philosophers) such Isaac Newton and Gottfried Leibniz began work that would ultimately disprove them.


Isaac Newton PRS MP above left (25 December 1642 – 20 March 1726/7) was an English physicist and mathematician who was widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution.

Gottfried Wilhelm von Leibniz above right (July 1, 1646 – November 14, 1716) was a German mathematician and philosopher. He occupies a prominent place in the history of mathematics and the history of philosophy.

They were actually great rivals and there is some debate on which of the two came up with calculus first.

Roger Joseph Boscovich, S.J.


Roger Joseph Boscovich (18 May 1711 – 13 February 1787) was a physicist, astronomer, mathematician, philosopher, diplomat, poet, theologian, Jesuit priest, and a polymath from the city of Dubrovnik in the Republic of Ragusa (modern-day Croatia), who studied and lived in Italy and France where he also published many of his works.

He developed the first coherent description of atomic theory in his work Theoria Philosophiae Naturalis, which is one of the great attempts to understand the structure of the universe in a single idea. He held that bodies could not be composed of continuous matter, but of countless “point-like structures”. In this work he states that the ultimate elements of matter are indivisible points “atoms”, which are centres of force and this force varies in proportion to distance. What is remarkable is that his works appeared well over a century before the birth of modern atomic theory.

In his atomic theory, Boscovich put together Newton’s 1718 atomic theory, which was where the world was made of up elementary particles that attract at close distances through the force of chemical attraction, and Gottfried Leibniz’s view of atomic particles as ‘points of energy’, to create his theory of ‘point atoms’ or ‘stationary atom theory’ in which atoms were thought of as centres of forces without spatial extent, and whereas Newton focused on attraction, Boscovich added to this the importance of repulsive forces, such that at short range, atoms attracted each other, but that at longer range, atoms pushed each other way, the latter aspect explaining gas pressure. In this model, the Democritus atom model was replaced by a region of equilibrium between the forces of attraction and repulsion associated with the dynamic field surrounding the atom collapsed into a material point.

John Dalton and J J Thomson by Sujeethan Gnanathas 13B

John Dalton


John Dalton FRS (6 September 1766 – 27 July 1844) was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness (sometimes referred to as Daltonism, in his honour).

He is known for the Atomic theory, Law of Multiple Proportions, Dalton’s Law of Partial Pressures and the Daltonism.

He won the Royal Medal in 1826 for his development of the Atomic Theory and his other important labours and discoveries in Physical Science.

In the first half of 1793, aged 26, Dalton took the position of teacher of mathematics and natural philosophy at Manchester’s New College, a dissenting college.

In 1794, he wrote his first scientific paper which he called: Extraordinary Facts Relating to the Vision of Colours.

This was the first ever paper to discuss colour blindness. Dalton had realized the condition was hereditary, because he and other members of his family had it.

Ultimately, Dalton’s theory for colour blindness was wrong, but as he was the first person ever to research it, the condition became known as Daltonism.

Further research papers followed, in the physical sciences: heat conduction, gas expansion by heat, the properties of light, the aurora borealis, and meteorology.

In 1800, Dalton began earning a living as a private tutor in science and mathematics.

In 1801, Dalton gave a series of lectures in Manchester whose contents were published in 1802. In these lectures he presented research he had been carrying out into gases and liquids. This research was ground breaking, offering great new insights into the nature of gases.

Firstly, Dalton stated correctly that he had no doubt that all gases could be liquefied provided their temperature was sufficiently low and pressure sufficiently high.

He then stated that when its volume is held constant in a container, the pressure of a gas varies in direct proportion to its temperature.

This was the first public statement of what eventually became known as Gay-Lussac’s Law, named after Joseph Gay-Lussac who published it in 1809.

In 1803, Dalton published his Law of Partial Pressures, still used by every university chemistry student, which states that in a mixture of non-reacting gases, the total gas pressure is equal to the sum of the partial pressures of the individual gases.

By now, Dalton’s work had distinguished him as a scientist of the first rank, and he was invited to give lectures to the Royal Institution in London.

His study of gases led Dalton to wonder about what these invisible substances were actually made of.

The idea of atoms had first been proposed more than 2000 years earlier by Democritus in Ancient Greece. Democritus believed that everything was made of tiny particles called atoms and that these atoms could not be split into smaller particles. He carried out countless chemical reactions, and in 1808 published what we now call Dalton’s Law in his book A New System of Chemical Philosophy:

If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.

For example, Dalton found that 12 grams of carbon could react with 16 grams of oxygen to form the compound we now call carbon monoxide or with 32 grams of oxygen to form carbon dioxide. The ratio of oxygen masses of 32:16, which simplifies to 2:1 intrigued Dalton. Analysing all of the data he had collected, Dalton stated his belief that matter exists as atoms. He went further than Democritus, by stating that atoms of different elements have different masses. He also published diagrams showing, for example:

Main points of Dalton’s atomic theory

1. Elements are made of extremely small particles called atoms.

2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.

3. Atoms cannot be subdivided, created, or destroyed.

4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

5. In chemical reactions, atoms are combined, separated, or rearranged.

When he was 71 years old, Dalton had a small stroke – or paralysis as it was known then. A year later, a more significant stroke left him unable to speak as clearly as he once could. In 1844, when he was 77, another stroke hit him. He died aged 77 on July 27, 1844.

His scientific reputation was so great that when his body was placed in Manchester Town Hall, it was visited by more than 40,000 people paying their respects. John Dalton was buried in Manchester in Ardwick cemetery.

J J Thomson

J. J. Thomson (Joseph John “J. J.” Thomson 18 December 1856 – 30 August 1940) was an English physicist. He is best known for the Plum pudding model, Discovery of electron, Discovery of isotopes, Mass spectrometer invention, First m/e measurement, Proposed first waveguide, Thomson scattering, Thomson problem, Coining term ‘delta ray’, Coining term ‘epsilon radiation’, Thomson (unit).

He won several awards like the Royal Medal (1894), Hughes Medal (1902), Nobel Prize for Physics (1906), Elliott Cresson Medal (1910), Copley Medal (1914), Albert Medal (1915), Franklin Medal (1922), Faraday Medal (1925).


J. J. Thomson was appointed a Fellow of the Royal Society 1865. He was a successor to Lord Rayleigh as Cavendish Professor of Experimental Physics (ironically he was actually incredibly accident prone and was often banned from entering his own lab for fear he would break something). His favourite student Ernst Rutherford later succeeded him in 1919. The early theoretical work of Thomson broadened the electromagnetic theories of James Clerk Maxwell’s, which revolutionized the study of gaseous conductors of electricity, as well as the nature of cathode rays.

Discovery of the electron

Several scientists, had suggested that atoms were built up from more than just one fundamental unit joined together, but they thought each of these fundamental units were the size of the smallest atom, hydrogen. Inspired by Wilhelm Röntgen’s 1895 discovery of X-rays, Thomson was the first to suggest that one of these fundamental units was actually more than 1,000 times smaller than an atom, suggesting the subatomic particle now known as the electron. He discovered this through his explorations on the properties of cathode rays and made his suggestion on 30 April 1897 following his discovery that cathode rays could travel much further through air than expected for an atom-sized particle.

Thomson demonstrated that cathode rays were actually some speedily moving particles. After measuring their speed and specific charge, he concluded that these “corpuscles” (electrons) were about 2000 times smaller in mass as compared to the hydrogen ion, the lightest-known atomic particle. The discovery, made public during Thomson’s 1897 lecture to the Royal Institution, was labelled as the most influential breakthrough in the history of physics since Sir Isaac Newton.

He estimated the mass of cathode rays by measuring the heat generated when the rays hit a thermal junction and comparing this with the magnetic deflection of the rays. His experiments suggested not only that cathode rays were over 1,000 times lighter than the hydrogen atom, but also that their mass was the same in whichever type of atom they came from. He concluded that the rays were composed of very light, negatively charged particles which were a universal building block of atoms. He called the particles “corpuscles”, but later scientists preferred the name electron which had been suggested by George Johnstone Stoney in 1891, prior to Thomson’s actual discovery.

In April 1897, Thomson had only early indications that the cathode rays could be deflected electrically (previous investigators such as Heinrich Hertz had thought they could not be). A month after Thomson’s announcement of the corpuscle he found that he could reliably deflect the rays by an electric field if he evacuated the discharge tube to a very low pressure. By comparing the deflection of a beam of cathode rays by electric and magnetic fields he obtained more robust measurements of the mass to charge ratio that confirmed his previous estimates. This became the classic means of measuring the charge and mass of the electron.


The above picture is a plan of J.J. Thomson’s experiment, which first demonstrated that atoms are actually composed of aggregates of charged particles. Prior to his work, it was believed that atoms were the fundamental building blocks of matter. The first evidence contrary to this notion came when people began studying the properties of atoms in large electric fields.

If a gas sample is introduced into the region between two charged plates, a current flow can be observed, suggesting that the atoms have been broken down into charged constituents. The source of these charged particles is a heated cathode that, in fact, causes the atoms of the sample to ionize. These were known as cathode rays. In 1897, Thomson set out to prove that the cathode rays produced from the cathode were actually a stream of negatively charged particles called electrons. From James Clerk Maxwell’s theory, he knew that charged particles could be deflected in a magnetic field.

Thomson believed that the corpuscles (now known as electrons) emerged from the atoms of the trace gas inside his cathode ray tubes. He thus concluded that atoms were divisible, and that the corpuscles were their building blocks. To explain the overall neutral charge of the atom, he proposed that the corpuscles were distributed in a uniform sea of positive charge; this was the “plum pudding” model — the electrons were embedded in the positive charge like plums in a plum pudding (although in Thomson’s model they were not stationary, but orbiting rapidly).


The plum pudding model on the left and a “plum” Christmas pudding on the right

The plum pudding model was a model of the atom that incorporated the recently discovered electron, and was proposed by J. J. Thomson in 1904. Thomson had discovered the electron in 1897. The plum pudding model was abandoned after discovery of the atomic nucleus. The plum pudding model of the atom is also known as the “Blueberry Muffin” model.

Thomson also researched on the nature of positive rays in 1911, which significantly helped in the discovery of Isotopes. He proved that isotopes could be broke by deflecting positive rays in electric and magnetic fields, which was later named mass spectrometry.

He published his autobiography “Recollections and Reflections” in 1936. Thomson is widely considered to be one of the greatest scientists ever, and the most influential pioneer of nuclear physics.

J. J. Thomson was made the Master of Trinity College, Cambridge in 1918, where he remained until his death. He died on August 30, 1940 at the age of 83. Thomson was buried close to Isaac Newton in Westminster Abbey.

Rutherford, Bohr and Chadwick By Abbas Mohsen 13G

Ernest Rutherford


Ernest Rutherford, 1st Baron Rutherford of Nelson, OM FRS (30 August 1871 – 19 October 1937) was a New Zealand-born British physicist who became known as the father of nuclear physics.

Rutherford scientific career began with much success in local schools including Nelson College and this led to a scholarship to Canterbury College, University of New Zealand. After gaining his BA, MA and BSc, and doing two years of research during which he invented a new form of radio receiver, in 1895 Rutherford was awarded an 1851 Research Fellowship from the Royal Commission for the Exhibition of 1851, to travel to England for postgraduate study at the Cavendish Laboratory, University of Cambridge

Initially he worked on the conductive effects of X-rays on gases with, his supervisor, J. J. Thomson which led to the discovery of the electron which Thomson presented to the world in 1897. Then hearing of Becquerel’s experience with uranium, Rutherford started to explore its radioactivity, discovering two types that differed from X-rays in their penetrating power.

By 1889 Rutherford was ready to earn a living and sought a job. With Thomson’s recommendation McGill University in Montreal accepted him as a professor of chemistry and In 1907 Rutherford returned to Britain to take the chair of physics at the University of Manchester.

In 1908 he was awarded the Nobel Prize in Chemistry “for his investigations into the disintegration of the elements, and the chemistry of radioactive substances”. This didn’t make him terribly happy as he considered himself a physicist. He said

All science is either physics or stamp collecting”.

After receiving the Nobel Prize, Rutherford, along with Hans Geiger and Ernest Marsden in 1909, performed his most famous work which demonstrated the nuclear nature of atoms by deflecting alpha particles passing through a thin gold foil.


Rutherford had found that a narrow beam of alpha particles was broadened when it passed through a thin film of mica or metal. He therefore had Geiger measure the angle through which these -particles were scattered by a thin piece of metal foil. Because it is unusually ductile, gold can be made into a foil that is only 0.00004 cm thick. When this foil was bombarded with alpha particles, Geiger found that the scattering was small, on the order of one degree.

These results were consistent with Rutherford’s expectations. He knew that the alpha particle had a considerable mass and moved quite rapidly. He therefore anticipated that virtually all of the alpha particles would be able to penetrate the metal foil, although they would be scattered slightly by collisions with the atoms through which they passed. In other words, Rutherford expected the alpha particles to pass through the metal foil the way a rifle bullet would penetrate a bag of sand.

One day, Geiger suggested that a research project should be given to Ernest Marsden, who was working in Rutherford’s laboratory, as he was now ready for a real research project. Rutherford responded, “Why not let him see whether any alpha particles can be scattered through a large angle?”

Rutherford was inspired to ask Geiger and Marsden in this experiment to look for alpha particles with very high deflection angles, of a type not expected from any theory of matter at that time. Such deflections, though rare, were found, and proved to be a smooth but high-order function of the deflection angle

Marsden found that a small fraction (perhaps 1 in 20,000) of the -particles were scattered through angles larger than 90 degrees. Many years later, reflecting on his reaction to these results, Rutherford said: “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

It was Rutherford’s interpretation of this data that led him to formulate the Rutherford model of the atom in 1911 – that a very small charged nucleus, containing much of the atom’s mass, was orbited by low-mass electrons.

The experiment’s apparatus consisted of Polonium in a lead box emitting alpha particles towards a gold foil. The foil was surrounded by a luminescent zinc sulphide screen to detect where the alpha particles went after contacting the gold atoms.


Rutherford’s 1911 atomic model, despite being backed by the proof of his Gold Foil Experiment, was not immediately accepted. He had greatly respected J.J. Thomson and had initially agreed with his atomic theory. This theory had proposed that the electrons were evenly distributed throughout an atom. Since an alpha particle is 8,000 times as heavy as an electron, one electron could not deflect an alpha particle at an obtuse angle. Applying Thomson’s model, a passing particle could not hit more than one electron at a time; therefore, all of the alpha particles should have passed straight through the gold foil. This was not the case – a notable few alpha particles reflected of the gold atoms back towards the polonium. Hence the mass of an atom must be condensed in a concentrated core. Otherwise the mass of the alpha particles would be greater than any part of an atom they hit. As Rutherford put it:

“The alpha projectile changed course in a single encounter with a target atom. But for this to occur, the forces of electrical repulsion had to be concentrated in a region of 10^-13cm whereas the atom was known to measure 10^-8cm.”

He went on to say that this meant most of the atom was empty space with a small dense core. Rutherford pondered for much time before announcing in 1911 that he had made a new atomic model—this one with a condensed core (which he named the “nucleus”) and electrons orbiting this core.

This experiment not only disproved Thomson’s atomic model but also paved the way for such discoveries as the atomic bomb and nuclear power. The atomic model he concluded after the findings of his Gold Foil experiment have yet to be disproven.


Niels Bohr


Niels Henrik David Bohr (7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922.

Bohr received an invitation from Rutherford to conduct post-doctoral work at Victoria University of Manchester and worked on Rutherford’s model of the atom combining it with Max Planck’s quantum theory to give us the Bohr Model of the atom. This model pictured the atom as a mini solar system, which consisted of the nucleus in the centre, but the electrons moved around the atom in fixed orbits. The electrons were able to move between the orbits through energy changes, but they would always end up in a fixed orbit.


The picture above shows Bohr’s model of the hydrogen atom. A negatively charged electron, confined to an atomic orbital, orbits a small, positively charged nucleus; a quantum jump between orbits is accompanied by an emitted or absorbed amount of electromagnetic radiation.

While working with Bohr, Rutherford theorized about the existence of neutrons, (which he had christened in his 1920 Bakerian Lecture), which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keep the nuclei from flying apart from the repulsion between protons. The only alternative to neutrons was the existence of “nuclear electrons” which would counteract some of the proton charges in the nucleus, since by then it was known that nuclei had about twice the mass that could be accounted for if they were simply assembled from hydrogen nuclei (protons). But how these nuclear electrons could be trapped in the nucleus, was a mystery.

Rutherford was still confused about one thing however. The nucleus itself held most of the atom’s mass, however there was quite a difference in the amount of protons there were and the mass of the atoms. This made Rutherford think that there were proton-electron pairs within the nucleus, which gave the atom mass, but no extra charge.


In 1930, Irene and Frederic Joliot- Curie found that alpha particles that struck beryllium caused an unknown radiation to be given off. This radiation was very hard to detect but it had the ability to knock protons out of Paraffin which were detected by a Geiger-Muller tube. They tried to explain that the radiation was gamma radiation. Rutherford, who had become Director of the Cavendish Laboratory at Cambridge University in 1919, and James Chadwick, disagreed as protons were too heavy for this to occur. But neutrons would need only a small amount of energy to achieve the same effect. The Joliot-Curies had discovered the neutron but did not know it.

James Chadwick

Sir James Chadwick, CH, FRS (20 October 1891 – 24 July 1974) was an English physicist who was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron in 1932.


James Chadwick decided to repeat the Joliot-Curie’s experiments, taking the conservation of momentum and kinetic energy into mind, which brought him to the final conclusion that the radiation was in fact an uncharged particle with no more than 1% more mass than a proton, and he called it the neutron, and in 1935, he received the Nobel prize for it.


The apparatus consisted of a neutron chamber in which neutrons were produced and an ionisation chamber, such as a Geiger-Muller tube, which detected protons.

Chadwick worked day and night to prove the neutron theory, studying the beryllium radiation with an ionisation counter and a cloud chamber. He found that the wax could be replaced with other light substances, even beryllium, and that protons were still produced. 

The alpha-particles from the radioactive source hit the beryllium nuclei and transformed them into carbon nuclei, leaving one free neutron. When this neutron hit the hydrogen nuclei in the wax it could knock a proton free, in the same way that a white snooker ball can transfer all its energy to a red snooker ball. Rutherford gave the best description of a neutron as a highly penetrating neutral particle with a mass similar to the proton. We now know it is not a combination of an electron and a proton. Quantum mechanics restricts an electron from getting that close to the proton, and measurements of nuclear ‘spin’ provide experimental proof that the nucleus does not contain electrons.

Chadwick explained in his Nobel lecture that it seemed ‘useless to discuss whether the neutron and proton are elementary particles or not’. He knew that a more powerful investigation of the neutron was necessary to decide if it was made up of anything else. We now believe that the neutron and the proton are made of even tinier particles called quarks.

To further confuse matters, free neutrons are not stable. If a neutron is outside the nucleus for several minutes it will transform into a proton, an electron, and an extremely light particle called a neutrino. The decay occurs because one of the quarks inside the neutron has transformed into a different quark, producing an additional positive charge in the particle.

Erwin Schrödinger and Werner Heisenberg By Sulaxan Shanmulgalingam 13V

These men were both pioneers in the new area of physics called Quantum Mechanics. Quantum mechanics is the science of the very small: the body of scientific principles that explains the behaviour of matter and its interactions with energy on the scale of atoms and subatomic particles. Quantum mechanics was necessary because classical mechanics could not explain the behaviour of the very small. The word “quantum” in this sense means the minimum amount of any physical entity involved in an interaction. Certain characteristics of matter can take only discrete values.

By 1926 both men were interested in improving Niels Bohr’s model of an atom where electrons were believed to orbit the nucleus like planets orbiting a star. However this model couldn’t support many of the new findings such as:

The model violated the uncertainty principle in that it considers electrons to have known orbits and locations, two things which cannot be measured simultaneously;

Bohr’s model cannot say why some energy levels should be very close together;

Atoms don’t have energy levels predicted by the model.

Erwin Schrödinger


Erwin Rudolf Josef Alexander Schrödinger (12 August 1887 – 4 January 1961), was a Nobel Prize-winning Austrian physicist who developed a number of fundamental results in the field of quantum theory, which formed the basis of wave mechanics: he formulated the wave equation (stationary and time-dependent Schrödinger equation) and revealed the identity of his development of the formalism and matrix mechanics. Schrödinger proposed an original interpretation of the physical meaning of the wave function.

In the first years of his career Schrödinger became acquainted with the ideas of quantum theory, developed in the works of Max Planck, Albert Einstein, Niels Bohr, Arnold Sommerfeld, and others. This knowledge helped him work on some problems in theoretical physics, but the Austrian scientist at the time was not yet ready to part with the traditional methods of classical physics.

He used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom.


This atomic model is known as the quantum mechanical model of the atom. Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron. This model can be portrayed as a nucleus surrounded by an electron cloud. Where the cloud is most dense, the probability of finding the electron is greatest, and conversely, the electron is less likely to be in a less dense area of the cloud. Thus, this model introduced the concept of sub-energy levels.

Schrödinger was not entirely comfortable with the implications of quantum theory. Schrödinger wrote about the probability interpretation of quantum mechanics, saying: “I don’t like it, and I’m sorry I ever had anything to do with it.

Werner Heisenberg

Werner Karl Heisenberg (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the key pioneers of quantum mechanics.


Heisenberg’s solution to the problems facing Bohr’s quantum model of the atom relied on a special type of algebra and provided a different approach in which discontinuities could occur.

His response to Schrödinger’s work was his second major breakthrough: The uncertainty principle that places a limit on the accuracy with which certain properties can be simultaneously known. In particular, the simultaneous measurement of both the position and the momentum of a particle can be known only to h /4 π (with h as Planck’s constant). One can measure the position of a particle to an infinite level of precision, but then its momentum has an infinite uncertainty and vice versa. This sets an absolute limit on human knowledge of the physical world and leads to the idea of quantum mechanical probability.

“If one wants to be clear about what is meant by the ‘position of an object,’ for example, of an electron…, then one has to specify definite experiments by which the ‘position of an electron’ can be measured; otherwise this term has no meaning at all” (Cassidy, “Werner Heisenberg [1901–1976]”). In effect, reality does not exist until measured. This concept not only reformulated physics, but also had a major impact on Western philosophy.

The joint findings

“It is impossible to measure simultaneously the exact position and exact velocity or momentum of a sub-atomic particle like electron and neutron”

Mathematics is used to calculate the behaviour of electrons, and subatomic particles that also make up an atom.

Surrounding the outside of an atomic nucleus is an electron cloud not orbiting electrons. These electrons will have a range of allowed energies. They mathematically determined regions in which electrons would be most likely found.

They contributed to the atomic theory by including quantum mechanics, the branch of mechanics, based on quantum theory, used for interpreting the behaviour of elementary particles and atoms.

“The region of space that surrounds a nucleus in which two electrons may randomly move.”

They built upon the ideas of Neils Bohr and took them in a new direction. They developed the probability function for the Hydrogen atom. The probability function basically describes a cloud-like region where the electron is likely to be found. It cannot say with any certainty, where the electron actually is at any point in time, can give an estimate of where it is. Due the electrons fuzziness we can conclude it to be a cloud model.

The Electron Cloud Model

This is the accepted model of the atom today. Electrons dart around the nucleus at a particular energy level. Rapid, random motion creates a “cloud” of negative charge around the nucleus. You can either know the speed or position of an electron, not both. The electron cloud gives the atom its size and shape and represents the probable location of the electrons.



The evolution of atomic models in the 20th century: Thomson, Rutherford, Bohr, Heisenberg/Schrödinger.

So by the end of the 1930s we have an image of an atom as a central nucleus consisting of fundamental particles of protons and neutrons with a cloud of fundamental particles called electrons whirling about outside. You either know the positions of the electrons or their speeds but not both (Heisenberg’s uncertainty principle).

However protons, neutrons and electrons were found not to be the only subatomic particles and after the Second World War, when greater and greater energies became available it was found that the atom wasn’t simple at all.

Sub-atomic particles

By Sujeethan Gnanathas, Sarangan Kathirgamanathan, Abbas Mohsen, Nicholas Savva and Sulaxan Shanmulgalingam

After we had done our research into the history of the structure of the atom Mrs Hare set us another task. At the time that Rutherford and Chadwick were investigating the structure of the nucleus other physicists were investigating some of the other sub atomic particles that were turning up.

The Positron

A positron is a sub-atomic particle with the same mass, spin of ½ and numerically equal but opposite charge to an electron. In other words it is the anti-matter equivalent of the electron. Like an electron it is low-energy but when it comes into contact with an electron both will annihilate each other producing gamma photons.

In quantum mechanics and particle physics, spin is an intrinsic form of angular momentum carried by elementary particles, composite particles (hadrons), and atomic nuclei.

In 1928 Paul Dirac was working on the mathematics of non-relativistic spin systems, (independently of Wolfgang Pauli) and he proposed the Dirac equation as a relativistic equation of motion for the wave function of the electron. This equation would only work if there were two solutions for its corresponding negative energy states. So the work led him to predict the existence of the positron, the electron’s antiparticle, which he interpreted in terms of what came to be called the Dirac Sea.

Paul Adrien Maurice Dirac OM FRS (8 August 1902 – 20 October 1984) was an English theoretical physicist who made fundamental contributions to the early development of both quantum mechanics and quantum electrodynamics.


The first person to observe the positron was Dmitri Skobeltsyn. In 1929 while using a Wilson cloud chamber to try to detect gamma radiation in cosmic rays, he detected particles that acted like electrons but curved in the opposite direction in an applied magnetic field.

Similarly in 1929 Chung-Yao Chao, a graduate student at Caltech, noticed some anomalous results that indicated particles behaving like electrons, but with a positive charge. Because the results were inconclusive the phenomenon was not pursued.

Officially Carl D. Anderson discovered the positron on August 2, 1932, for which he won the Nobel Prize for Physics in 1936. Anderson did not coin the term positron, but allowed it at the suggestion of the Physical Review journal editor to which he submitted his discovery paper in late 1932. The positron was the first evidence of antimatter and was discovered when Anderson allowed cosmic rays to pass through a cloud chamber and a lead plate. A magnet surrounded this apparatus, causing particles to bend in different directions based on their electric charge. The ion trail left by each positron appeared on the photographic plate with a curvature matching the mass-to-charge ratio of an electron, but in a direction that showed its charge was positive.

Below is the cloud chamber photograph by C. D. Anderson of the first positron ever identified. A 6 mm lead plate separates the upper and lower halves of the chamber. The deflection and direction of the particle’s ion trail indicate the particle is a positron.


Carl David Anderson (September 3, 1905 – January 11, 1991) was an American physicist. He is best known for his discovery of the positron in 1932, an achievement for which he received the 1936 Nobel Prize in Physics, and of the muon in 1936.

Anderson wrote in retrospect that the positron could have been discovered earlier based on Chung-Yao Chao’s work, if only it had been followed up. Frédéric and Irène Joliot-Curie in Paris had evidence of positrons in old photographs when Anderson’s results came out, but they had dismissed them as protons.

Cosmic rays are immensely high-energy radiation, mainly originating outside the Solar System.


Primary cosmic particle collides with a molecule of atmosphere. The e+ is the positron.

The Muon

The muon (symbol m) is a negatively charged sub-atomic particle like the electron but two hundred times heavier. It is very unstable and only lasts about 2.2 micro seconds before decaying into a positron and two neutrinos. Like the electron it is a lepton with unitary negative electric charge of −1 and a spin of ½.

The physicists credited with the discovery of the muon are Carl D. Anderson and Seth Neddermeyer at Caltech in 1936. They had noticed particles that curved differently from electrons and other known particles when passed through a magnetic field. They were negatively charged but curved less sharply than electrons, but more sharply than protons, for particles of the same velocity. It was assumed that the magnitude of their negative electric charge was equal to that of the electron, and so to account for the difference in curvature, it was supposed that their mass was greater than an electron but smaller than a proton. Thus Anderson initially called the new particle a mesotron, adopting the prefix meso- from the Greek word for “mid-“. The existence of the muon was confirmed in 1937 by J. C. Street and E. C. Stevenson’s cloud chamber experiment.

The eventual recognition of the “mu meson” muon as a simple “heavy electron” with no role at all in the nuclear interaction, seemed so incongruous and surprising at the time, that Nobel laureate I. I. Rabi famously quipped, “Who ordered that?

Isidor Isaac Rabi (29 July 1898 – 11 January 1988) was a Polish-born American physicist and Nobel laureate, recognized in 1944 for his discovery of nuclear magnetic resonance, which is used in magnetic resonance imaging. He was also involved in the development of the cavity magnetron, which is used in microwave radar and microwave ovens.


The neutrino

A neutrino is a neutral fundamental sub-atomic particle with a mass close to zero. It has a half-integer spin and it rarely interacts with normal matter. Because it is electrically neutral it cannot be affected by electromagnetic forces and is only affected by the weak force. It belongs to the same family of particles as the electron, the leptons. There are in fact three types of neutrino; the electron neutrino (associated with the electron); the muon neutrino (associated with the muon) and the tau neutrino (associated with the tau).

Neutrinos are created as a result of certain types of radioactive decay, or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms.

The neutrino was first suggested by Wolfgang Pauli in 1930 to explain how beta decay could conserve energy, momentum, and angular momentum (spin). He considered that the new particle was emitted from the nucleus together with the electron or beta particle in the process of beta decay.

Wolfgang Ernst Pauli (25 April 1900 – 15 December 1958) was an Austrian-born Swiss theoretical physicist and one of the pioneers of quantum physics.


Enrico Fermi, who developed the theory of beta decay, coined the term neutrino (the Italian equivalent of “little neutral one”) in 1933.


Enrico Fermi (29 September 1901 – 28 November 1954) was an Italian physicist.

In 1942 Wang Ganchang first proposed the use of beta-capture to experimentally detect neutrinos. In the 20 July 1956 issue of Science, Clyde Cowan, Frederick Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire published confirmation that they had detected the neutrino, a result that was rewarded almost forty years later with the 1995 Nobel Prize.

In this experiment, now known as the Cowan–Reines neutrino experiment, antineutrinos created in a nuclear reactor by beta decay reacted with protons to produce neutrons and positrons:


The positron quickly finds an electron, and they annihilate each other. The two resulting gamma rays (γ) are detectable. The neutron can be detected by its capture on an appropriate nucleus, releasing a gamma ray. The coincidence of both events – positron annihilation and neutron capture – gives a unique signature of an antineutrino interaction.

Reines and Cowan used two targets containing a solution of cadmium chloride in water. Two scintillation detectors were placed next to the cadmium targets. Antineutrinos with an energy above the threshold of 1.8 MeV caused charged current interactions with the protons in the water, producing positrons and neutrons. This is very much like β+ decay, where energy is used to convert a proton into a neutron, a positron (e+) and an electron neutrino (νe) is emitted:

From known β+ decay:


In the Cowan and Reines experiment, instead of an outgoing neutrino, you have an incoming antineutrino from a nuclear reactor:



The above photograph is of first use of a hydrogen bubble chamber to detect neutrinos, on November 13, 1970. A neutrino hit a proton in a hydrogen atom. The collision occurred at the point where three tracks emanate on the right of the photograph.

Neutrinos cannot be detected directly, because they do not ionize the materials they are passing through. All detection methods require the neutrinos to carry a minimum threshold energy. So far, there is no detection method for low-energy neutrinos, in the sense that potential neutrino interactions cannot be uniquely distinguished from other causes. Neutrino detectors are often built underground in order to isolate the detector from cosmic rays and other background radiation. Super Kamiokande is a large volume of water surrounded by photomultiplier tubes that watch for the Cherenkov radiation emitted when an incoming neutrino creates an electron or muon in the water. The Sudbury Neutrino Observatory is similar, but uses heavy water as the detecting medium, which uses the same effects, but also allows the additional reaction any-flavour neutrino photo-dissociation of deuterium, resulting in a free neutron which is then detected from gamma radiation after chlorine-capture.

Super-Kamiokande (full name: Super-Kamioka Neutrino Detection Experiment, abbreviated to Super-K or SK) is a neutrino observatory located under Mount Kamioka near the city of Hida, Gifu Prefecture, Japan. The observatory was designed to search for proton decay, study solar and atmospheric neutrinos, and keep watch for supernovae in the Milky Way Galaxy.

Cherenkov radiation is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium (a type of electrical insulator) at a speed greater than the phase velocity of light in that medium.

The Sudbury Neutrino Observatory (SNO) is a neutrino observatory located about 2 km underground in Vale Inco’s Creighton Mine in Sudbury, Ontario, Canada.



The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964.


Murray Gell-Mann (born September 15, 1929) is an American physicist who received the 1969 Nobel Prize in physics for his work on the theory of elementary particles.

George Zweig (born May 30, 1937) is an American physicist.

In 1968, deep inelastic scattering experiments at the Stanford Linear Accelerator Center (SLAC) showed that the proton contained much smaller, point-like objects and was therefore not an elementary particle. The objects that were observed at SLAC would later be identified as up and down quarks as the other flavours were discovered.


A proton composed of two up quarks and one down quark. (The colour assignment of individual quarks is not important, only that all three colours be present.)


Six of the particles in the Standard Model are quarks (shown in purple). Six more are the leptons (shown in green). Each of the first three columns forms a generation of matter.

In particle physics, a gauge boson is a force carrier, a bosonic particle that carries any of the fundamental interactions of nature. Elementary particles, whose interactions are described by a gauge theory, interact with each other by the exchange of gauge bosons—usually as virtual particles.

The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles.

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