The History of the Atom by Year 13 Physics Students

Democritus and Aristotle by Sulaxchane Balachanthiran



Democritus was born in Abdera at around 460 BCE (he died around 370 BCE), in Thrace, ancient Greece. Even though he is classified as a philosopher he was more of scientist.

He was the first person to formulate an atomic theory and was one of the earliest Greeks to develop the theory of atomism.

Democritus’ ideas were largely ignored in Athens. Although many now consider him the “father of modern science”

Democritus theory was that everything is composed of “atoms” which cannot be physically divided. “atom” comes from the Greek word “a-tomos” which means “not divisible” meaning that it cannot be cut down. He also said that atoms are and will always be in motion and that between atoms is empty space, which he called it “void”. The number of atoms and type of atoms are infinite and that they are indestructible


Democritus also said that atoms are impenetrable and their density is proportional to their volume. He also suggested that the atoms had a shape that caused their properties e.g. Iron is strong because its atoms had hooks that linked each atom together and gave iron its strength.

He believed that the universe had no beginning and that nothing happened at random, there will always be a material cause. He also said poverty and democracy was far better than wealth. He believed that prevailing religion at his time was not evil and that no mortal God existed.

People at his time believed that the moon and sun were God but Democritus disagreed by saying that the moon is just an ordinary place made up of atoms and that the sun was a red giant molten stone far away in space. He was imprisoned for this and was said to have committed a religious crime.


Book the atomists: Leucippus and Democritus- fragment



Aristotle was born after Democritus (about 384 BCE) in Chalcidice, ancient Greece.

He believed that there was one world and that what ever formed in this existed in that world. He was more of a scientist than philosopher and wanted to find the form of what existed in this world.

He came up with the idea that things consists of substance and what he meant by substance is a combination of both potential (matter) and act (form).

Unfortunately for Democritus’ ideas Aristotle was such a revered philosopher that his wrong ideas about matter were accepted right up until the seventeenth century.

Aristotle believed that all matter was made up of five elements (first proposed by Empedocles): Earth (corresponding to the modern idea of a solid), water (corresponding to the modern idea of a liquid), air (corresponding to the modern idea of a gas), fire (corresponding to the modern idea of heat) and aether (corresponding to the stars and planets).


Robert Boyle  by Mrs Hare


It wasn’t until 1661 that natural philosopher Robert Boyle (1627-1691) published The Sceptical Chymist in which he argued that matter was composed of various combinations of different “corpuscules” or atoms, rather than the classical elements of air, earth, fire and water. Democritus’ ideas were starting to be accepted again.

John Dalton (1766-1844) by Abdiqani Yusuf

John Dalton was an English physicist, meteorologist and chemist and he is best known for his pioneering work in the development of modern atomic theory.

He was born to a Quaker family in Eaglesfield, Cumbria, England (Quakers are Christians who are not part of the Church of England and during Dalton’s time were barred from attending or teaching at English universities). Being a Quaker he could not attend an English University so was educated informally by a blind philosopher and polymath called John

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.

John Dalton studied and expanded upon this previous work and developed the law of multiple proportions: if two elements came together to form more than one compound, 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 integers. For instance, Proust had studied tin oxides and found that their masses were either 88.1% tin and 11.9% oxygen or 78.7% tin and 21.3% oxygen (these were tin(II) oxide and tin dioxide respectively). Dalton noted from these percentages that 100g of tin will combine either with 13.5g or 27g of oxygen; 13.5 and 27 form a ratio of 1:2. Dalton found an atomic theory of matter could elegantly explain this common pattern in chemistry – in the case of Proust’s tin oxides, one tin atom will combine with either one or two oxygen atoms.

Dalton also believed atomic theory could explain why water absorbed different gases in different proportions: for example, he found that water absorbed carbon dioxide far better than it absorbed nitrogen. Dalton hypothesized this was due to the differences in mass and complexity of the gases’ respective particles. Indeed, carbon dioxide molecules (CO2) are heavier and larger than nitrogen molecules (N2).

Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures (chemical compounds). This marked the first truly scientific theory of the atom, since Dalton reached his conclusions by experimentation and examination of the results in an empirical fashion.

In 1803 Dalton orally presented his first list of relative atomic weights for a number of substances. This paper was published in 1805, but he did not discuss exactly how he obtained these figures. The method was first revealed in 1807 by his acquaintance Thomas Thomson, in the third edition of Thomson’s textbook, A System of Chemistry. Finally, Dalton published a full account in his own textbook, A New System of Chemical Philosophy, 1808 and 1810.

Dalton estimated the atomic weights according to the mass ratios in which they combined, with the hydrogen atom taken as unity. However, Dalton did not conceive that with some elements atoms exist in molecules — e.g. pure oxygen exists as O2. He also mistakenly believed that the simplest compound between any two elements is always one atom of each (so he thought water was HO, not H2O). This, in addition to the crudity of his equipment, flawed his results. For instance, in 1803 he believed that oxygen atoms were 5.5 times heavier than hydrogen atoms, because in water he measured 5.5 grams of oxygen for every 1 gram of hydrogen and believed the formula for water was HO. Adopting better data, in 1806 he concluded that the atomic weight of oxygen must actually be 7 rather than 5.5, and he retained this weight for the rest of his life. Others at this time had already concluded that the oxygen atom must weigh 8 relative to hydrogen equals 1, if one assumes Dalton’s formula for the water molecule (HO), or 16 if one assumes the modern water formula.

1. Two pages from John Dalton’s 1808 book A New System of Chemical Philosophy in which he proposed his version of atomic theory based on scientific experimentation.

2. Andrew G. van Melsen (1952). From Atomos to Atom. Mineola, N.Y.: Dover Publications. ISBN 0-486-49584-1.

3. Dalton, John. “On the Absorption of Gases by Water and Other Liquids”, in Memoirs of the Literary and Philosophical Society of Manchester. 1803. Retrieved on August 29, 2007.

4. Johnson, Chris. “Avogadro – his contribution to chemistry”. Archived from the original on 27 June 2009. Retrieved 2009-08-01.

5. Alan J. Rocke (1984). Chemical Atomism in the Nineteenth Century. Columbus: Ohio State University Press.

Joseph John Thomson  by Balakrishnar Murasoli


J.J. Thomson wasborn in Cheetham Hil, Manchester on the 18 December 1856. He attended Owens College in Manchester where his professor told him to apply for a scholarship to Trinity College Cambridge.

He successfully won the scholarship and in 1880 came second in his class in the mathematic graduation exam.

The Cavendish laboratory

The Cavendish laboratory in Cambridge was founded in 1871, and in 1884 J.J Thomson was chosen to be its third Cavendish Professor. When he was chosen some people found it rather amusing because he was rather inexperienced at experiments (rather accident prone in fact). Whilst he was there many experiments on electromagnetism and atomic particles were carried out and Thomson was interested in this work. He helped the researchers by checking their progress and making some suggestions for improvement in the research.


Several scientists had suggested that atoms were built up from more than one unit but Thomson, in 1897, was the first to suggest that one fundamental unit of the atom was over 1000 times smaller than an atom, suggesting the subatomic particles now known as electrons. He discovered this through his explorations on the properties of cathode rays and made his suggestion on 30 April 1897 following his discovery that Lenard rays could travel much further through air than expected for an atom-sized particle. 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 1000 times lighter than the hydrogen atom, but also that their mass was the same whatever 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 1894, prior to Thomson’s actual discovery.

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 deflect the rays reliably by electric fields if he evacuated the discharge tubes to very low pressures. By comparing the deflection of a beam of cathode rays by electric and magnetic fields he was then able to get 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.


Thomson’s illustration of the Crookes tube by which he observed the deflection of cathode rays by an electric field (and later measured their mass to charge ratio). Cathode rays were emitted from the cathode C, passed through slits A (the anode) and B (grounded), then through the electric field generated between plates D and E, finally impacting the surface at the far end.

The cathode is negatively charged and the anode is positively charged. When the cathode ray hits the fluorescent screen light is produced. It was given the name cathode ray because the beam flowed from the negatively charged to the positively charged electrode. Thomson took a side of a magnet and placed it next to the cathode ray. The cathode ray deflected at a larger angle away from the magnet. When he flipped the sides of the magnet the cathode ray attracted towards the magnet. From this he proved that the cathode rays are negatively charged particles. However in order for it to be a particle it should have a mass. He used the oil drop experiment originally done by Millikan to prove that the charge of the particle was -1.602 x 10ˉᶦ⁹C. Mathematically, Thomson used the charge to mass ratio and multiplied it by the charge to get the mass of the particle in the cathode ray tube (electron). He proved that the electron had a mass of 9.109 x 10^-31kg. This is around 2000 times smaller than a hydrogen ion (proton).

Thomson believed that the corpuscles 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).


Plum pudding model

This model was later proved incorrect when Ernest Rutherford showed that the positive charge is concentrated in the nucleus of the atom.


Edexcel A2 Physics by Miles Hudson ISBN 978-1-4082-0608-9 PAGE 78

Article “Cathode ray” published by Joseph Battell in 1901 6pages viewed on 08/11/12 at 21.10 pm published by Cambridge

Ernest Rutherford  by Thinesh Aruleeswaran


Ernest Rutherford has been called the ‘Father of Nuclear Physics’.

He was born on August 30, 1871 in Nelson, New Zealand and he was the fourth child and second son in a family of seven sons and five daughters. His father James Rutherford, who was a Scottish wheelwright, had immigrated to New Zealand with Ernest’s grandfather and the whole family in 1842. His mother, named Martha Thompson, was an English schoolteacher, who, with her widowed mother, also went to live there in 1855.

Ernest received his early education in Government schools and at the age of 16 entered Nelson Collegiate School. He was awarded a University scholarship and entered the University of New Zealand, Wellington (Canterbury College). He graduated with an M.A. in 1893 gaining a double first in Mathematics and Physical Science and he continued with research work at the College for a short time, receiving the BSc. degree the following year.

In 1907, Rutherford moved to England to become professor of physics at Manchester University. One year later, he was awarded the Nobel Prize in Chemistry. He was furious about this and he is quoted as saying: ‘All science is either physics or stamp collectingwhich implied thatPhysics is the only real science. The rest is just stamp collecting.

In 1911, although he could not prove that it was positive or negative; he theorized that atoms have their charge concentrated in a very small nucleus, and thereby pioneered the Rutherford model of the atom, through his discovery and interpretation of Rutherford scattering in his gold foil experiment. He asked Hans Geiger and Ernest Marsden to do the actual experiment to give them something to do.

The gold foil experiment consisted of a series of tests in which positively charged alpha particles (helium nuclei) were fired at a very thin sheet of gold foil. If Thomson’s Plum Pudding model was to be accurate, the big alpha particles should have passed through the gold foil with only a few minor deflections. This is because the alpha particles are heavy and the charge in the “plum pudding model” is widely spread. However, the actual results surprised Rutherford. Although many of the alpha particles did pass through as expected, many others were deflected at small angles while others were reflected back to the alpha source.

In detail, a beam of alpha particles, generated by the radioactive decay of radon, was directed normally onto a sheet of very thin gold foil in an evacuated chamber. A zinc sulphide screen at the focus of a microscope was used as a detector; the screen and microscope could be swivelled around the foil to observe particles deflected at any given angle. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees; measuring the pattern of scattered particles was expected to provide information about the distribution of charge within the atom. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. According to Rutherford:

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. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge. —Ernest Rutherford


Top left: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed.

Top right: Observed results: a small portion of the particles were deflected, indicating a small, concentrated positive charge. Note that the image is not to scale; in reality the nucleus is vastly smaller than the electron shell

He was demonstrating the nuclear nature of atoms.

During the experiment, Rutherford discovered and named alpha and beta decay and created the terms alpha, beta, and gamma rays. He demonstrated radioactivity was the spontaneous disintegration of atoms and was the first person to artificially break up an element. He identified alpha particles as helium nuclei.

The data generated from the gold foil experiment demonstrated that the plum pudding model of the atom was incorrect. The fact that many of the alpha particles were deflected or reflected meant that the atom had a concentrated centre of positive charge and of relatively large mass. The alpha particles had either hit the positive centre directly or passed by it close enough to be affected by its positive charge. Since many other particles passed through the gold foil, the positive centre would have to be a relatively small size compared to the rest of the atom – meaning that the atom is mostly open space.

Because the majority of the positive particles continued on their original path unmoved, Rutherford was forced to conclude that most of the remainder of the atom was a region of very low density. A great deal of charge was also associated with the central region of high density. Rutherford hypothesized that these two properties resided in the same physical structure and termed his discovery “the central charge”, a region later named the nucleus. Thus the current view of the nuclear atom – a structure with a positively charged centre (nucleus) of high density and negatively charged electron particles moving around the nucleus at relatively large distances compared to the nuclear radius – was created.

Rutherford interpreted the experimental results in a famous 1911 paper. He was able to definitively reject J.J. Thomson’s plum pudding model of the atom, since none of Thomson’s negative “corpuscles” (i.e. electrons) contained enough charge or mass to deflect alphas strongly, nor did the diffuse positive “pudding” or cloudlike positive charge, in which the electrons were embedded in the plum pudding model. Instead, Rutherford suggested that a large amount of the atom’s charge and mass is instead concentrated into a very physically small (as compared with the size of the atom) region, giving it a very high electric field. Outside of this “central charge” (later termed the nucleus), he proposed that the atom was mostly empty space. Rutherford was able to say from the experiment that the nuclear charge was positive and used the following language for pictorial purposes:

“For concreteness, consider the passage of a high speed Alpha particle through an atom having a positive central charge Ne, and surrounded by a compensating charge of N electrons.”

From energetic considerations of how far alpha particles of known mass and kinetic energy would be able to penetrate toward a central charge of 100 e (1.6022×10−17 C), Rutherford was able to calculate that the radius of his gold central charge would need to be physically smaller (how much smaller, could not be told) than 3.4×10^−14 metres (the modern value for the actual radius is only about a fifth of this). The figure applied in a gold atom which was itself known to be much larger: 1.5×10−10 metres or so in radius – a very surprising finding, as it implied a strong central charge less than 1⁄4000 of the diameter of the atom.

Rutherford had used strictly Newtonian methods to analyse the relatively low-energy alpha-scattering of this experiment. Later, when full quantum mechanical methods were available, it was found that they gave the same scattering equation which had been derived by Rutherford by classical means.

Although Rutherford’s model of the atom itself had a number of problems with electron charge placement and motion, which were only resolved following the development of quantum mechanics, the central conclusion from the Geiger–Marsden experiment, and the existence of the nucleus, still holds. Below is a picture of Rutherford’s view of the atom.


Rutherford’s description of the atom set the foundation for all future atomic models and the development of nuclear physics. Rutherford’s model was later elaborated into the Bohr model by physicist Niels Bohr in 1913. The Bohr model, in turn, was soon replaced by the Schrodinger model of the atom, as the basic atomic model used today.

With his results, they helped us to describe the nuclear structure of the atom. The deflection of the alpha particles suggested that the existence of a dense positively charged central region containing most of the atomic mass. In 1920, he assumed the existence of the neutron.


Sir James Chadwick  by Aaron Edwards


(20th October 1891- 24th July 1974)

James Chadwick was born in Cheshire, England on the 20th October 1891, to John Joseph Chadwick (father) and Anne Mary Knowles (mother). During his school life, James attended Manchester High School, which led to his entrance into Manchester University in 1908 and in 1911 he graduated with an honours degree from the school of physics.

After graduating, Chadwick spent the next two years working under Rutherford, where he gained a Masters in Science degree for his work on radioactivity. After gaining this qualification, he moved to Berlin to work in the Physikalisch Technische Reichsanstalt at Charlottenburg (Physical Technical Institute) where he worked under Professor H. Geiger.

Soon after he moved to Berlin, World War 1 broke out and Chadwick was held as a prisoner of war for the entirety of the war. After the war had ended, Chadwick returned to England. He continued his work under Rutherford but this time in Caius College, Cambridge. In 1921, Rutherford oversaw Chadwick’s PhD. During this second term under Rutherford, Chadwick was promoted to being Assistant Director of Research in the Cavendish Laboratory. In 1932 Chadwick made a discovery which finished the ‘puzzle’ of the make-up of an atom, he discovered the existence of the Neutron. And not soon after, Chadwick was awarded the Nobel Prize for physics.

In 1920, Ernest Rutherford conceived the possible existence of the neutron. In particular, he considered that the disparity found between the atomic number of an atom and its atomic mass could be explained by the existence of a neutrally charged particle within the atomic nucleus. He considered the neutron to be a neutral double consisting of an electron orbiting a proton.

In 1930 Viktor Ambartsumian and Dmitri Ivanenko in USSR found that, contrary to the prevailing opinion of the time, the nucleus cannot consist of protons and electrons. They proved that some neutral particles must be present besides the protons.

In 1931, Walther Bothe and Herbert Becker in Germany found that if the very energetic alpha particles emitted from polonium fell on certain light elements, specifically beryllium, boron, or lithium, an unusually penetrating radiation was produced.


At first this radiation was thought to be gamma radiation, although it was more penetrating than any gamma rays known, and the details of experimental results were very difficult to interpret on this basis. The next important contribution was reported in 1932 by Irène Joliot-Curie and Frédéric Joliot in Paris. They showed that if this unknown radiation fell on paraffin, or any other hydrogen-containing compound, it ejected protons of very high energy. This was not in itself inconsistent with the assumed gamma ray nature of the new radiation, but detailed quantitative analysis of the data became increasingly difficult to reconcile with such a hypothesis.

In 1932, James Chadwick performed a series of experiments at the University of Cambridge, showing that the gamma ray hypothesis was untenable. He suggested that the new radiation consisted of uncharged particles of approximately the mass of the proton, and he performed a series of experiments verifying his suggestion. These uncharged particles were called neutrons, apparently from the Latin root for neutral and the Greek ending -on (by imitation of electron and proton).

He 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.



Within a month Chadwick had conclusive proof of the existence of the neutron. He published his findings in the journal, Nature, on February 27, 1932.

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 knew the neutron wasn’t formed from an electron and a proton, and 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.

When World War 2 started, Chadwick moved to America and was part of what is known as the ‘Manhattan Project’. His job was to help to create the first ever nuclear weapons. After the war (1945) James Chadwick was knighted. And then on the 24th July 1974, he passed away in Cambridge. The discovery of the neutron had led to the creation of nuclear fission.

Bohr, Heisenberg and Schrodinger by Sinduran Sivrajan

Niels Bohr


Niels Bohr was a Danish Physicist. He was born on the 7th October 1885, in Copenhagen, Denmark. He attended the University of Copenhagen in 1903, where he did his master’s degree and PhD. He then moved to Cambridge University in 1911 where he conducted experimental work in the Cavendish Laboratory under the guidance of Sir J.J. Thomson.

In 1912 he moved to Manchester University where he worked with Rutherford. Whilst there, he adapted Rutherford’s nuclear structure to Max Planck’s quantum theory and so obtained a model of atomic structure which, with later improvements – mainly as a result of Heisenberg’s concepts – remains valid to this day. Bohr published his model of atomic structure in 1913. Here he introduced the theory of electrons traveling in orbits around the atom’s nucleus, the chemical properties of each element being largely determined by the number of electrons in the outer orbits of its atoms. Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a photon (light quantum) of discrete energy. This became a basis for quantum theory. He devised the model, now known as the Bohr model, which stated that:

1. Electrons orbit the positive nucleus in orbits that have specific size and energy.

2. The energy that an orbit contained is proportional to its size.

3. When an electron moves between the orbits they need energy which is absorbed/ emitted in the form of radiation.

Below is a diagram of the Bohr model of the hydrogen atom, showing an electron jumping between fixed orbits and emitting a photon of energy with a specific frequency


The electrons orbited the nucleus due to electromagnetic force between the protons and the electrons. However this model couldn’t answer some of the basic questions like: ‘Why should electrons occupy only certain energy levels with definite orbits with definite radius?’

Werner Heisenberg


Werner Heisenberg was born on the 5th December 1901, at Würzberg, Germany and became a renowned theoretical physicist. He attended the University of Munich to study physics under Wien, Sommerfeld, Rosenthal, and Pringsheim, attended lectures given by Niels Bohr in 1922 and then went on to take his Ph.D. at the University of Munich in 1923. He became an assistant to Max Born later that year at the University of Göttingen and continued working with Bohr. In 1927 he proposed the ‘uncertainty principle’, which states that electrons do not travel in a neat orbit. He also said that the more precise we are of the electron’s position, the less precise we are about its momentum/velocity. This is because, to observe, we need light which are photons and when we are observing electrons, the photons will interact/collide with the electrons we are observing and as a result transfer some of their momentum to the electron. He also determined that the only way to describe the location of an electron in an atom is through probability distribution. This formed the basis of the electron cloud model.

Here you can see Heisenberg’s uncertainty theory



Erwin Schrodinger


Erwin Schrodinger was an Austrian Physicist and he was born on the 12th August 1887 in Vienna, Austria. He went to the University of Vienna in 1906 and it was Fritz Hasenöhrl’s lectures on theoretical physics which had the greatest influence on him. He was awarded his PhD in 1910 and obtained a further degree in 1914. During WWI he served as an artillery officer but was still able to continue with his research and between 1917 and 1924 he worked on many fields within physics. In 1925 Schrodinger took Bohr’s Model of an atom and used mathematical calculations to produce his own model. Using De Broglie’s theory he deduced a set of equations or wave functions which he then used to predict that finding the position of an electron is a probability, as electrons are waves and they could form standing waves in the orbit. He also said that the probability of finding electrons in a region of space called an orbital was distributed along the orbital. He described the orbital as an electron density cloud and that the denser it is, the higher the probability of finding an electron. Schrödinger published his revolutionary work relating to wave mechanics and the general theory of relativity in a series of six papers in 1926. Wave mechanics, as proposed by Schrödinger in these papers, was the second formulation of quantum theory, the first being matrix mechanics due to Heisenberg. Many scientists preferred Schrödinger’s theory since it could be visualized, while Heisenberg’s was strictly mathematical. A split threatened among physicists, but Schrödinger soon showed that the two theories were identical, only expressed differently.


The picture above is a depiction of the atomic structure of the helium atom. The darkness of the electron cloud corresponds to the line-of-sight integral over the probability function of the 1s atomic orbital of the electron. The magnified nucleus is schematic, showing protons in pink and neutrons in purple. In reality, the nucleus (and the wave function of each of the nucleons) is also spherically symmetric and 1s, and the four particles, each with a different quantum number, like the electrons in the helium atom, are all most likely to be found in the same space, at the exact centre of the nucleus. (For more complicated nuclei this is not the case.)


New scientist magazine – publish date 27/04/2002 issue number: 2340


Author: Oxlade, Chris.

Publisher: Heinemann Library,

Pub date: 2002

ISBN: 0431136009

So by 1932 our picture of the atom consisted of a central nucleus containing positive protons and neutral neutrons with a cloud of electrons surrounding it, but this is not the end of the story.

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