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History of the Atom Discovery

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  • Mohammad Shahraan
  • Khan Phys
  • Helen O’Keefe

The secrets of atom

Democritus was the first one to suggest that objects are made from something called atoms. Although Democritus was an ancient Greek philosopher, the word ‘atoms’ is from the Greek word atoma which means individual. Democritus was around from 460 – 370 BC and he further deduced that atoms are solid spheres and that they can’t be split anymore.

Next, Aristotle who was also an ancient Greek philosopher, offered that items or objects were made from ‘Elements’. He said that the elements are either earth, wind, fire, and water and the properties to go with it for example dry, cold, hot, wet. Something could be made by joining elements together and could be converted into other things by adding other elements. Aristotle’s atomic theory was to show that anything made with fire could be either hot or dry or if anything was made with earth, this could be either dry or cold.

At that time arguments were established by thought, reason and debate, there were no experiments as experiments were thought to be vulgar. Now as Aristotle was a wealthy man and was treated as a celebrity, his explanations were based on familiar experiences, he made conclusions based on what he saw for example snow and fire joined together makes water. Whereas Democritus was not as popular and nobody wanted to believe him as his theory was saying that atoms couldn’t be seen. Basically Aristotle’s theory was the basis of atoms all the way to the middle ages.

In addition, Robert Boyle who was around from 1627 – 1691, studied gases and conferred the likelihood of atoms existing. He predicted that elements are made from something called ‘corpuscles’. He stated that atoms are organised in groups and that different groups are different chemical substances. It was around his time that experiments has started to come around.

Moreover, Isaac Newton was another scientist who was around 1643 – 1727, he also studied gases. He is famous for being the one who discovered gravity. He proposed a mechanical universe where solid masses were in movement. Also that atoms/particles are not stationary.

Furthermore, Antoine Lavoisier who was around 1627 – 1691 became known as the father of modern chemistry. He was an excellent experimentalist, and as such he assembled an accurate and precise balance to investigate oxidation. He demonstrated that when a substance is oxidised, the increase in its mass is equal to the mass lost by the surrounding air. He stated one of the most fundamental laws of science which is the mass conservation law and it states that matter cannot be created nor destroyed.

Additionally, John Dalton was around from 1766 – 1844 and he suggested a theory of atoms, which are that elements consists of tiny particles called atoms. Atoms of the same element are alike whereas atoms of different elements vary in size, mass and other properties. Atoms cannot be divided, created or destroyed. Compounds (molecules) are made when different elements are joined together in whole-number ratios. In a chemical reaction, atoms are linked, separated or rearranged.

Likewise, during the 19th Century, people were eager to find new elements and by 1860, 60 new elements had been discovered. Then a scientist called Dimitri Mendeleev who was around in the time of 1834 – 1907, had a concept of classifying the elements. He rearranged the elements in order of ascending atomic weight, he discovered consistent patterns and he invented a table to predict presence of numerous elements. The modern version of the periodic table organises elements according to an ‘atomic number’. An atomic number is number of protons added with number of neutrons. Changes that are given to the modern periodic table are the positions of some elements.

Also then in 1857, Heinrich Geissler experiments on whether electricity can still travel, if the air was taken away. So when most of the air was sucked out, the tube still glowed. This attributed the small amount of air left in the tube. He discovered different gases generated different colours of light. People liked this a lot and so used them for entertainment for example neon lights. Energy saving lightbulbs are an example of gas discharge tubes.

Then another scientist named Crookes who made a vacuum tube and he made a better vacuum tube than Heinrich Geissler as it sucks more air out. As a result there is no glow in the tube but on the glass at the end of the tub glowed green. There a cross was produced a shadow on the screen. For the vacuum tube, whatever moved the current, travels in a straight line. Crookes designed a lightweight wheel to see if the rays made it turn. The experiment method was to apply a voltage to the apparatus, the wheel moved away from the cathode but the light wouldn’t turn this wheel. Cathode rays must be some kind of small particle.

J.J. Thomson who was around 1856 – 1940 wanted to see if the particles could be strayed by a magnet and also if another voltage was applied to the tube. He designed an even better vacuum than Crookes’ and Heinrich Geissler’s vacuum tubes. He observed that the rays bounced towards the positive plate. Particles are negatively charged. He hypothesised that these particles are part of the atom. “… the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification, …” 1^ and 1* [Thomson, 1904] This was compared to a British dessert at the time so it became known as the plum pudding model.

Ernest Rutherford was around from 1871-1937, was accountable for discoveries in radioactivity and nuclear physics. He was a student of J.J. Thomson and wanted to determine the size of the atom. He fired positively charged particle at a thin gold foil. He anticipated positively charged particles would not deviate as they passed through the positive sphere. Rutherford actually observed that about 2 in every 7 positive particles deflected back. “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.Ernest Rutherford. 2^ and 2* [Rutherford, 1964] He discovered alpha and beta rays that pioneered the laws of radioactive decay, and acknowledged alpha particles as helium nuclei. This showed that there is something in the centre of the atom and it contains most of the atomic mass. Rutherford clarified his results by saying that atoms is made up of mainly empty space, they are small, dense and that there is a positive sphere at the centre known as the nucleus. The positively charged particles are redirected if they are close enough to the nucleus and also that electrons orbit the nucleus.

There is a theory known as the electromagnetic theory which states that any charged particle in a circular orbit radiates electromagnetic energy. The electron loses energy as it orbits the nucleus. The radius of its orbit decreases as the energy decreases. The electron should spiral towards the nucleus. The electron should emit electromagnetic waves as it loses energy at a mixture of frequencies over a certain range. The radiation spectra were not continuous. The emission spectra couldn’t be resolved with the Rutherford model, no one really understood why the formula worked. Although a scientist named J.J. Balmer has studied the emission spectrum for several elements. Spectra for other elements could be predicted using the formula. A mathematical model could be made on observations from hydrogen.

Neils Bohr who was around at the time of 1885 – 1962 came with a revolutionary proposal which states energy of an orbiting atom is quantized i.e. only particular types of energies are allowed. Energies must be multiple of a base unit, he also proposed that the electrons could jump between orbits. He was the one that pioneered the quantum theory. The Bohr model shows that electrons orbit in shells of definite energy. If an electron changes from a higher to a lower energy state, the change in energy is proportional to the frequency f of the emitted photon. The energy is given off as a photon of definite energy. This relates line spectra to atomic model. Energy is only released when electrons moves to a lower energy state. Photon represents the “spare energy”.

Planck proposed light travels in discrete packets of energy which is quanta. Quanta is photons. Photons move at the speed of light and they have an associated frequency. For the electron to emit light, minimum energy is required. Quantum theory explains the photoelectric effect. Einstein’s equation E = mc2 relates matter and energy.

In conclusion the atomic structure and the atom itself is so interesting to learn about and you could spend millenniums studying about it. It built the way to radioactivity, x ray treatment, matter and anti-matter particles even dark matter just with atoms. This pioneered nanotechnology and most of the stuff we see today. The theory of relativity could be related to this in a way because atoms were the cause of all these theories we see today. Richard Feynman who became one of the best-known scientists in the world remarks about the atom in this statement “If all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis that all things are made of atoms – little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied…” 3* and 3^ [Feynman, 1998] this quote demonstrates that how atoms are so interesting and so interesting that even in millennia people will be studying it.


1* Thomson, J.J. (1904). On the Structure of the Atom: an Investigation of the Stability and Periods of Oscillation of a number of Corpuscles arranged at equal intervals around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure (extract of paper). Philosophical Magazine, p.237 (British science journal)

2* Rutherford, E. (1964). Rutherford and the Nature of the Atom by E. N. da C. Andrade, p.111, and quoted in Nobel Laureates in chemistry (1901-1992) by Laylin K. James, p.57.

3* Feynman, R. (1998). Six Easy Pieces: Fundamentals of Physics Explained (Penguin Press Science, Paperback), p.4


1^ Plum Pudding Model. (n.d.). In Wikipedia. Retrieved 12th February 2015, from http://en.wikipedia.org/wiki/Plum_pudding_model

2^ Ernest Rutherford. (n.d.). In Wikiquote. Retrieved 12th February 2015, from http://en.wikiquote.org/wiki/Ernest_Rutherford

3^ Richard Feynman. (n.d.). In The Information Philosopher. Retrieved 12th February 2015, from http://www.informationphilosopher.com/solutions/scientists/feynman/

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