A key concept of science taught in middle school is atomic theory, everything in our universe is composed of tiny particles called atoms. As we progress through high school and further education, the atom still remains relevant, yet more and more complex ideas begin to emerge about this dependable concept that creates the mysteries that scientists strive to solve. This article goes back to the very beginning, when the period of time where the atom and the three subatomic particles came into existence and even earlier when the idea of the atom was born.
Like many scientific concepts with origins in Greece, the atom is not an exception. The idea of an atom was conceived by Democritus, who believed that all things were composed of tiny invisible things that could not be divided any further. An analogy for his ideas is to cut an apple until one piece is so small that it cannot be further divided. In Greek, the root of what we call indivisible matter is ‘atomos’, which clearly links to the English word for the atom. However, at the time Aristotle’s ideas on the four humours composing all objects in our universe began to become more popular leaving Democritus' idea of the atom left in history.
However, in the 1800s, John Dalton reconsidered and brought back Democritus’ idea of the atom. Dalton was a chemist who experimented on various elements and compounds, realising that reactants combine in ratios to form certain products. His experiment was proven to be suspicious in the 20th century because it was difficult to recreate his results but the experiments proved to Dalton that a theory about atoms could be created. Thus atomic theory was born.
Based on the following principles
All matter is composed of extremely small particles called atoms.
Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties.
Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements can combine in simple whole number ratios to form chemical compounds.
In chemical reactions, atoms are combined, separated, or rearranged.
In 1897, J.J Thomson discovered the electron, as an esteemed professor at Cambridge University, he had spent a few years determining what was inside a cathode ray tube, a device that had wires on each side and when connected to a voltage, the ray would form patterns. The question that remained was what was the ray inside the tube? He conducted a few experiments which manipulated the ray by adding magnetic or electric fields and measuring the deflection. The particles were emitted by the negatively charged end of the tube and repelled by a negative electric field when added. Since like charges repel, he concluded that the particle was negatively charged. Initially, the electron was called corpuscles, which mean small bodies, but after contributions from other scientists such as G. Johnstone Stoney and George Francis Fitzgerald, the particle was named an electron.
Credit to chem.libretexts.org
The discovery of the electron did pave the way for a different model of the atom: the Plum Pudding Model. In this model, the electrons were inside a positive body, some consider it like a chocolate chip cookie, where the chocolate chips are electrons while the cookie is the positive surrounding. Since matter is electrically neutral, Thomson believed that the negative charges needed to be cancelled.
Initially, some felt that Thomson was crazy for suggesting the existence of the electron but some people did acknowledge his discovery and wanted to test the Plum Pudding Model. Those scientists were Hans Geiger and Ernest Marsden under the instruction of Ernest Rutherford. Geiger and Marsden fired alpha particles (helium nucleus with two protons and two neutrons) from a radium source at gold foil, this was surrounded by a screen coated in fluorescent substance. When the alpha particles hit the screen they leave streaks that the scientists could see. Rutherford and his team noticed 3 observations: most of the particles went straight through the gold foil, some were deflected at very sharp angles while some were deflected at more gradual angles. From this, they concluded that Thomson’s Plum Pudding model was incorrect.
Since a majority of the particles went straight through the foil, it suggested that there must be a lot of empty space within the atom. While some of the alpha particles deflected back, suggesting there was something at the centre. The alpha particles have a 2+ charge and since some of the particles were deflected at large angles, Rutherford hypothesised that the thing in the center was positively charged. Thus, the atom nucleus was discovered.
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The initial understanding was that despite the existence of a positive nucleus, Rutherford still believed that negative charges would still be evenly distributed in the atom. Yet, Marsden and Rutherford decided to use a cloud chamber filled with hydrogen and fire alpha particles at the gas, he saw traces of the radiation in the chamber and discovered the proton. A few years later, he theorised the neutron in order for the atom’s mass to be balanced but the discovery was done by James Chadwick.
James Chadwick initially had gone to Germany to further his studies in 1914 but due to the outbreak of WWI, he spent much of his time in a prison camp. Famous stories of him asking guards to buy him toothpaste that was mildly radioactive in the German market, which he used to make an electroscope with wood and tin foil, an instrument used to detect electric charge on an object. After the war, he was inspired by Walter Bothe and his student Becker who were bombarding beryllium with alpha particles and studying the radiation. He replicated their experiment but rather than considering the radiation emitted as photons ( a mass-less quantum particle that carries the energy in EM waves), he believed that it was a particle with the mass of the proton but neutrally charged. In 1935, he was awarded a Nobel Prize.
Infographic from Labster Theory
The components of the atom that we know today were ingeniously discovered but what about the atomic structure? After Rutherford’s model, quantum physicist Niels Bohr modified his idea by adding electrons moving in fixed shells around a nucleus, this was through very complex experiments and hypothesising. As quantum physics progressed, Erwin Schrodinger and Werner Heisenberg added a probabilistic element to the atom, electron clouds surrounding a nucleus, which remains the current model of the atom. As technology progresses, who knows what the atom will look like in the future?
Sites
https://www.aps.org/apsnews/2007/05/may-1932-chadwick-reports-the-discovery-of-the-neutron
https://www.jstor.org/stable/4027867
https://spark.iop.org/rutherfords-alpha-scattering-experiment
https://www.phys.ufl.edu/~korytov/phz6355/note_A14_e_p_photon_n.pdf
https://www.compoundchem.com/2016/10/13/atomicmodels/
https://www.bbc.co.uk/bitesize/guides/z29rsrd/revision/1
https://www.biography.com/scientist/john-dalton
https://www.aps.org/apsnews/2000/10/discovery-of-the-electron
https://scienceready.com.au/pages/rutherfords-atomic-model#:~:text=orbit%20the%20nucleus-,Geiger%2DMarsden's%20Gold%20Foil%20Experiment,when%20alpha%20particles%20hit%20it.
https://www.livescience.com/gold-foil-experiment-geiger-marsden