The Atom 

Have you ever wondered what the things around you are made of? What, at its core, is the universe made of? If every object and substance can be broken down into just a few, or less, fundamental things that compose them? These are age long questions that have been asked ever since humans first began to develop complex thought. It's only natural for us, blessed with advanced intellects, that we ask ourselves how the world works and what is our fundamental place in it. In the quest for this fundamental truth about the universe, the problem of what matter is made of, has sprung up several times. However, it wasn't until very recently in human history that we have come up with an accurate model of what composes things. In this article, you will walk the road it took to get here, and how we currently think that matter is structured.

 

Our story begins in ancient Greece. It is here that Greek philosophers Leucippus and Democritus first came up with the idea that matter is made up of tiny particles; they believed that if you were to split matter up into smaller and smaller portions, surely, there must be a point in which you can no longer divide it; an indivisible particle. These particles were given the name 'A Tomos', which translates from Greek to 'indivisible'. Leucippus and Democritus then took this a step further by saying that the indivisible particle, or atom, of each material had the properties of said material, so clay atoms would be soft, moldable and squashy while the atoms of something like rocks would be hard.

 

This idea, however, wasn't developed for the next 2,300 years or so, and it wasn't until the 1800s that this atomic theory would be developed, this time with much more objective evidence. It was during this time that French chemist Antonie Lavoisier first came up with a theory for the conservation of matter. After running an experiment in which he heated a sample of tin and air in a sealed glass container, he observed that the mass of the container and its contents was exactly the same before and after he heated them up. This then led him to the conclusion that nothing in the universe is either made or destroyed, things can change shape or form, but their actual mass, the amount of matter that they contain, must remain the same given that mass is not lost to the surroundings. British lecturer John Dalton later built upon Lavoisier's principle and proposed that elements exist as discrete packets of matter, he did so by performing experiments with samples of different gasses. He noted that when gasses reacted and combined, they would always end up doing it in specific ratios. So, for instance, Oxygen and Hydrogen would always combine to make up water in a 2:1 ratio. In doing this, he devised the idea of the atom being a solid sphere. Each element would have a uniquely structured sphere. These contributions to science made it clear that matter was indeed composed of atoms, but even despite this, several questions still remained, mainly, why do atoms behave the way they do? This began the question of atomic structure: what in the structure of atoms causes them to behave the way they do?

The next big discovery regarding the atom which attempted to solve the question of atomic structure would involve cathode rays. A cathode ray tube, in the chance you are not aware, is a bottle shaped, sealed piece of glass with two pieces of metal on one end through which one can run a current. A diagram of a cathode ray tube is shown in Figure 1.1. 

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​The metal plates labeled 'cathode' and 'anode' respectively are both connected to a power supply. When turned on, this power supply gives the anode a positive charge and the cathode a negative charge. In 1897, J.J. Thomson conducted this experiment, and by doing so, he was able to observe that, as can be seen in the diagram, a beam shot through the plates and into the other end of the cathode ray tube. Upon seeing this, J.J. Thompson got curious about what the cathode ray was made up of, and so as any scientist would, he ran a series of experiments in an attempt to reach the truth. Eventually, by placing two metal plates next to the cathode ray, J.J. Thomson found that it bent away from the negatively charged plate and towards the positively charged plate. This can be seen in Figure 1.2. 

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After observing the shift of the cathode ray, J.J. Thomson deducted that given its behavior, the cathode ray must have a negative charge. He verified this deduction by performing another series of experiments involving magnets, and after measuring the mass of the cathode ray by measuring its deviation in a magnetic field and the heat it emitted, Thomson calculated that the mass of the ray was about 1,000 times lighter than that of a hydrogen atom, the smallest bit of matter known at the time.

 

 

It was well known that Hydrogen was the smallest, and thus, lightest atom, and yet, if atoms were indivisible and the smallest thing that existed, then how could there be something lighter? What Thomson had just inadvertently proven was that atoms were not indivisible, since in light of the evidence provided by this experiment, Thomson reasoned that these tiny particles with a negative charge –which we now call electrons- must be coming from atoms because everything is made of atoms, hence, atoms had to be divisible and made up of these smaller particles which carried a negative charge. This conclusion didn't exactly align with other observations, however. It was clear that not all matter had a charge, as everyday things generally have a neutral charge; not everything is attracted to positive or a negative plate. And so, considering these two pieces of evidence, Thomson argued thus: atoms must be made up of a very large positive charge plastered with very small negative charges to counteract it, thus making the atom of a neutral charge. This reasoning can be summed up in the Plum Pudding model of the atom, as can be seen in Figure 1.3.

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