The Story of the Atom

Possibly the earliest scientific question we as a race attempted to answer was ‘what is our world made of’. People reasoned that everything had to be made of something- all the machines and things we build have different components in them that we can identify, so it seemed natural that those materials and components were in turn made of some ‘stuff’ or other. Some reasoned that everything was made up of the most common things present in our earth; the classical ‘elements’ of earth, air, fire and water, but throughout the latter stages of the last millennia the burgeoning science of chemistry began to debunk this idea. People sought for a new theory to answer what everything consisted of, what the building blocks were, and hoped to find in this search an answer to several other questions; why chemicals that reacted together did so in fixed ratios, for example. For a solution to this problem, they returned to an idea almost as old as science itself; that everything consisted of tiny blobs of matter, invisible to the naked eye, that joined to one another in special ways. The way they joined together varied depending on the stuff they made up, hence the different properties of different materials, and the changing of these ‘joinings’ was what was responsible for chemical reactions and their behaviour. The earliest scientists who theorised the existence of these things called them corpuscles; nowadays we call them atoms.

By the turn of the twentieth century, thanks to two hundred years of chemistry using atoms to conveniently explain their observations, it was considered common knowledge among the scientific community that an atom was the basic building block of matter, and it was generally considered to be the smallest piece of matter in the universe; everything was made of atoms, and atoms were fundamental and solid. However, in 1897 JJ Thomson discovered the electron, with a small negative charge, and his evidence suggested that electrons were a constituent part of atoms. But atoms were neutrally charged, so there had to be some positive charge present to balance out; Thomson postulated that the negative electrons ‘floated’ within a sea of positive charge, in what became known as the plum pudding model. Atoms were not fundamental at all; even these components of all matter had components themselves. A later experiment by Ernest Rutherford sought to test the theory of the plum pudding model; he bombarded a thin piece of gold foil with positively charged alpha particles, and found that some were deflected at wild angles but that most passed straight through. This suggested, rather than a large uniform area of positive charge, a small area of very highly concentrated positive charge, such that when the alpha particle came close to it it was repelled violently (just like putting two like poles of a magnet together) but that most of the time it would miss this positive charge completely; most of the atom was empty space. So, he thought the atom must be like the solar system, with the negative electrons acting like planets orbiting a central, positive nucleus.

This made sense in theory, but the maths didn’t check out; it predicted the electrons to either spiral into the nucleus and for the whole of creation to smash itself to pieces, or for it all to break apart. It took Niels Bohr to suggest that the electrons might be confined to discrete orbital energy levels (roughly corresponding to distances from the nucleus) for the model of the atom to be complete; these energy levels (or ‘shells’) were later extrapolated to explain why chemical reactions occur, and the whole of chemistry can basically be boiled down to different atoms swapping electrons between energy levels in accordance with the second law of thermodynamics. Bohr’s explanation drew heavily from Max Planck’s recent explanation of quantum theory, which modelled photons of light as having discrete energy levels, and this suggested that electrons were also quantum particles; this ran contrary to people’s previous understanding of them, since they had been presumed to be solid ‘blobs’ of matter. This was but one step along the principle that defines quantum theory; nothing is actually real, everything is quantum, so don’t even try to imagine how it all works.

However, this still left the problem of the nucleus unsolved; what was this area of such great charge density packed  tightly into the centre of each atom, around which the electrons moved? What was it made of? How big was it? How was it able to account for almost all of a substance’s mass, given how little the electrons weighed?

Subsequent experiments have revealed an atomic nucleus to tiny almost beyond imagining; if your hand were the size of the earth, an atom would be roughly one millimetre in diameter, but if an atom were the size of St. Paul’s Cathedral then its nucleus would be the size of a full stop. Imagining the sheer tinyness of such a thing defies human comprehension. However, this tells us nothing about the nucleus’ structure; it took Ernest Rutherford (the guy who had disproved the plum pudding model) to take the first step along this road when he, in 1918, confirmed that the nucleus of a hydrogen atom comprised just one component (or ‘nucleon’ as we collectively call them today). Since this component had a positive charge, to cancel out the one negative electron of a hydrogen atom, he called it a proton, and then (entirely correctly) postulated that all the other positive charges in larger atomic nuclei were caused by more protons stuck together in the nucleus. However, having multiple positive charges all in one place would normally cause them to repel one another, so Rutherford suggested that there might be some neutrally-charged particles in there as well, acting as a kind of electromagnetic glue. He called these neutrons (since they were neutrally charged), and he has since been proved correct; neutrons and protons are of roughly the same size, collectively constitute around 99.95% of any given atom’s mass, and are found in all atomic nuclei. However, even these weren’t quite fundamental subatomic particles, and as the 20th century drew on, scientists began to delve even deeper inside the atom; and I’ll pick up that story next time.

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