I just can’t get it how did they know the atom and the details of it.
In: 14
Rutherford directed alpha particles from a decaying radioactive material to a piece of foil and observed the way in which these particles reflected and scattered. The only way the particular scattering made sense was if almost all the atomic matter was concentrated in a tiny volume situated at the atome center, the atomic nucleus.
ELI5 version:
If you close your eyes and throw tennis balls a vehicle you could probably figure out if if was a truck or a car or a motorcycle based on where the tennis balls ended up.
They used many techniques, starting with shooting electrons through thin gold foil and noticing that while most of the electrons flew right through, some of them were deflected as if they had hit something hard. That’s how they discovered that atoms are mostly empty space with a nucleus. Then there is a technology called x-ray crystallography, where they were able to scan crystals and read the patterns to reconstruct the three-dimensional shape of the atoms in the crystal. That Parts too complicated to do an eli5 explanation.
Scientists discovered atoms by observing the effects of them on things around them. For example, they noticed that when atoms collide, they cause things to move. By studying these effects, they were able to infer the existence of atoms.
The modern atomic model is the result of about 70 years of many different attempts to understand the nature of atoms. The basic approaches involved people a) speculating about how they _might_ work (theory) and b) doing experiments that would either confirm or deny some of these speculations, or give information that could prompt more speculations (experiment). Sometimes theory went first, sometimes experiment went first, but the back-and-forth was how they figured it out.
For example. The idea of atoms — of there being “units” of chemistry — became popular in the 18th century as part of the Chemical Revolution. It was a very useful way to understand how chemical reactions worked. But nobody had much of an idea of what these atoms _were_ other than a way to understand chemistry. So you could say, if I have some atoms of hydrogen and some atoms of oxygen and I subject them to a spark, it will cause a (pretty violent) reaction that will end up with water, which is made of both hydrogen and oxygen somehow merged together.
There was a scientist named William Prout who in the early 19th century basically said, I bet that the difference between a hydrogen atom and an oxygen atom is related to them being made out of different amounts of the same stuff. That is, that atoms were themselves made of pieces (“subatomic particles”), and that hydrogen might have some number of these and oxygen might have another number. Prout theorized that there was only one type of subatomic particle.
Later in the century, an experimental physicist, J.J. Thomson, became interested in weird “rays” that seemed to be produced through experimental devices called cathode ray tubes (the same kinds of tubes that made old TVs work). People had already discovered, sort of through tinkering, that if you passed an electric current through a vacuum in the right way, it made some kind of rays come out the other side that could be detected with the right equipment. But nobody knew why this worked or what the rays were.
Thomson said, ah, let’s study the rays and see (based on a hypothesis) if they might be something like Prout’s hypothetical subatomic particle! So he did some very careful observations that proved that the rays coming out were tiny particles with strong electric charges that could be bent by a magnet. He declared that what was happening in a cathode ray tube is that it was breaking down atoms into their subatomic pieces, and what was being shot through them was a Proutian particle that Thomson dubbed the corpuscle. Everybody thought that was a dumb name and used “electron” instead.
To accompany this idea, Thomson suggested that maybe atoms were just big lumps of electrons that were somehow all rotating around each other. A single atom might have hundreds of electrons in it in Thomson’s model. This, he argued, explained the cathode ray tubes, and also explained (in its own way) the link between matter and electricity.
Cool idea, said other scientists, but they were never totally sold on it. Ernest Rutherford and his students tried an experiment to see if atoms worked the way Thomson thought. They used a source of alpha radiation (which had been recently discovered, but itself not understood) to shoot particles through a very thin foil of gold. If Thomson was right, and atoms were composed of just a bunch of swirling little electrons, the alpha particle (which was a positively-charged, relatively large particle) should just fly right through them. And most of them did fly right through. But some of them got reflected backwards at a very sharp angle. It is sort of like what would happen if you through a tennis ball at a single metal pole: most of the time you’d miss, but if you hit the pole, it’d bounce right back at you. (How’d they “see” these particles? They had a setup by which they could tell how many particles hit a detector, basically, so they could just count the “hits” and where they were.)
From this data, Rutherford reasoned that Thomson’s model couldn’t be right. That there probably was a cloud of electrons, but that something very small, but very hard, was located in the center of the atom — the thing the alpha particles were being reflected by. He dubbed this the “nucleus,” which is just a Greek word that means “kernel” (like the seed at the center of a peach), and posited that it probably contained most of the mass of the atom and that it was positively charged, unlike the electron. The electrons, he guessed, might rotate around the nucleus like planets rotating around a Sun. So this is sometimes called the “planetary” model of the atom.
And so on, and so on. Later theory figured out that electrons can’t work like planets; they’d run out of energy. Explaining how they could work required inventing quantum theory and quantum mechanics. Experiments showed how that did and didn’t work as well. The nucleus itself became something to investigate, and various experiments showed that it contained subatomic particles of its own (protons, neutrons) and, by the 1960s, that even these contained their own sub-subatomic particles (quarks). In this time they also figured out how to split heavy nuclei (fission) and fuse light nuclei (fusion), and figure out in great detail how these different parts of the atom worked, what forces they were subjected to, and so on.
Obviously I’m summarizing a lot, but you can see that a) they didn’t need to “see” an atom at any time in this, because they could infer things from experiment, b) there was a complicated and intertwined back and forth between theory and experiment the whole way through, and c) ultimately they ended up with a model that does a _very_ good job of answering questions we might have of it, and which is built into all sorts of applied settings, so it must be somewhat accurate, though there always may be something new to learn.
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