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It can happen, while we are studying any subject at school, to find out the atom.
Teachers and books tell us it's a part of the matter so small that it is not visible to the naked eye or even by the optical microscope.
And then, we obviously wonder, how can they say that it exists? How can they say what it's like? Do they just guess?
The truth is nowadays there are lots of machines that allow us to study the whole world, and also the matter.
But when they did not exist, what did people know? What about atoms and cells and blood and heart? What did they know about it?
There were people that, supposing reasonable hypothesis and trying to prove they were right with experiments and so on, had contributed with their theories to write the story I'm talking about: the history of the atom.
Just to make an example: Greek philosophers from the V century b.C., the atomists, didn’t have such a technology as we are used to nowadays.
They thought about how the matter could be divided, and in how many pieces. They knew they could divide numbers, as they are abstract symbols, so they can be divided as much as you want. But you can’t divide anything real unless you have to stop. And there, where you have to stop, there’s the atom.
They gave it this name (atom -> a-tomos = not divisible) because you can’t obtain anything as you are dividing matter. If you obtain nothing, you can’t reform the matter. And so: how can then matter exist?
That’s why they used this term.
Therefore, this was just a convention. There wasn’t any way to prove what the atomists, and Democritus, said. We know: when there’s something we don’t understand, we try to guess. And that’s how we make hypothesis. But there’s something true in what we think, because we use our reason, so we can approach the truth. So, even if Democritus and the atomists were philosophers, not scientists, it’s as they opened the way to chemistry. Because reason is on the base of science, but also of philosophy. But while science tries to check the answers, philosophy tries to answer the questions.
The atomists continued their theories saying that the atoms’ order influenced matter features. So, changing the atoms’ order, the matter would change.
Of course there wasn’t any way to prove this... and, while some philosophers supported Democritus's ideas, others didn’t. For example: Aristotle, whose theory was accepted by the Church.
Long time after, Isaac Newton specified that atoms didn’t bind physically, as the atomists said, but there were attractive forces that allow atoms to bind to each other.
Let’s skip then to the beginning of the XIX century a.D when John Dalton, a chemist, said that:
matter is formed by particles named atoms, which are not-divisible and can neither be created nor disrupted;
the atoms which form any stuff have all the same characteristics;
atoms with different characteristics make out different kind of stuff;
matter change as various atoms bind together.
I don’t mean he tried to demonstrate atomists’ theories. Even if there are people who say he did, and people who say he didn’t. However, there were for sure many people who tried to do that, sometimes cooperating, sometimes even denying each other.
As it happened between John Dalton and Joseph Louis Gay-Lussac, who, helped by Alexander von Humboldt, said that different atoms could bind together, even if their number didn’t match.
In fact, Dalton thought that the ratio between different atoms had to be the same.
Now we can understand why they thought this way: they didn’t know that matter isn’t formed just by atoms. Atoms are the smallest part of matter, and can be clustered forming molecules.
Matter is formed by molecules, molecules are formed by atoms. So, if anyone assumes atoms are like molecules, nothing would come out but wrong results.
That’s because you can divide matter until there is nothing more than one molecule, and still have matter features; but if you continue dividing even the molecule, you won’t have the same stuff anymore.
That’s what Amedeo Avogadro found out.
Chemistry research about the atom has surely continued as well as research about its physical proprieties.
Let’s start with J. Thomson’s atomic model. Thomson said that the atom is a spherical particle and within it there are electrical charges.
But one of Ernest Rutherford’s experiment demonstrated that the electrical charges are situated within the nucleus, but only the positives ones, while the negatives surround them.
How could he say this? In his experiment, so-called alpha particles were sent against atoms. Some particles passed through, while others changed their way. That means that alpha particles which hurt the nucleus were discarded.
If the particles passed through, that means that there’s a space between the nucleus (positive) and the electrons (negative).
Rutherford’s model is also called planetarium because the ratio between protons and electrons is like the ratio between a planet and its satellites.
In conclusion: for what we know today we should be thankful to many people, who, with their knowledge, research and theories, had contributed century by century to give us the definition of the atom that we still use.