For example, if you take a sample of human bone and put it under a microscope, how do you know if the atoms you’re seeing are calcium atoms? You can’t exactly count the protons on the inside, can you? Also, how do you distinguish between protons and neutrons? Do they reflect different wavelengths of light and so have different colours or something? I’ve also heard people saying that we can’t actually see atoms using microscopes, is that true? If so, how can we say something is made out, say, carbon, when we can’t see it? If the answer to that is that we have tests (flame tests for metals, precipitate tests, pH tests, etc…), then how did we know it is that element/compound that results in the test turning out a certain way? I have so many questions!
P.S. I know that nuclei aren’t really perfect spherical balls, but rather collections of protons and neutrons, which are spheres, in a classical, non-quantum-mechanical sense.
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We can tell the total number of protons and neutrons in an atom once we know its atomic mass. Electrons do have a mass, but it’s very, very small, and while neutrons are heavier than protons the difference is tiny, so the mass of an atom can mostly be used as a proxy for the number of protons and neutrons it has.
We can then determine the number of protons by measuring the charge in the nucleus. Protons have a positive charge, while neutrons have no charge, so the charge in the nucleus tells us the number of protons. We can then get the number of neutrons just by subtracting the number of protons from the total mass.
We can’t see atoms using *optical* microscopes, it’s true. They’re just too small: they can’t reflect light in the way larger objects can. But it’s not quite correct to say that we can’t see atoms using microscopes at all. What we can’t see atoms with is light. If we build microscopes that “see” with something else -something that can be used on small enough scales to see atoms- and then translate that to light so that our eyes know what to do with it, that works. A scanning tunneling microscope, for example, uses electrons and the strange ways they interact with objects to build up a picture of an atom.
How do we identify a substance? There a number of different ways, but the general idea is that we use chemistry: we see how an element reacts to substances we already know. The periodic table is useful here, because there turn out to be strong similarities between elements in the same groups. Even before we discovered germanium, for example, we knew that there should be an element with 32 protons in its nucleus, and we were able to guess (correctly, as it turned out) that it should have a high boiling point, a pH slightly above 7, and so on, because it was in thw same group as carbon and silicon. We even predicted its color correctly. We just hadn’t isolated it yet.
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