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.
In: 0
So many confusions here.
Nuclei are not spheres, they are clumps of protons and neutrons, like grapes.
Optical microscopes can’t see atoms, the wavelength of light determines the smallest thing you can see. We have other microscopes, like scanning/tunneling electron microscopes that can image a single atom, but “see” is in the scientific instrument sense of the work, not the looking at it with your eye sense.
Protons have a positive change, neutrons have neutral charge, you can’t see either of them.
Different atoms absorb and radiate light at specific wavelengths, as electrons shift between orbital states. You can see this light and measure it with a spectroscope.
You know something is made of carbon/calcium through chemistry, and each test has it’s own “justification”.
This part of science has been very thoroughly studied for hundreds of years. Maybe you could search for your questions one at a time, and only post the ones you can’t find ELI5s for.
You can’t see a nucleus with a microscope. The short answer is that you know how many protons + neutrons there are by the mass of the atom. And you know how many are protons or neutrons by its charge – protons have positive charge, and neutrons have no charge.
The short answer is that they’re too small to be seen with microscopes and they’re not spheres. The longer answer is that you’re expecting an entire Intro to Chemistry course out of ELI5 and this may not be the best venue for that. Maybe something like brilliant.org would help?
They’re not *really* spheres. It’s just convenient to call them that. In reality, the particles are constantly moving around at such a high speed all we can ever really say about where they are is that they are “somewhere within this sphere”.
The way we know what we’re dealing with is the way elements behave when we do things like heat them up or combine them is described by the number of electrons they have, and THAT is most strongly influenced by their number of protons and neutrons. Atoms can carry a charge and be ions, but in general we can do other things to understand if we’re working with ions or not and remove the charge to make them the element they are.
If the numbers of electrons for elements are wildly different than what we expect, we’ll find we can’t make certain reactions work.
That’s how flame tests, precipitate tests, pH tests, etc. work. We’ve spent hundreds of years studying what happens if we do those things to “pure” samples of the element and our model of atoms has a kind of algebra to it that describes the expected results. If we perform an experiment the algebra tells us what we should get, and if we get a different result the only explanation can be “this sample is not made of the element we thought it was”.
A lot of this was done without seeing them, purely using that algebra. A lot of Physics is similar: it uses math to describe the results we’ve observed but we can’t actually see the things the math implies exists even with our best microscopes. This is why sometimes things like Relativity come along and cause a HUGE leap in our knowledge: before Relativity our math for how things move was “good enough” but there were a lot of things going on in the universe we couldn’t explain. Relativity explained almost all of those things and why the math that was “good enough” wasn’t working for them.
So it’s also possible that one day, we might figure out that the whole proton/neutron/electron model is wrong and there’s some other explanation. But maybe not. Right now chemistry has far fewer “we don’t know why this happens” situations than astrophysics or even biology. Those holes in our knowledge represent places that what happens in our experiments is not what our math says should happen. If we don’t have many of those holes, it implies that our math is accurate, which implies the model is close to reality.
If you look out of the 2nd floor window at a lawn, chances are you can’t make out individual blades of grass. But you can still see the lawn. The same way we see things made of atoms even though we can’t see the atoms.
Some microscopes can actually make out atoms. I think nuclei, much less protons and neutrons, are still out of reach, but in any case we knew about them way before microscopes got even close to being able to see them.
> how did we know it is that element/compound that results in the test turning out a certain way
You got it the wrong way. First we observed that certain materials behave in a certain way. There’s a reddish metal that gets a green coating if left out in the air, and we decided to call it “copper” way before we knew about copper atoms. Only in the 18th and 19th century did scientists come to the conclusion that all matter is made up of atoms, and copper metal is made of copper atoms.
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.
Yes you can use emission spectra (set of wavelengths emitted) to identify chemical species including elements.
As for determining which isotope (variant on the number of neutrons) you’re looking at, basically you have to determine the average mass of each atom.
You can do that by vaporizing a substance and turning it into a gas. With a gas you can use PV = nRT which depends on the number of atoms (n) instead of the mass of atoms.
You can use that gas equation to count atoms, and then you can divide the mass of your substance by the counter number of atoms, to get average atomic mass and that gives you isotope ratios.
>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 isolate them and subject them to various tests and see that they react the way calcium atoms react.
>You can’t exactly count the protons on the inside, can you?
You can, actually. They have mass, and therefore weight. And the protons have electric charge, so can be deflected by an electric field. So we can determine how much of that mass is protons and (by extension) how much of that mass is neutrons.
>Also, how do you distinguish between protons and neutrons? Do they reflect different wavelengths of light and so have different colours or something?
Protons have a positive charge and neutrons have no charge.
>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?
Well, you *can* see carbon. Look in the mirror, see yourself? You’re largely made of carbon. Go outside. Look at a tree. A lot of carbon there. Have a wife or morther? Check out the bling on her ring finger. Pure, concentrated carbon.
How do we know it’s carbon? Well, like calcium above, because it has all the properties we understand carbon to have.
> 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!
Because that’s how we defined it. We take a substance that we don’t know what it is. See how it reacts, slap a label on it, then anything else that tests in the same way we deduce is the same substance with the same label.
So if something has the same weight as carbon, same number of protons and neutrons, reacts with the same way with the same kinds of substances and, basically, is the same in all distinguishable ways, then it’s the same substance.
To figure out a number of protons and neutrons, we can first find atomic mass. That will give us the total number (protons + neutrons). We can figure out atomic mass from statistics of reactions (that’s how it was calculated by John Dalton in 1803), or we can make atom to have unit charge (by giving or taking an electron) and observing its behavior in magnetic field (mass spectrometry).
Figuring the number of protons is harder. We can strip all electrons from the atom and measure the charge. But it is not practical – stripping electrons is a lot of work.
In practice, atoms are identified by a spectroscopy. Instead of measuring the nucleus, we measure an electron cloud instead. Electrons cannot be seen, but “distances” between them can be – they consume wavelengths that just “right enough”. “Distances” between electrons (and therefore – absorbed wavelengths of light) depend on the number of protons, so if two atoms have the same absorption lines – they have the same number of protons.
So you’re basically asking, at the heart of it, “what is the history of people understanding atomic properties and how they relate to their subatomic particles?” Which is a big question!
There’s no single experiment or tool that gives you all of this information. Rather, the understanding we have is the accretion of over a century of different experiments, observations, and so on, which rule out some ideas and encourage others. At the moment, our model of this is good-enough to answer almost any questions we have about it, so we judge that as being as close to accurate as we can be, and some of these fundamental questions seem rather settled. But there are surely things we don’t completely understand, because there are places where our present models break down (e.g., protons and neutrons are made of quarks, but what are quarks made of? This is where something like string theory comes in as a possible answer, but we don’t yet have any way to test that).
Think of it this way, in broad strokes. People became aware, in the 17th and 18th centuries, that the elements of the world could be categorized broadly in how they reacted chemically, and that there were in fact a great many elements (not just four or five, as had been thought before). The developed the techniques still used in modern chemistry today for isolating specific elements — that is, for isolating a chemical composition to the point where it couldn’t be rendered into anything else, and always behaved the same way.
They would then give these elements a name, like “oxygen” or “carbon.” And they could describe how they each had different properties, e.g., oxygen at room temperature is a gas, carbon at room temperature is a solid. And in exploring these properties they found other interesting things, like the fact that if you burn these elements they give off different colors, and in fact if you look at the light from those flames with a prism you see that each element has its own individual spectra. And so they could then take these observations and start looking for more elements (e.g., they used spectroscopy, as this was called, to note that the Sun seemed to have elemental lines that didn’t look like any other they had seen, and thus inferred there was some “new” element in the Sun which they called “helium,” after the Sun).
Over time they did other experiments that showed that you could more or less “weigh” these elements, and that’s when (in the 19th century) they got the idea that you could put all of the elements in order by weight, and that if you did that right you’d see that some elements that were quite different from others in terms of weight actually had very similar properties as other elements (the periodic table). They also were able to see “holes” in this ranking that told them about elements that ought to exist but had not yet been observed.
Another thing they discovered is that if you took a sealed glass vessel and filled it with a gas, or a vacuum, and then ran an electric current through it, it might make interesting colors or, even more curiously, generate rays that could be detected with specialized film or other chemicals, but were invisible to the human eye and had various properties (like being able to pierce human flesh, but not bone — hence X-rays). This stimulated a lot of thought as to what the nature of matter was, and the nature of electricity.
In one of these experiments, one scientist found that electrical current, when passed through a specific device called a cathode tube, produced rays that he was able to measure. These rays had a very strong electric charge and a very low mass. He theorized that this was a fundamental component of atoms that was being broken down by the tube, and called it a “corpuscle.” Everyone thought that was a lousy name and went with “electron” instead. And so the first really serious subatomic model was born: the idea that atoms are made up of different tiny particles, and the number of these it had depended on what kind of atom it was and what its chemical properties were.
Lots of other experiments over the first couple of decades of the 20th century showed that a) there were more to atoms than just electrons (protons, neutrons, etc.), and b) that the structure of the atom was as we understand it today, with protons and neutrons in the central nucleus and electrons on the outside. One could continue the story as I have been writing it, but it would get even more detailed and esoteric as one gets into nuclear and then quantum physics and particle accelerators and so on.
The general point is the same, though: these models that we have are the end product of a multi-century cycle of new ways of thinking about atoms, new tools for probing into them, new experiments for determining whether one model was a better model than another.
It isn’t the sort of thing where we just got very good at “looking” at atoms — they are too small for that kind of straightforward visual observation. It is lots of inferences from theories and devices. Even electron scanning microscopes, which can visualize individual atomic shells, are built-up on those kinds of inferences. What is remarkable is that these inferences have allowed us to build up a model of the world that is both remarkably resilient to any kind of challenge (it satisfies any experiment we can think of) and can be put into direct application (if quantum mechanics or our nuclear model was totally wrong, the computer you are reading this on would not function, because those models were used to develop everything from the transistors that make it “think” to the screen on which you are looking at this).
Latest Answers