PhD., University of Maryland
Jonathan is a published author and recently completed a book on physics and applied mathematics.
The atomic nucleus was discovered by Earnest Rutherford in his Gold Foil Experiment in which he shot alpha particles at a thin piece of gold foil. Most of the particles went straight through the foil, but a few bounced straight back. This told him that there must be small compact positive units inside the atom. This is the atomic nucleus, a small core which contains most of an atom's mass but makes up only a 100,000th of the atom's total volume. The ratio of protons to neutrons in an atomic nucleus affects an atom's stability and can cause radioactive decay.
So let's talk about atomic nucleus now the atomic nucleus was something that was discovered by Rutherford in 1909 when he did a very famous experiment called the gold foil experiment. Now what he did was he had a piece of gold foil they used gold because you can make it really, really thin it's one of the most malleable of metals you can really hammer it down and make it extremely thin and make it do what you want it to do. So he had this piece of gold foil and then he had an alpha emitter, so this is a radioactive isotope that spits out a bunch of helium nuclei. Alright so these alphas come many of them just go right through it. And so that indicates there's a lot of empty space in the gold foil so when something is small as an alpha particle as a helium nucleus goes through it just doesn't even touch anything. But then sometimes he found that the alphas were just back scattered right back at him and that's something that could not be explained by contemporary models of the atom because people were thinking there's got to be all this positive stuff but they just thought that it was just this positive goo that was just the size of the atom and then you had electrons kind of put in and that's called the plumb putting model.
But this experiment shows that, that can't be the case. So this is like firing a gun at a pillow and having the bullet come right back at you. It doesn't work that way, what does it mean if that happens? It means there's a rock in the pillow okay so that's what Rutherford said, there's a rock in the pillow. Now most of the atom's mass is contained at a very, very small core of the atom. How small? A hundred thousand times smaller than the atom itself, so the vast majority of the atom consists basically of empty space and then you've got this tiny little central area that the vast majority of the mass. So rounding that you have something called an electron cloud and you may have seen those weird system models that show like this and then the electrons orbiting. They don't do that, because of quantum mechanics they kind of spread out, they're just part of that space, they don't have a lot of mass but they're spread out. You can think of it like a wave because that's what it is, so this is the contemporary version of atom. We've got a little nucleus in the center and then we've got electron cloud. Alright so what is this little nucleus in the center made of? Well it's made of neutrons and protons.
Neutrons don't have any charge at all protons have the positive charge of an electron. So electrons have a negative charge, protons have the same charge but positive. Alright both of these guys have about the same mass. Not exactly the same and actually neutrons are a little bit heavier which is responsible for a lot of the radioactivity that you already know about but it's about the same. So for that reason, what we're going to do is we're going to call the number of protons which is the total charge of the nucleus, the atomic number and we're going to call the number of neutrons plus the number of protons the mass number. So that tells us the mass of the nucleus because neutrons and protons are about the same mass. Alright so the atomic number tells you the charge of the nucleus and that tells you how many electrons you need to make that atom neutral. So since the electrons are what govern all the chemical reactions the atomic number governs all the chemical reactions it tells you which element you've got. So that's the only thing that tells you the element, how many protons you got.
If you change the mass number, the number of neutrons but you don't change the atomic number then you've got exactly the same element it'll have a different mass number but it'll be the same element. So these two things for example are both carbon, but they've got a different mass number, this one has a mass number of 12 and this one has a mass number of 14. So these are called different isotopes of carbon, an isotope has the same atomic number but a different mass number. Alright so what if I wanted to know how many neutrons are in carbon 12. Well it's carbon so that means that I already know the number of protons 6 and that's for both of them. This 12 is the mass number of carbon 12, so the mass number is the number of protons plus the number of neutrons. The number of protons is 6 and the sum is 12 then this guy has 6 neutrons. Alright what about this guy? Well this guy again 6 protons but protons plus neutrons gives 14 so that means that this guy has 8 neutrons.
Alright what if I ask in a different way? What would I write down for the symbol for an atom who's nucleus consists of 8 protons and 12 neutrons. Alright 8 protons so that tells us the atomic number is 8, so that means it's oxygen now in order to know that stuff you kind of have to have a periodic table in front of you but I imagine you'll have one. So we've got our oxygen there we're going to put a little 8 down in the subscript just to remind us that the atomic number of oxygen is 8 that doesn't actually need to be there just like we didn't have it up here with the carbon as soon as you tell me it's carbon atomic number 6 but we'll write it there just to be happy. And then what about the mass number well, if I got 8 protons and 12 neutrons, add those together the mass number is 20 so this is oxygen 20. Now oxygen 20 is not a stable nucleus why not? Well here's the issue when you've got a lot of protons all together in the same tiny enclosed space there're all positively charged. Positive charges don't like each other, so you would expect this nucleus to just explode.
But there's lots of nuclei right? I mean you know oxygen certainly does exist in some form and it doesn't explode so how can you have those 8 protons sitting right next to each other? Well the reason is the strong force what happens is there's another force that combats this electric force that wants everybody to fly apart and it pulls everybody together. Now the neutrons participate in this strong force they're actually part of it but they've got 0 charge. So they don't hurt the electric problem so we have what's called a strong force balancing act. What we want to do is we want to have enough neutrons to stabilize the electric repulsion of all the protons but we don't want to have too many neutrons because like I said earlier neutrons weigh a little bit more than protons. So it costs more to have neutrons. So the issue is that the more protons you have the more of a problem you've got with the electric repulsion. But the more neutrons you've got the more of a problem you've got with mass. He weighs too much so the issue is if you want to have this nice balancing act oxygen 20 has too many neutrons, too many I mean jeez 12 neutrons that's way more than I need to stabilize it. In fact I really only need about 8 neutrons to stabilize oxygen, so the issue is and this is just experimental, when you have a large atomic number like uranium 92 protons you need about one and a half times as many neutrons as you got protons about one and a half.
Alright when you have a small atomic number like oxygen 8, you need the number of protons to be about the same as the number of neutrons and then you'll have a stable nucleus. And that's atomic nuclei.