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Relative Size of Atoms

Teacher/Instructor Jonathan Osbourne
Jonathan Osbourne

PhD., University of Maryland
Published author

Jonathan is a published author and recently completed a book on physics and applied mathematics.

Nuclear reactions are reactions between nuclei which involve tremendous amounts of energy and in which mass number and charge are conserved. Alpha and beta decay, fission and fusion are all types of nuclear reactions. Unlike chemical reactions, atomic number is usually changed.

Alright. So let's talk about the relative size of atoms. Alright and you might think that's easy. If it's got a big atomic number it's big. If it's got a small atomic number, it's small. So for example uranium should be bigger than helium. Duh. Alright, that is kind of true. But it gets much more iffy and kind of strange when the atomic numbers are close together. So atom size is really associated with the electrons. It's not really associated with the nuclei. So we got to be very very very careful in thinking about how are the electrons arranged and how big is that electron cloud.

So first thing that we come to is mass number is totally unimportant because that only tells you how big the nucleus is. So when you're talking about the size of the atom, it's the electron. I mean that electron cloud's 100,000 times bigger than the nucleus. So it doesn't really matter whether it's helium 3 or helium 4, because they both got two electrons and they both have got a charge of +2 in the nucleus. So those are the only thing that's are important. Whether or not there are, there's only one neutron or two, nobody cares. Alright. So that's the first thing.

Now, second thing, when you have the same number of electrons, you've got to notice that the larger z will actually give you a smaller atom. So if you've got let's say, lithium plus and helium, you might think, well jeez. Lithium plus that's lithium, it's got 3 protons, so, that means it's got a bigger nucleus, it's going to be a bigger atom. But no. That's actually not true because lithium has 3 protons, that means that its charge is 3+ in the center so it's pulling on those electrons harder. So it will make the atom smaller if it's got the same number of electrons because it pulls closer, it pulls harder on the electrons and makes them closer. But what about lithium, not plus versus helium, not plus?

Alright. Here to get the 3 electrons, the first two go on the ground state but that next one goes in the next shell out and that's a much bigger shell. So that makes the lithium atom much much much bigger than helium. So this last electron goes into the next shell, that's what makes it bigger. So when we look at the periodic table, what happens is going this way across a period actually makes the atom smaller because there's more charge and the electrons are going into the same shell. As opposed to that, when I go down here, I, from period to period, I actually get bigger. Alright. So it's kind of strange. Now when we're looking at ions, like we did up here, we need to look at the number of electrons because remember it's the electrons that determine the size of the atom.

Alright. So let's do some practice. Suppose that I'm asked to rank beryllium, carbon and fluorine, in terms of which one's the biggest, which one's the smallest. alright. So I first locate them on the periodic table. Beryllium, carbon, fluorine Beryllium, carbon, fluorine. And look, as I go across I get smaller. So that means that the smallest is fluorine and the biggest is beryllium. And so that's the way that they're going to rank. Fluorine, carbon, beryllium in order from smallest to biggest. Alright? Let's go and look at fluorine, neon and sodium. Alright.

So now I've got fluorine, then neon then down here to sodium. Alright. So, like we saw before, here fluorine will be the smallest, I'm sorry. Neon will be the smallest, then fluorine because neon's got more protons that's going to pull harder on those electrons but it's in the same shell. So that means that neon's going to be the smallest, then fluorine then sodium, because sodium has this extra electron in the next shell. So even though it's pulling harder on the electrons, this last eleventh electron goes in the next shell which makes it much bigger. Alright.

Let's now look at fluorine, iron, neon and sodium plus. So now we're looking at ions, so we need to be careful. This is fluorine minus, so that means it's got 10 electrons. This is neon, so it's got 10 electrons and this is sodium plus. So it's got 10 electrons. So now, because they all have the same number of electrons, the one with the biggest z is the smallest, because here I don't have that extra electron that goes into the next shell. I've kicked it off. So that means that this ion is actually smaller. That one's the second smallest and that one's the third. So see it's totally different when you're looking at ions. You have to be very very very careful if your teacher gives you something with a minus or a plus.

Alright. what about here? Now it's all just crazy everywhere. So now we've got magnesium + +, we've got aluminum nothing and then we've got chlorine minus. Alright. So let's look and see how this goes. Magnesium + +. So magnesium is over here but I kicked off two electrons, so really, I'm like neon. But I've got magnesium, so that's a bigger atom. so this guy, I'm sorry, bigger charge in the center so he's going to pull closer, so this guy is actually going to be the smallest. Alright? Then we look at aluminum. Aluminum's over here so it's smaller than magnesium would be if it wasn't ionized But it's still bigger than what the chlorine is. Because the chlorine's over here. It's ionized so it's actually kind of taking the place of the argon. It's got a bigger positive charge so this chlorine is actually smaller, so it's going to go 2 and then 3 like that.

So we always have to think very very very carefully. Where is it on the periodic table and what is its charge in the nucleus. And that's relative size of atoms.