So the atomic number is symbolized by Z and it refers to the number of protons in a nucleus. And you can find the atomic number on the periodic table, so we're gonna talk about hydrogen in this video. So for hydrogen, hydrogen's atomic number atomic number is one, so it's right here. So there's one proton in the nucleus of a hydrogen atom. In a neutral atom, the number of protons is equal to the number of electrons, because in a neutral atom there's no overall number of electrons.
overall charge and the positive charges of the protons completely balance with the negative charges of the electrons. So let's go ahead and draw an atom of hydrogen. So we know the atomic number of hydrogen is one, so there's one proton in the nucleus. So there's my one proton in the nucleus.
And we're talking about a neutral hydrogen atom, so there's one electron. I'm gonna draw that one electron somewhere outside the nucleus and I'm going to use the oversimplified Bohr model, right? So this isn't actually what an atom is. an atom looks like, but it's a very simple view that helps you get started.
So this is one version of hydrogen. So this is one isotope of hydrogen. So this is called protium.
So let me go ahead and write that here. So this is protium. And let's talk about isotopes.
And isotopes are atoms of a single element, so we're talking about hydrogen here, that differ in the number of neutrons in their nuclei. And so let's talk about the about the next isotope of hydrogen. So this is called deuterium.
So let me go ahead and write deuterium here. Deuterium is hydrogen, so it must have one proton in the nucleus, and it must have one electron outside the nucleus. But if you look at the definition for isotopes, atoms of a single element that differ in the number of neutrons, protium has zero neutrons in the nucleus.
Deuterium has one. So let me go. Let me go ahead and draw in deuterium's one neutron.
I'll use blue here, so neutrons are going to be blue. So deuterium has one neutron, and since neutrons have mass, deuterium has more mass than protium, so isotopes have different masses because they differ in terms of number of neutrons. Notice though that they have the same atomic number. They have the same number of protons in the nucleus.
It's one proton in the nucleus, and that's important because if you change the number of protons, you're changing the element. And that's not what we're doing here. We're talking about atoms of a single element.
Deuterium is still hydrogen. It's an isotope. And finally, our last isotope, which is tritium. So tritium has one proton in the nucleus, one electron outside the nucleus. Let me draw that in here.
And it must differ in terms of number of neutrons. So tritium has two neutrons. Let me go ahead and draw the two neutrons here in the nucleus. And so those are the isotopes.
isotopes of hydrogen. So how do we distinguish between the different isotopes? Well we're gonna write little symbols to represent these isotopes.
And so the symbol that we'll draw here for protium is going to have the element symbol, which is of course hydrogen. And then down here we're going to write the atomic number. So the subscript is the atomic number, which is one because there's one proton in the nucleus. And then for the superscript, we're going to write in the mass number, so let me move down here so we can look at the definition for the mass number. The mass number is the combined number of protons and neutrons in a nucleus, so it's protons and neutrons, and it's symbolized by A.
So A is the mass number, which is equal to the number of protons, that's the atomic number, which we symbolize by Z, plus the number of neutrons neutrons, so A is equal to Z plus N. And for protium, let's look at protium here, right? So in the nucleus, there's only one proton and zero neutrons. So one plus zero gives us a mass number of one. And I'll use red here for mass number so we can distinguish.
So mass number is red. And let me use a different color here for the atom. Atomic number, let me use magenta here.
So the subscript is the atomic number, and that's Z. And the superscript is the mass number, and that's A. Alright, so this symbol represents the protium isotope. So let's draw one for deuterium. Alright, so it's hydrogen, so we put an H here.
There's still one proton in the nucleus, right? One proton in the nucleus, so we put an atomic number One, the mass number is a superscript. It's the combined number of protons and neutrons. So we look in the nucleus here, there's one proton and one neutron. So one plus one is equal to two.
So we put a two here for the superscript. And finally for tritium, it's still hydrogen, so we put hydrogen here. Alright, there's one proton in the nucleus, atomic number of one, so we put a one here.
And then the combined number of protons and... and neutrons, that would be three. So one proton plus two neutrons gives us three.
So there's the symbol for tritium. So here are the isotopes of hydrogen, and using these symbols allows us to differentiate between them. Alright, let's take what we've learned and do a few more practice problems here.
So let's look at a symbol for carbon. So here we have carbon with... with subscript six, superscript 12. Alright, and if we wanna know how many protons, electrons, and neutrons there are, so let's first think about protons. Well, we know that the subscript is the atomic number, and the atomic number is equal to the number of protons. So there are six protons in this atom of carbon.
And if it's a neutral atom of carbon, the number of electrons must be equal to the number of protons. So if there are six protons, there must also be six electrons. And finally, how do we figure out the number of neutrons? Well, let's go ahead and write down the formula we discussed.
The mass number is equal to the atomic number plus the number of neutrons. Alright, so the mass number was right here, right? That's 12, so we can put in a 12. The atomic number was six, right here, so we put in a 12. We put in a six, plus the number of neutrons, plus the number of neutrons.
So the number of neutrons is just equal to 12 minus six, which is of course six, so there are six neutrons. So just subtract the atomic number from the mass number and you'll get the number of neutrons in your atom. All right, let's do another one. So this is carbon, and this time we have a superscript of 13. So the atomic number. doesn't change when you're talking about an isotope.
If you change the atomic number, you change the element. So there's still six protons in the nucleus of this atom. And in a neutral atom, there must be the equal number of electrons, so six electrons.
And then finally, how many neutrons are there? Well, just like we did before, we subtract the atomic number from the mass number. So we just have to do. Let's do 13 minus six to figure out the number of neutrons.
So 13 minus six is of course seven. So there are seven neutrons in this atom. Alright, another way to represent isotopes, let's say we wanted to represent this isotope in a different way.
Sometimes you'll see it where you write the name of the element, so this is carbon, and then you put a hyphen here, and then you put the mass number. So carbon hyphen 13 refers to... this isotope of carbon, and this is called hyphen notation. So let me go ahead and write this hyphen notation.
Alright, let's do one more example here. Let's do one that looks a little bit scarier. So let's do uranium, so U is uranium.
The atomic number of uranium is 92. The mass number for this isotope is 235. So how many protons, electrons, electrons and neutrons in this atom of uranium. Alright, so once again, for protons, we look at the atomic number, that's 92, so there must be 92 protons. In a neutral atom, the number of electrons is equal to the number of protons, so there are 92 electrons. And then finally, to figure out the number of neutrons, we subtract this number from the mass number. So, we just need to do...
235, 235 minus 92, and that gives us 143. So there are 143 neutrons.