 In a past video, we discussed the structure of the atom. Just to review, protons and neutrons are in the nucleus of the atom and electrons surround the nucleus in orbits if we're using the Bohr model or in clouds of probability if we're using the more accurate quantum mechanical model. In today's video, we will be using the Bohr model because it is convenient for what we are doing. As you should recall, the number of protons determines what element an atom is. For example, an oxygen atom always has 8 protons. An atom of magnesium always has 12 protons, etc. But the number of neutrons in an atom can vary. In a previous video, we talked about isotopes, which are atoms with different numbers of neutrons. Electrons have identical chemical properties, and they can participate in the same chemical reactions. However, they have different masses, and the mass of an isotope is approximately equal to the mass of the protons plus the mass of the neutrons. The number of electrons in a particular element can vary as well. In a previous video, we talked about ions, which are atoms of a particular element that have either lost or gained electrons. Isotope notation is one method we can use to keep track of things, like the number of protons, neutrons, and electrons, in a given atom that we are studying. Even though isotope notation looks a little like the squares we see on the periodic table, it is not the same. Here is how it works. So let's take a look at this picture. You'll see we have an X. Well that big letter there is the chemical symbol. Could be carbon, could be hydrogen, it could be molybdenum. Whatever the element is, it'll go there. That little Z in the lower corner, that's going to be the atomic number. That's the number of protons that are in the nucleus of the atom. Now up on the top left, that's the mass number. The mass number is the total number of neutrons and protons that are in the nucleus. Because if we add up the protons and neutrons, we get about the mass of the atom. And the units we use for mass are atomic mass units. We'll get into what that is in a future video as well. So let's take a look at this example of isotope notation. First we're going to look at that big letter in the middle of the box. That is F and it stands for fluorine. F is our chemical symbol. Next we're going to look in the lower left. That is our atomic number. Some people use the abbreviation Z. And our atomic number is 9. That means we have 9 protons in this atom. If we go to the upper left, we see the number 18. This is our mass number. And our mass number is the total number of protons and neutrons. Now how can we figure out how many neutrons we have? Well it's pretty easy. We just take the mass number and we subtract the number of protons or the atomic number. And that will give us the number of neutrons. In this case, 18 minus 9 equals 9. So this atom has 9 protons and 9 neutrons. Now let's talk about electrons. In the upper right, we have a symbol for charge. In a neutral atom, the number of protons is the same as the number of electrons. The charge is zero. But in this case, we have a charge of negative one. This is an ion and because it's a negative ion, we call this an anion. So how can we get a charge of negative one? Well this could happen if we had one extra electron. So if we wanted to find out the total number of electrons in this atom, we would take the number of electrons we would have for a neutral atom, that's 9, and we'll add one more electron. And that gives us 10 electrons total. So let's take a look at another example of isotope notation. Let's start by looking at that big letter in the middle. That is going to be the symbol for our element. And if we looked at a periodic table, we would see that U stands for uranium. In order to figure out how many protons we have, we look at that number in the lower left. The number of protons is 92. To find the number of neutrons, we're going to look in the upper left. That's our mass number and that is the number of protons plus neutrons. So to find the number of neutrons, we're going to take our mass number and we're going to subtract the number of protons. 238 minus 92 is 146. That's a lot of neutrons. How many electrons do we have? If we look at that symbol up in the right, there's a zero, which means we have a neutral atom. Well, in a neutral atom, we would have the same number of electrons as we have protons. So we would have 92. Now let's look at another example. So first we'll look at that big symbol in the middle, NA. If we looked at our periodic table, we would see that that is the symbol for the element sodium. How many protons do we have? Well, to figure this out, we look in the lower left. We look at the atomic number and that is 11 and that means we have 11 protons. How many neutrons do we have? Well, we have to look in the upper left to find the mass number and since the mass number is the protons plus the number of neutrons, we're going to take 23 and we're going to subtract our number of protons, that's 11, so we get 12 neutrons. Now we have to figure out how many electrons. Well, if this were a neutral atom, we would have 11 electrons to balance out the number of protons. However, we see that we have a one plus charge. That means there are more protons than there are electrons. In order to get a one plus charge, we would have to be missing an electron. So we have 10 electrons. 11 positive charges plus 10 negative charges is going to leave us with one extra positive charge. So you should now be able to look at isotope notation and determine what the element is, determine how many protons it has, determine the mass number, determining how many neutrons it has by taking the mass number and subtracting the number of protons, determining how many electrons it has and determining the charge if it has one.