Hi, and welcome back to my videos for General Chemistry 1. In this video, we'll talk about the sizes of atoms and ions, and we'll find out that just because an atom has a higher atomic number doesn't always mean it's larger. How can we have more protons but still end up with a smaller atom? The secret has to do with the effective nuclear charge which we talked about in the last video. It's the charge that electrons feel pulling them toward the nucleus. We said that the effective nuclear charge is lower for the outer electrons of an atom because they're shielded by the inner electrons. Those electrons block some of the positive charge of the nucleus. It turns out that the effective nuclear charge that the valence electrons feel has a huge impact on many properties of atoms and ions. We'll talk about four of those properties, the atomic radius, the ionization energy, the electron affinity, and the electronegativity. We'll talk about the first one, atomic radius, in this video, and the others in the next two videos. As you might expect, the radius of an atom increases as we go down a column at the periodic table. This is because, as we go down, the value of n increases, which means the number of electron shells increases. So for example, if we look at the atoms in this column, the value of n increases from n equals 2, up to n equals 7, and the atomic radius also increases from 82 pk meters, up to 180 pk meters. A pk meter is a trillionth of a meter. So atoms get larger as we go down the periodic table. But what about when we move from left to right? This time to understand the change in the atomic radius, we need to think about the effective nuclear charge on the valence electrons. Take the second row of the periodic table, for example. Each of these atoms has its valence electrons in the n equals 2 shell. Lithium has three protons in all, so its nucleus has a charge of plus 3. However, as we found out in the last video, the two inner electrons shield some of this charge so that the valence electron only feels an effective nuclear charge of plus 1. On the other hand, the nucleus of neon has 10 protons, so it has a charge of plus 10. But the valence electrons are shielded by the two electrons closer to the nucleus, so they only feel an effective nuclear charge of plus 8. If we apply the same logic to all the atoms in the row, you can see that the effective nuclear charge increases as we go left to right. So how does that affect the radius? Well, the more the nucleus pulls on the valence electrons, the closer those electrons move to the nucleus. So the atom is smaller, the more the nucleus pulls on the electrons. So neon, which feels a strong pull for the nucleus, is smaller than lithium. As you can see, lithium has a radius of 134 ppm and the radius decreases to 69 ppm as we go to the right. So the atomic radius is higher as we go to the left on the periodic table and as we go down. So let's try an example. Suppose we have atoms of strontium, phosphorus, aluminum, cesium, and fluorine. Let's put these in order from smallest radius to largest. The first thing we need to do is find each of these on the periodic table. So here are strontium, phosphorus, aluminum, cesium, and fluorine. Now that we've done that, we just need to remember that the atoms get larger as we go down into the left. So fluorine is the smallest and then phosphorus, then aluminum, then strontium, and cesium is the largest. And we find that the actual data confirms it. Of these five, fluorine is the smallest at 71 ppm and cesium is the largest at 225. Now what happens when we create ions instead of neutral atoms? For example, suppose we have a neptunium atom, which has a radius of 155 ppm. In order to create a cation, we have to remove electrons. As you might expect, when we remove electrons, the radius of the ion decreases. So in this series of neptunium ions, the radius decreases until it reaches 85 ppm for the neptunium plus 7 ion. The reverse is true when we make anions. We do that by adding electrons to an atom. So for example, the radius of an iodine atom is 133. So when we add an electron to it, the radius of an I minus ion is 206 ppm. And that's it for now. Next time, we'll talk about a couple more properties that are affected by the effective nuclear charge, and those properties are especially useful for determining the energies of certain chemical reactions. So until next time, have a good week.