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Trends in Ionic Radius

Concept #1: Understanding ionic radius

Transcript

In this new video, we're going to take a look at ionic radius.
Atomic radius deals with the neutral forms of elements. Ionic radius deals with the ionic forms of elements. We're going to say that positive ions are called cations. We're going to say cations tend to be smaller than their parent atoms. Because when it comes to ionic radius as well as atomic radius, it's all about electrons. The more electrons you have, then the bigger you are, especially for ionic radius.
Neutral lithium has three electrons, but lithium plus one means it loses one electron, so now it only has two electrons. Also, just notice this, that third electron lithium has when its neutral is in the 2s shell. But when we lose that electron, that means we actually lose the second shell, so as a result, lithium gets a lot smaller. It's only comprised now of one shell, instead of two.
Think about it. You take away electrons from shells, the shells are going to disappear, therefore, what's left are the smaller shells. In the opposite way, if I add electrons to a shell, it actually makes the shell a little bit bigger. We're going to say in the opposite way anions tend to be bigger than their parents.
Neutral fluorine has 9 electrons. When we add an additional electron, we are adding mass to it, maybe a small amount because electrons are very, very small. They're times 10 to the negative 31 kilograms, but every little bit counts. As a result, you'd expect F- to be bigger than neutral F.
We're going to say the pattern for ionic size correlates with the following trend when comparing ions with the same number of electrons. What does that mean? Well, it means this. Let's say I was ranking different elements together. Let's say I was ranking Mg2+, neon and let's say one more here, F-. Let's just say that. Mg2+, magnesium neutral has 12 electrons, plus 2 means it only has 10 now. Neon, neon is neutral, so it has 10 electrons. Neutral fluorine, like we said, has 9 electrons minus 1 means it gains 1, so it has 10.
When they have the same number of electrons, we use this pattern to break the tie because they all can't be the same size. Someone has to be bigger than someone else. When they all have the same number of electrons, we're going to say minus 3 charge, would mean it's bigger than minus 2 charge, than minus 1, all the way down to plus 3. Obviously, if you have a number that's even more negative than minus 3, then that will be bigger than minus 3. If you have a number that's even more positive than positive 3, then that would even be smaller still.
We're going to say here since they all have the same number of electrons, we're going to say F- is the biggest because it has a negative 1 charge. Neon would be next because it's neutral. Then magnesium is plus 2, so it would be the smallest.
Again, we only do this if they have the same number of electrons. If they have different number of electrons, then we simply say, the one with the most electrons is the biggest because again, we said, for ionic radius, more electrons equals bigger ion.

Example #1: Rank each set of ions in order of increasing ionic size. 

a)  K+ , Ca2+, Ar

b)  Sr2+, Na+, I 

c)   V5+, S2-, Cl