Valence electrons and ionic compounds | AP Chemistry | Khan Academy

In this video, we're going to get even more appreciation for why the periodic table of elements is so useful. In particular, we're going to focus on groups of the periodic table of elements. When we talk about a group, we're just talking about a column. As we'll see, even though the elements in a given column might have very different atomic numbers, they all have similar properties.

The reason why they all have similar properties is, in most cases, they have the same number of valence electrons. Remember, valence electrons are the reactive electrons, the ones that might interact with other things. Because elements with similar valence electrons will have similar reactivities, they will form similar ions, similar ions, and they will have similar roles, similar roles in ionic compounds.

Now, for the sake of this video, I'm going to focus most on the extremes of the periodic table: the groups at the left and the right, because those are the closest to having a full outer shell, either by losing electrons or by gaining electrons. So just to remind ourselves, what does it mean to have a full outer shell? Well, in general, people will refer to the octet rule. For our second, third, fourth, fifth, and on and on shells, you're full when you have eight electrons.

The major exception to the octet rule is the first shell, where it is full with two electrons. So, helium, even though it only has two electrons, is very, very, very stable. The major data point that we have around this octet rule are the group 18 elements, right over here, also known as the noble gases. They're known as the noble gases because they're very unreactive. They're very content; they don't want to mess around with anyone else.

That's because all of the noble gases have full outer shells. Helium's outer shell is the first shell, and it's full. Neon’s outer shell is the second shell; it's full. Argon's outer shell is the third shell, and it's full, and so on and so forth. Now, if we go one group to the left of the noble gases, we get to the halogens. The halogens have seven valence electrons, so you can imagine they're only one electron away from having an electron configuration like the noble gas to the right of each of them.

So these halogens right over here really like to attract electrons to form a negative ion or an anion. You will oftentimes see fluorine as a fluoride anion, so it has a negative one charge, or you also see chlorine with a negative one charge as the chloride anion. And I could go on and on; you'll often see iodine gain an electron and have a negative one charge.

If you go one step to the left, the oxygen group—oxygen, sulfur, and so on—these elements have six valence electrons. So, still easier for them to have a full outer shell by gaining two electrons than by losing six electrons. So these elements also like to attract electrons. You can see oxygen as an oxide anion, having gained two electrons; it swiped it from somebody else. Sulfur as a sulfide anion.

Now, if you go to the other extreme of the periodic table, if you look at group one elements, they have one valence electron. Especially the ones that you look in red here, which are known as the alkali metals, it's much easier for them to lose an electron to have a full outer shell than for them to gain seven electrons. The reason why hydrogen's a bit of an exception is that it doesn't have to gain seven electrons to have a full outer shell; it has to gain one.

So, hydrogen could lose one and essentially have no electrons, or it could gain one electron, and it would have a full outer shell like helium. But when we think about ionic compounds, these alkali metals are really some of the most interesting participants because, as you can imagine, for them to get stable, they want to give away an electron.

So you're very likely to see them having given away an electron and having a positive one charge. You'll oftentimes see a lithium ion with a positive one charge, a sodium ion with a positive one charge, a potassium ion with a positive one charge, and that's in general true of all of these group one elements.

Now, what about these group two elements, also known as the alkaline earth metals? Well, once again, it's easier for them to lose two electrons than for them to gain six to have a full outer shell. So, you'll typically see beryllium having a positive two charge; it would have lost those two electrons. Magnesium as having a positive two charge, calcium as having a positive two charge.

Now, given that, how would you expect things on the left and things on the right to form ionic compounds? So you might guess if you have an alkali metal in the presence of a halogen, things could get very reactive. In fact, things will get very reactive because these want to give away an electron, and these want to take an electron, and that's what will happen.

The electrons will leave the group one element, and then they will go to the halogen. In the process, it might release a lot of energy, but what you would be left with is an ionic compound. For example, lithium loses an electron and has a positive one charge. That positive ion would be very attracted to a chlorine anion that has just gained an electron; maybe it's the same electron or swiped that electron from another lithium atom.

So these two things would be attracted, and they could form lithium chloride. All of these alkali metals could play that same role in this ionic compound as lithium. So it's also typical to see sodium chloride, that is table salt; it's also typical to see potassium chloride, so on and so forth.

On the other hand, fluorine or bromine or iodine can play a similar role as chlorine, so you could see something like sodium iodide or potassium iodide. Once again, the alkali metal would have lost an electron, the halogen would have gained an electron, and then they are attracted to each other, forming these ionic compounds.

What kind of ionic compounds might be formed with these group two elements? Let's take calcium for example. It's not unreasonable for calcium to lose two electrons to have a stable outer shell, to have an electron configuration like argon. So if it loses two electrons, it has a positive two charge, and you can imagine those two electrons get lost to two different iodine atoms, so each of them have a negative one charge.

So, times two. And then what type of ionic compound could they form? Well, you could have one calcium and then two iodines. So, calcium iodide is actually an ionic compound you would see; it has a neutral charge overall because calcium has a positive two charge, and each of the iodines have a negative one charge, but then you have two of them, so it is neutral overall.

What might calcium do with oxygen? While calcium likes to lose two electrons, oxygen likes to gain two electrons. So, you could see something like calcium oxide. So, I will leave you there. The big picture here is that the column in which an element is tells you a lot about its reactivity because it tells you in general how many valence electrons it has.

Atoms are most stable when they have a full outer shell, and so that helps you predict, hey, is it easier for them to lose electrons and form a positive ion or gain electrons and form a negative ion? From that, you can make predictions as to the types of ionic compounds that could be formed with the different elements.