Valence electrons | Atomic structure and properties | AP Chemistry | Khan Academy

We are now going to talk about valence electrons and non-valence electrons, which are known as core electrons. One question that you might have been asking yourself this whole time that we've been looking at electron configurations is: what is the point? The point of electron configurations is they can give us insights as to how a given atom, or how a given element, is likely to react with other atoms.

To make that point a little bit clearer, let's look at the electron configuration of an element that we'll see a lot of in chemistry—oxygen. So, oxygen's electron configuration is what? Pause this video and see if you can work through that.

Well, in a neutral oxygen atom, you have eight protons and eight electrons. So first, you're going to fill the one shell. Then, you are going to start filling the second shell. So, you're going to go 2s². Right now, I have four. I have to have four more, so then you're going to have 2p⁴. Notice, if I add up all of the electrons here, I have exactly eight electrons.

Now, if I'm thinking about how might oxygen react, it's interesting to look at the outer oxygen electrons—the electrons that are in the outermost shell. The outermost shell is being described right over here, the second shell. So how many electrons are in the outermost shell? You have six electrons here, so oxygen has six valence electrons.

Now, how many core electrons does it have? The core electrons generally aren't reactive or don't involve as much in reactions. It has two core electrons. Now, why are six valence electrons interesting? Atoms tend to be more stable when they have a filled outer shell, or in most examples at least filled s and p sub shells in their outer shell.

In this situation, you can say, "Okay, oxygen has six valence electrons." Oftentimes, that's drawn with a Lewis structure, and it might look something like this, where oxygen has one, two, three, four, five, six valence electrons. You might say, "Hey, it would be nice if oxygen somehow were able to share or get a hold of two more electrons," because then that outermost shell will have a full number of eight electrons. The 2s and the 2p would be filled in; we'd have 2p⁶.

So, you say, "All right, well, maybe they can grab those electrons from something else." That's actually what oxygen does a lot of; it grabs electrons from other things. You could look at something like calcium. Pause this video, think about what the electron configuration of calcium is, and then think about how calcium is likely to react, given that atoms tend to be more stable when they have a full outer shell where both their s and p subshells are completely filled.

Well, calcium's electron configuration, I could do it in noble gas notation or configuration; it would have the electron configuration of argon. One of the reasons why the noble gases are so stable is that they have a completely full shell. Argon, for example, has a completely full first shell, second shell, and third shell. Then, to build calcium, we'll have two electrons in that fourth shell, so it is argon and then 4s².

How many valence electrons does calcium have? Well, you could see it right over there: it has two valence electrons. What about its core electrons? A neutral calcium atom would have 20 electrons because it has 20 protons. So, it would have 18 core electrons—electrons that are less likely to react.

So, you could say, "What's the easiest way for calcium to get to a full outer shell?" Instead of trying to gain six electrons, it might be a lot easier to just lose these two electrons. It is actually the case that many times calcium will lose electrons and become ionized, will become, will get a positive charge.

The big picture here is one of the values of electron configurations is to think about which of your electrons are most likely to react. Those are your valence electrons. In most cases, your valence electrons are going to be your outermost electrons. They're going to be the electrons in that outermost shell. Generally speaking, if you're talking about elements that are in the s block or the p block, you can think about how many valence electrons they have just based on what column they're in.

This column right over here has one valence electron. This column over here has two valence electrons. This column out here has three valence electrons, four valence electrons, five valence electrons, six valence electrons, and seven valence electrons. The noble gases here are very unreactive. So one way to think about it is they are very, very, very stable; they have filled their outer shell.