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Valence electrons

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Valence electrons are the electrons in the outermost shell, or energy level, of an atom. For example, oxygen has six valence electrons, two in the 2s subshell and four in the 2p subshell. We can write the configuration of oxygen's valence electrons as 2s²2p⁴. Created by Sal Khan.

Video transcript

- [Instructor] We are now going to talk about valence electrons, and non-valence electrons, which are known as core electrons and so 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? And the point of electron configurations is, is they can give us insights as to how a given atom or how a given element is likely to react with other atoms. And so just to make that point, or make it a little bit clearer, let's look at the electron configuration of an element that we'll see a lot of in chemistry, of 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 gonna fill the one shell, then you are going to start filling the second shell, so you're gonna go 2s2, so I have four right now, I have to have four more, so then you're going to have 2p4. And then notice, if I add up all 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. So the outermost shell is being described right over here, this second shell. So how many electrons are in the outermost shell? You have six electrons here. So oxygen has six valence, valence electrons. And how many core electrons does it have? And the core electrons generally aren't reactive, or aren't involved as much in reactions? It has two core, two core electrons. Now, why is six valence electrons interesting? Well, atoms tend to be more stable when they have a filled outer shell, or in most examples, at least a filled SNP subshells in their outer shell. And so in this situation, you say, okay, oxygen has six valence electrons, and 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, and you might say, hey, it would be nice if oxygen somehow were able to share, or get ahold 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 then, we would have 2p6. And so you'd say, alright, well maybe they can grab those electrons from something else and that's actually what oxygen does a lot of, it grabs electrons from other things. You can 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'd have the electron configuration of argon, and 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, and then to build calcium, will then have two electrons in that fourth shell, so it is argon and then 4s2. So 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? Well, a neutral calcium atom would have 20 electrons, 'cause it has 20 protons, so it would have 18 core electrons. Electrons that are less likely to react. And so you can say, what's the easiest way for calcium to get to a full outer shell? Well, 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 get a positive charge. So the big picture here is, one of the values of electron configuration 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, they are very unreactive, so one way to think about it is they are very very very stable, they have filled their outer shell.