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AP.Chem:
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Video transcript

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 insight 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 will see a lot of in chemistry of oxygen so oxygens 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 were going to fill the one shell then you are going to start filling the second shell so you're going to go to s 2 so I have 4 right now I have to have 4 more so then you're going to have to P 4 and notice if I add up all of the electrons here I have exactly 8 electrons now if I'm thinking about how my 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 don't aren't involved as much in reactions it has two core to 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 filled s and P sub shells 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 in we'd have two p6 and so you say all right 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 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 sub shells 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 and one of the reasons why the noble gases are so stable is that they have a completely full shell argon 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 for s2 so how many valence electrons does calcium have well you could see it right over there it has two valence valence electrons what about its core electrons well a neutral calcium atom would have twenty electrons because it has 20 protons so it would have 18 18 core electrons electrons that are less likely to react and so you could 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 be caught 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 case your valence electrons are going to be your outermost electrons they're going to be the electrons in that outer most shell generally speaking if you're talking about elements that are in the s block or the P block you can think about with 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 gas is here they are very unreactive so one way to think about is they are very very very stable they have filled their outer shell
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