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- [Instructor] In this video,

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we're gonna introduce[br]ourselves to a new way

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of visualizing atoms.

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And as you can imagine[br]from the title here,

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that's going to be Lewis diagrams.

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But before I even get into that,

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let's do a little bit of[br]review of what we already know

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about Bohr models.

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So let's say we take an[br]arbitrary element here.

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Let's say we take nitrogen.

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Nitrogen, by definition,[br]has seven protons.

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And so if it's neutral, it's[br]going to have seven electrons.

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So a Bohr model for nitrogen,

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in our first shell, that[br]first shell is going to look

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just like helium and it's[br]going to have two electrons.

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So let me draw it like that.

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And then in its second shell,

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its second shell, it is going[br]to have the remaining five

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of the seven electrons.

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And we are going to make[br]them unpaired at first.

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So one, two,

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three, four, and then five.

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The reason why I did it this way is,

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a full valence shell is[br]going to have eight electrons

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or four pairs.

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But if the electrons can spread apart,

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they like to spread apart.

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So that's why I did one, two, three, four,

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and then I paired this last one[br]because there's nowhere else

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for it to actually go.

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Now, I just touched on this[br]issue of valence electrons.

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Those are the electrons[br]in your outermost shell,

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and they tend to be the ones[br]that are involved in reactions.

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So chemists said, "Hey,[br]just for shorthand,

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instead of having to draw[br]all of this every time,

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why don't we just visualize[br]the valence electrons?"

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And so let's do that in[br]this nitrogen example.

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So a Lewis diagram,

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which is I'm just going to draw right now,

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is that simplified visualization

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where you write the[br]symbol for that element,

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and you just depict its valence electrons.

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We just saw that there[br]are five valence electrons

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for nitrogen, seven[br]total, but five valence,

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five electrons in that outermost shell.

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So it is going to be one, two,

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three, four, and then five.

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So that's a Lewis diagram[br]for a neutral nitrogen atom.

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It turns out we can also do this for ions.

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So let's say that we had[br]a nitride ion over here.

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Now, a nitride ion has[br]gained three electrons.

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So it actually has[br]eight valence electrons.

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So if you gain three from five,[br]you're going to have eight.

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So I'll go one, two,

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three, four, five,

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six, seven, eight.

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And because it gained three[br]electrons from being neutral,

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it now has a negative three charge.

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And so you'll often see[br]it written like this

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where they put brackets around it,

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and you would see three minus.

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Now, the last thing that[br]you might wonder about is,

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"Okay, I kinda understood

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how you got the valence[br]electrons for nitrogen.

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Is there just some general[br]pattern in the periodic table?"

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And the simple answer is yes.

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And that's one of the useful things

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about the periodic table.

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Or as we'll learn,[br]there's many, many other

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really interesting things about it.

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If you look at the groups, in general,

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you're going to have one valence electron

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for Group One elements,[br]for this column over here.

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You're going to have two valence electrons

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for these Group Two elements.

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And I know what you're thinking,

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"Okay, is just the group the[br]number of valence electrons?"

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Well, unfortunately, it doesn't[br]exactly work out that way.

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I'm going to skip the[br]transition metals here

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because those get a little[br]bit more complicated.

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It's a little bit more advanced.

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But then if we go over[br]here to, what is this,

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Group One, Two, Three,[br]Four, Five, Six, Seven,

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Eight, Nine, 10, 11, 12, 13,

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Group 13 over here is going to[br]have three valence electrons.

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Group 14, four valence electrons.

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Five valence electrons in Group 15,

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and that's why we saw[br]five valence electrons

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for nitrogen here.

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Six for Group 16.

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Seven for Group 17.

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And then 18 for, or sorry, (laughing)

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I should say eight valence[br]electrons for Group 18.

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So one way to remember it is,

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for Groups 13 through 18,

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you take the group number[br]and you subtract 10,

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and you're going to get the[br]number of valence electrons.

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And hopefully that made sense based

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on how we were able to figure[br]out the valence electrons

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for example nitrogen.