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- [Instructor] In this video,

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we're gonna introduce
ourselves to a new way

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of visualizing atoms.

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And as you can imagine
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
review of what we already know

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about Bohr models.

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

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Let's say we take nitrogen.

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Nitrogen, by definition,
has seven protons.

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

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So a Bohr model for nitrogen,

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in our first shell, that
first shell is going to look

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just like helium and it's
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
to have the remaining five

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of the seven electrons.

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And we are going to make
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
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
because there's nowhere else

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for it to actually go.

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

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Those are the electrons
in your outermost shell,

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

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So chemists said, "Hey,
just for shorthand,

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instead of having to draw
all of this every time,

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

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And so let's do that in
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
symbol for that element,

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and you just depict its valence electrons.

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We just saw that there
are five valence electrons

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for nitrogen, seven
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
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
a nitride ion over here.

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Now, a nitride ion has
gained three electrons.

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So it actually has
eight valence electrons.

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So if you gain three from five,
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
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
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
you might wonder about is,

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"Okay, I kinda understood

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how you got the valence
electrons for nitrogen.

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Is there just some general
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,
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,
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
number of valence electrons?"

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

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I'm going to skip the
transition metals here

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because those get a little
bit more complicated.

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It's a little bit more advanced.

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

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Group One, Two, Three,
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
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
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
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
and you subtract 10,

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and you're going to get the
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
out the valence electrons

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for example nitrogen.