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- [Lecturer] As we continue
into our journey of chemistry,

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we're gonna gain more
and more appreciation

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for the periodic table of elements.

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We're gonna realize that it
gives us all sorts of insights

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about how different elements
relate to each other.

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And we're gonna talk about
one of those insights,

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and that's atomic radii trends.

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So we're gonna talk about,

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by looking at the act table of elements,

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can we deduce how the
different sizes of these atoms

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might relate to each other?

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So let's just start with
the group one elements

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right over here.

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So we're in this first column.

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What do you think is going
to happen to the radius

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of these atoms as we go
down this first column?

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As we go from hydrogen to
lithium, sodium, potassium,

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so on and so forth?

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Well, you might be thinking,
"Well, as we go down this,

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we're adding a lot more electrons."

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The outermost electrons,

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even though we have the same
number of valence electrons,

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we have one valence electron
for everything in group one,

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that one valence electron is at a higher

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and higher energy level.

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It is at a further and
further out energy shell.

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And so one way to think about
it is, if you have the nucleus

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of an atom here

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and you have that one
valence electron out here,

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well, the more that
you go down this group,

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you're gonna have more
electrons in between,

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in between... (chuckles)

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This is a pretty messy drawing.

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In between that nucleus
and that valence electron.

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And that valence electron
is going to be further

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and further out because it's
at a higher energy level.

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So because of that,
one, you have shielding

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from these inner electrons
from that positive nucleus,

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and this is at a higher
and higher energy level.

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As you go down this group,
the radius increases.

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So let me write that down.

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Increases.

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What are we talking about?

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We're talking about
atomic radii increases.

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So for example, cesium or,
well, let's go with francium.

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Francium is a much larger
atom than hydrogen.

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Now what happens if we
were to go horizontally?

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What happens if we were to
go across a period here?

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So let me do it in different color.

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What if we were to go, if
we were to look at, say,

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period four, and if we
were to go from potassium

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to the right all the way to krypton?

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What do you think is going to happen here?

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I mean, think about it for a second

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before I explain it to you.

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All right, so this is the situation

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where we're going to keep adding electrons

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as we move to the right,

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but you're not going
to be adding electrons

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to higher and higher energy levels.

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You're either going to be backfilling

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in the transition elements,

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or you're going to be adding electrons

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to your valence shell.

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So you're not having higher
and higher energy electrons,

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so they're not gonna be any
further away from that nucleus.

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But as you go from left to right

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across one of these periods,
you're adding protons.

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So you're making the center of that atom

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more and more positively charged.

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So it's going to pull in
those outer shell electrons

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more and more and more.

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So based on that, you would expect to see

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that the radius decreases
as you go from left to right

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along the periodic table of elements.

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And we can confirm this intuition

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by looking at this plot here.

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So what this is doing is

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it's plotting every element
in the periodic table

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of elements based on its atomic number

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and its atomic radius.

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So for example, this right
over here is hydrogen,

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and then your atomic number
increases, you're at helium,

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and our intuition is correct.

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It looks like the radius has decreased.

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And then we go into the second period.

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And actually, let me just
show each period here.

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So if we go into period two here,

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lithium has the largest radius.

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And as we go from left
to right in period two,

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we get to smaller and smaller radii.

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Now if we go to period three,
we see the same trend again.

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So we see, confirmed in the
actual data, that trend,

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that as you go from left
to right on a period,

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the radii or the radius decreases.

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Now let's think about a group,
which is where we started.

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Well, across or up, down, any group,

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if we go to group one right over here,

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we see that intuition.

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You go from hydrogen to
lithium to sodium to krypton,

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all the way to cesium here,

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we have our radius is increasing

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as we're adding higher
and higher energy shells.

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You see the same thing with group two.

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This is the second column in
the periodic table of elements.

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So the data confirms our intuition.