- [Lecturer] As we continue into our journey of chemistry,
we're gonna gain more and more appreciation
for the periodic table of elements.
We're gonna realize that it gives us all sorts of insights
about how different elements relate to each other.
And we're gonna talk about one of those insights,
and that's atomic radii trends.
So we're gonna talk about,
by looking at the act table of elements,
can we deduce how the different sizes of these atoms
might relate to each other?
So let's just start with the group one elements
right over here.
So we're in this first column.
What do you think is going to happen to the radius
of these atoms as we go down this first column?
As we go from hydrogen to lithium, sodium, potassium,
so on and so forth?
Well, you might be thinking, "Well, as we go down this,
we're adding a lot more electrons."
The outermost electrons,
even though we have the same number of valence electrons,
we have one valence electron for everything in group one,
that one valence electron is at a higher
and higher energy level.
It is at a further and further out energy shell.
And so one way to think about it is, if you have the nucleus
of an atom here
and you have that one valence electron out here,
well, the more that you go down this group,
you're gonna have more electrons in between,
in between... (chuckles)
This is a pretty messy drawing.
In between that nucleus and that valence electron.
And that valence electron is going to be further
and further out because it's at a higher energy level.
So because of that, one, you have shielding
from these inner electrons from that positive nucleus,
and this is at a higher and higher energy level.
As you go down this group, the radius increases.
So let me write that down.
Increases.
What are we talking about?
We're talking about atomic radii increases.
So for example, cesium or, well, let's go with francium.
Francium is a much larger atom than hydrogen.
Now what happens if we were to go horizontally?
What happens if we were to go across a period here?
So let me do it in different color.
What if we were to go, if we were to look at, say,
period four, and if we were to go from potassium
to the right all the way to krypton?
What do you think is going to happen here?
I mean, think about it for a second
before I explain it to you.
All right, so this is the situation
where we're going to keep adding electrons
as we move to the right,
but you're not going to be adding electrons
to higher and higher energy levels.
You're either going to be backfilling
in the transition elements,
or you're going to be adding electrons
to your valence shell.
So you're not having higher and higher energy electrons,
so they're not gonna be any further away from that nucleus.
But as you go from left to right
across one of these periods, you're adding protons.
So you're making the center of that atom
more and more positively charged.
So it's going to pull in those outer shell electrons
more and more and more.
So based on that, you would expect to see
that the radius decreases as you go from left to right
along the periodic table of elements.
And we can confirm this intuition
by looking at this plot here.
So what this is doing is
it's plotting every element in the periodic table
of elements based on its atomic number
and its atomic radius.
So for example, this right over here is hydrogen,
and then your atomic number increases, you're at helium,
and our intuition is correct.
It looks like the radius has decreased.
And then we go into the second period.
And actually, let me just show each period here.
So if we go into period two here,
lithium has the largest radius.
And as we go from left to right in period two,
we get to smaller and smaller radii.
Now if we go to period three, we see the same trend again.
So we see, confirmed in the actual data, that trend,
that as you go from left to right on a period,
the radii or the radius decreases.
Now let's think about a group, which is where we started.
Well, across or up, down, any group,
if we go to group one right over here,
we see that intuition.
You go from hydrogen to lithium to sodium to krypton,
all the way to cesium here,
we have our radius is increasing
as we're adding higher and higher energy shells.
You see the same thing with group two.
This is the second column in the periodic table of elements.
So the data confirms our intuition.