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Course: High school chemistry > Unit 1
Lesson 5: Periodic trendsAtomic radii trends
Atoms are not all the same size. The relative size of the atoms follows a set of trends on the periodic table. Going across a period, the main group elements tend to decrease in atomic radius due to the increased nuclear charge. Going down a group, the main group elements tend to increase in atomic radius due to increased shielding and the addition of new shells to the structure of the atom. Created by Sal Khan.
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how come as the protons increase going down the periodic table the radius increases, but as the protons increase from left to right the radius decreases(4 votes)
When moving down a group, you’re adding more protons, but you’re also adding more electrons, filling up the lower shells and forcing the electrons into higher energy shells that stay further away from the nucleus.
When you move right across a period, you’re adding more protons and electrons, but you stay at the same number of shells, rather than forcing electrons into higher shells, allowing the magnetic attraction between the protons and electrons to keep the radius of the atom smaller.(7 votes)
- If I am understanding correctly, as the number of electrons increases in the same energy level, the radius of the atom decreases. But why?(3 votes)
Where is the Atomic Radius in a element?(2 votes)
Technically, the atomic radius is simply the radius of the atom.(1 vote)
- Why are Francium and some other elements not on the graph?(1 vote)
Why are some numbers in brackets?(1 vote)
Video transcript
- [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.