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Lecture Comments (21)

1 answer

Last reply by: Professor Hovasapian
Wed Nov 25, 2015 12:41 AM

Post by Jason Smith on November 23, 2015

Hi professor. Do we have any observational evidence for the geometries of molecules as well as their bond angles? Or is this something that, similar to electron orbitals, we've simply figured out mathematically? Thanks!

1 answer

Last reply by: Professor Hovasapian
Wed Jan 28, 2015 12:42 PM

Post by CHARINA TECSON on January 25, 2015

Great teacher, great lecture, great course! Thanks for everything!

0 answers

Post by Rafael Mojica on April 27, 2014

You should teach every single course. Specially Organic Chemistry.You are great!

5 answers

Last reply by: Tim Zhang
Sat Feb 1, 2014 5:45 PM

Post by Tim Zhang on January 29, 2014

Hi Professor Hovasapian, I am having trouble on drawing the molecular geometry and the electron-domain geometry. For instance should I call H2O a tetrahedral or a bent? I mean when I draw the electron-domain geometry of H20, should I put 2 lone pair electrons dots around oxygen? if that's correct, will the molecular geometry just be without these 2 lone pair electron dots? Also, Do I need to show the number of bonds between 2 atoms in a VSEPR model ?

1 answer

Last reply by: Professor Hovasapian
Wed Dec 18, 2013 7:41 PM

Post by Arvind S R on December 18, 2013

Shouldn't you consider the magnitude of the dipole moment of each bond in NO3 before concluding that it's non-polar? Because one of the bonds is a double bond, I'd expect that the magnitude of dipole moment is different.

1 answer

Last reply by: Professor Hovasapian
Mon Dec 9, 2013 5:01 AM

Post by Yu Zhang on December 7, 2013

For Ap exam, we need to memorize the VSEPR table, right?

4 answers

Last reply by: Arvind S R
Wed Dec 18, 2013 4:26 AM

Post by Julie Mohamed on December 1, 2012

what about trigonal pyramidal

Shapes of Molecules

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Shapes of Molecules 0:35
    • VSEPR
    • Steps in Determining Shapes of Molecules
    • Linear
    • Trigonal Planar
    • Tetrahedral
    • Trigonal Bipyramidal
    • Octahedral
    • Table: Shapes of Molecules
    • Example: CO₂
    • Example: NO₃⁻
    • Example: H₂O
    • Example: NH₃
    • Example: PCl₃⁻
    • Example: IF₄⁺
    • Example: KrF₄

Transcription: Shapes of Molecules

Hello, and welcome back to Educator.com, and welcome back to AP Chemistry.0000

Today, we are going to talk about the shapes of molecules.0004

We are finally going to get to what these molecules actually look like in three-dimensional space.0007

Fortunately, there are only a handful of shapes that molecules do take on, and they are completely intuitive, based on...there is nothing fancy about them; there is nothing strange about them.0014

As it turns out, atoms arrange themselves around a central molecule, exactly like you think they would.0023

That is nice; so there is a lot of intuition here.0028

Let's just go ahead and jump in and get started, and see what we can do.0032

OK, so this is called the VSEPR model.0037

Under normal circumstances, I wouldn't mention it, because I myself don't really care about names and what things are called; what is important is that you actually understand what it is that is going on.0043

However, on the test (on the AP test and some occasional other test), they will actually mention VSEPR, so it would be nice to know exactly that it refers to this.0053

Let's go ahead and...VSEPR...now, I did mention it, actually, in the last lesson, when I was discussing one of the molecules; I mentioned VSEPR and the shape; I hope that didn't confuse you--here is where we are actually going to discuss it formally.0065

VSEPR stands for Valence Shell Electron Pair Repulsion.0083

Now, that is just a fancy way of saying that negative charges want to be as far away as possible from each other.0094

We know that a bond consists of an electron pair; those electrons that are not bonded are lone pairs on the particular atoms.0101

These pairs of electrons--they don't just want to bunch up on one side of an atom; they want to be as far apart as possible from each other.0110

Well, you know that already, because that is what your intuition tells you: negative repels negative, so you're not going to have a lone pair sitting next to a lone pair; they are going to want to be as far away as possible, but still be attached to that atom.0118

If you have three lone pairs, they are going to arrange themselves 120 degrees away from each other; that is the whole idea.0131

This is just a fancy way of saying that electron pairs will be as far away as possible, but still within the molecule; that is it.0139

That is what that means; the name isn't altogether that important.0149

Now, I am going to introduce a notation here, and this is a notation...this is a way...again, we are talking about molecules in three-dimensional space.0153

But we can't represent three-dimensional space on a two-dimensional piece of paper (or this thing that you are looking at--this electronic tablet).0161

It is a flat, two-dimensional surface, so we need a notation that represents three dimensions on two dimensions.0168

Here is what the notation is going to look like; so we are going to take the methane molecule, CH4, and use that as our model to designate our notation.0175

CH4: OK, when you see an atom and a straight line (single bond, OK) to another atom, this means that this and this are right on top of each other.0186

They are in the plane of the page, OK?0206

Now, this dashed line means that this hydrogen is actually behind the page, and this hydrogen is behind the page; this one coming forward with this solid wedge--that means it's in front of the page.0210

So, if I were going to take the page that I am writing on, and if I turned it like this, what you would see--this representation--it would be: the carbon is in the page; there is one hydrogen on top of it; this one right here is actually coming forward; this one right here is actually going back; and this one right here is going back.0223

This is like a...imagine, if you will, a tripod, and there is one H on top of it.0248

Again, it is a three-dimensional molecule, but we need a way of representing it in two dimensions, so we use these dashes to show that things are going back away from us, these solid wedges to show that things are coming towards us, and this straight line to show us that things are straight up and down, in the plane of the page; that is it.0256

Another way of representing it: if I were to take this molecule and rotate it slightly so that one of these hydrogens...now, if they are both in the page, you will also see it represented this way.0278

H, H; now we have a C; there is an H up here, and there is an H here, exactly in the line of this page.0290

And then, they have one of the H's going back and one of the H's coming forward; this and this are equivalent--this is still just a tripod that has been twisted, in order to make two of these things line up in the plane of the page.0301

If I were to turn it like this, I would have a carbon here, hydrogen here, hydrogen here, one coming out this way, and one going that way.0317

I prefer this notation, myself; but again, as long as you understand--if you have some models, that would be really, really great to work with, but it absolutely is important that you be able to recognize the structure as it's written on a two-dimensional piece of paper.0326

These structures are going to be profoundly important: for those of you that go on to organic chemistry, this is going to be your life, because organic chemistry is not about math; it's about structures of molecules in three-dimensional space.0341

You will live this, every day; it's all about drawing structures in three-dimensional space.0353

Models will help, but as long as you understand what this means, you should be OK.0359

So again, a solid wedge means it's coming out toward you; the dashed wedges mean it's going away from you; and the straight line means it's right there in the plane of the page--same plane.0364

The carbon and the hydrogen are in the same plane.0376

So, having said that, now we are going to describe the process; I am going to write down the process that you go through, in order to find out what shape a molecule has in space.0378

It is going to be the same thing over and over again--the first thing you do is: you draw the Lewis structure.0389

The second thing you do is: you count the objects around the central atom (and I'll tell you what I mean by "objects" in just a second).0400

You might have one central atom; you might have other central atoms (more than one).0417

Now, "objects" is lone pairs (as pairs--in other words, one lone pair is one object), plus atoms.0421

In other words, if I look at this methane molecule, I have 1, 2, 3, 4 objects around here, because there are no lone pairs--so the atoms...that is the atoms, but there are no lone pairs, so the number of objects around this carbon is 4.0443

If I took a look at ammonia (let me do this one in blue), which has one lone pair; it has 3 hydrogens (like that, arranged in a tripod); well, again, it has 4 objects around it.0461

3 of those objects are atoms; one of those objects is a lone pair.0477

They both have what you will see in a minute as a tetrahedral arrangement.0481

So again, count the objects around the central atom; "objects" means lone pairs, plus the atoms around that central atom.0486

OK, #3 (let me go back to black here): Use the table of geometries to decide the shape; and this table is the table that I'm going to actually draw next.0493

It is the table that you are going to use as a reference; you can find it in your books--different books have different ways of describing this particular table: some of them do it horizontally/vertically; some of them...there are different ways of arranging it.0519

But this table that you are going to use, which is going to be your reference table for geometric shapes--I'm going to be drawing it out for you in just a minute.0534

It is going to be the next page.0540

OK, so you use the table of geometries to decide the shape, and the last thing that you do is: you determine if the molecule is polar or not.0543

You determine if the molecule has a permanent dipole by adding up (let's make this a little bit clearer here) the directions of the dipoles for each individual bond.0552

And remember what we talked about when we talked about dipole and dipole moments.0601

For example, in a CO2 molecule, the difference in electronegativity between oxygen and carbon is noticeable--oxygen is very, very electronegative; carbon is not very electronegative.0606

So, the electrons in those bonds are actually going to be pulled towards the oxygen.0618

Each individual bond has a dipole moment: in other words, it is more negative toward the oxygen end, positive toward the carbon end.0625

However, since CO2 (which we will see in a moment) is a linear molecule (carbon is at the center; oxygen here; oxygen there; the electrons are being pulled this way)--well, since this is an oxygen atom, and it's being pulled that way, the two individual dipoles actually end up canceling, so CO2 as a whole molecule has no net dipole.0632

It does not have a permanent dipole, because the two dipoles of the individual bonds cancel out.0650

The vector sum cancels out, which is why, if you put CO2 in a magnetic field, it is not going to behave the way another molecule that does have a permanent dipole would behave in a magnetic field.0657

It can't align itself with a magnetic field, because it doesn't have a positive end and a negative end.0669

The negatives and positives cancel out, even though each individual bond has a positive end and a negative end.0674

We will see that in a minute; don't worry.0679

OK, so now, let me discuss...oh, actually, I'm just going to draw out, using the notation, the shapes; because again, I want you to get used to the shapes and used to the notation, and then I'll go ahead and draw out the table.0682

There are going to be 1, 2, 3, 4, 5 basic shapes.0695

All right, the first one is going to be linear, and a linear molecule consists of a central atom that has two objects around it: this and this.0699

We are going to have triagonal planar (or, if you want, trigonal planar; it doesn't matter--it's only words; words are irrelevant; it's the concepts that are underneath them that matter).0716

Trigonal planar means that it is a central atom; you have an x; you have an x; and you have an x--there are three objects around it, arranged in a...they are 120 degrees apart.0734

Right?--electron pair, electron pair, electron pair: they want to get as far away as possible from each other, but still stay in the bond, so they have to be 120 degrees apart.0749

OK, let me put this in parentheses, so that you know that this added a third one.0760

We have...the third basic shape is going to be tetrahedral--a very, very common shape, and that is going to be a central atom (this is what we drew for the methane and the ammonia...oops, not H; we are just using generics here; x, x, x) with four objects around it.0766

And again, objects can be lone pairs or atoms; so, in this case, we are not specifying which yet--we will get to that when we get to the table, when we decide on the final shape of the molecule, not counting the lone pairs.0791

OK, so the next one is going to be triagonal (or trigonal) bipyramidal, and this is a central atom, one on top, one on bottom, one over here, one going back, and one going forward.0803

Basically, it is the shape of a triangle; one here, one here; the base is a triangle and has faces--it's a pyramid, basically.0827

It is a double pyramid where the base is a triangle.0839

Just take a look at this notation; that is what is important.0842

OK, and let me see here: you know what, I'm actually going to move this one over, because I want to do all of these on one page.0846

I don't want to write the last one...so I'm going to move this one over to the right; I'm going to do it this way.0855

This is x; this is x; this is x; this is x; and this is x.0862

And now, the last one is going to be octahedral.0871

By the way, -hedral is from hedron; it means face in Greek; so triagonal bipyramidal...octahedral means 8 faces.0875

You have a central atom, and then you have x on top, x on bottom; you have two of them going backward (oops, we don't want these stray lines--definitely not here; if we're writing words, it's not a problem, but not with structures), and two coming forward.0884

It is like having a square; these 4 form a square, and then, on top and on the bottom...the central atom is here; there is one here, one here, one back here, one back here, one up there, one up there.0908

It is a pyramid that has a square base.0924

OK, x...octahedral: these are the five basic shapes.0930

And then, there are sub-shapes, and this is what the table is going to do.0936

OK, so now, let's go ahead and draw this table.0940

All right, let's see if we can actually fit this on one page.0943

Let's go ahead and write "Objects" here: objects are going to be this direction, and then we'll have lone pairs in this direction; so 2, 3, 4, 5, 6.0949

Yes, I think I should be able to fit them in.0971

2, 3, 4, 5, and 6: that is correct; that is the number of objects.0974

Now, I'm going to draw a little line; actually, let me make a little box here; OK.0985

This; this is called...let me...yes, that is fine; I'll go ahead and do that.0995

This is linear; this is triagonal planar; this is tetrahedral; this is...I'll put trigonal bipyramidal; and this is octahedral.1008

OK, now, we have 0, 1, 2, 3, and 4.1039

Let's separate these out, and get this little table going here, and let's separate these--not the greatest artistic rendition, but aesthetics is not what we are concerned with; understanding is what we are concerned with.1055

OK, here is the relationship: let me fill these in, and then I will tell you what they mean.1083

I'm going to put T/P for triagonal planar, bent, and I'm going to put Tet for tetrahedral, triagonal pyramidal, bent; this is going to be triagonal bipyramidal (I'll just write Bipy); this is going to be a seesaw shape; this is going to be a T shape.1092

This is going to be linear, and this is going to be octahedral; this is going to be square pyramidal; this is going to be square planar; this is going to be T-shape; and this is going to be linear.1136

Now, let me show you how I use this table.1170

OK, let's say I draw--remember the steps that I wrote the first page of this thing?--we draw the Lewis structure; OK.1173

Next, we count the number of objects; let's say, for example, that I counted four objects, OK?--lone pairs and atoms.1180

I count the number of objects; objects is in this row right here--I'm doing objects this way, lone pairs this way.1189

OK, I counted 4 objects; it's tetrahedral--this is the basic shape.1199

That means the objects are arranging themselves in a tetrahedron around the central atom.1206

Now, I count the number of lone pairs around that central atom; if I have, let's say, no lone pairs, then the whole molecule itself is tetrahedral shape.1211

If there is 1 lone pair on the central atom, and 3 atoms, it is triagonal planar shape.1223

If it is 4 objects, but 2 of them are lone pairs, the molecule itself is bent.1231

These things in here actually give me the shape of the molecule, not counting the lone pairs.1239

The lone pairs are used to find the basic shape; but then, underneath the basic shape, depending on how many lone pairs I have, I have the actual shape of the molecule that counts just the central atom and the other atoms around it.1249

This is your reference: now, we can go ahead and start the examples.1263

OK, the first example we are going to do is CO2.1269

Examples: CO2: OK, by now, I am not going to go ahead and go through the actual process of doing the Lewis structure; I am hoping that you are good with that.1278

The Lewis structure of CO2 is going to be like this; OK, that is our Lewis structure.1290

We count the number of objects, including lone pairs, around the central atom; the central atom is the carbon; there are 2 objects: 1 and 2 (don't worry about the bonds)...1 and 2--there are no lone pairs.1299

We have 2 objects.1309

Well, 2 objects; 0 lone pairs; that means we go to the top row.1314

We look under 2, for objects; we go down to 0 lone pairs; and we end up with a linear molecule.1324

That means the molecule is linear; that means this is the actual arrangement in space.1332

It is a carbon with oxygen over here and oxygen over here; the angle between one oxygen and the other is exactly what you think it is: 180 degrees.1338

That is it; so, in this case...now notice: the Lewis structure didn't tell me what the shape was; I used the Lewis structure; I found the objects, the number of lone pairs; that is what told me what the shape was in space: it's linear.1348

I just happened to arrange it that way; I could have written this Lewis structure this way--that is a perfectly valid Lewis structure.1360

But, the actual shape in space is this: it is a straight line.1367

Now that we know that it is linear...OK, so we have our shape; now, we do our final part.1374

We have to see if the molecule has a permanent dipole moment; well, take a look: this is a linear molecule; oxygen is more electronegative than carbon; that means that this bond--the dipole moment is that way; this bond--the dipole moment is that way.1379

Because it is pulling this way and pulling this way (these are the same atom)--these pulls, like a tug-of-war, cancel out.1396

There is no net dipole; this is not a polar molecule--in other words, it is a non-polar molecule.1404

Each individual bond is polar, but adding all of the bonds together as a whole...the molecule has no net dipole moment.1412

Non-polar: very, very important for those of you that go on to work on spectroscopy--the idea of a polar molecule is at the very heart of how a molecule behaves in a magnetic field.1425

OK, let's do NO3.1439

We did NO3 last lesson: this is NO3-.1445

We had a Lewis structure...one of the Lewis structures; one of the resonance structures.1450

Now, that is the nice thing about this: when you are deciding geometry, you don't have to take all of the resonance structures; all of the resonance structures are essentially the same shape...again, because remember, only electrons are moving.1454

You only have to use one of the resonance structures to decide shape.1465

0, 0...OK, I'll go ahead and put the lone pairs on the oxygens; we might as well be consistent.1470

OK, number of objects around the central atom: 3 (1, 2, 3).1479

Number of lone pairs around the central atom: 0 lone pairs.1486

Well, when we go to the table, the number of total objects, including lone pairs, is 3.1497

So, if we go to the table, it's going to be triagonal planar.1504

If we go down to 0 on the right; on the top, we go to 3; that is triagonal planar--that is the general shape; now, from your perspective, on the left-hand side of the table, we go down to 0 lone pairs, and we go across under the column for triagonal planar, and it turns out that the actual shape is triagonal planar.1508

In other words, this molecule looks like this: N, O, O, O; and remember, all of the bonds are equivalent, so it's not like one is stronger than the other.1534

That is the actual shape in space: it is arranged in the form of a triangle, and it is a plane: there isn't one that is...you know, it isn't puckered or bent; it's just one nitrogen with three oxygens around it, and they are all in the same plane.1555

Well, let's see: oxygen is more electronegative than nitrogen, so let's check its dipole moment.1568

Let's do it this way: here is our central nitrogen; electrons are being pulled that way for that bond; electrons are being pulled that way for that bond; and electrons are being pulled that way for that bond.1573

What happens if you are in the center, and there are 3 people, 120 degrees angle between them, all pulling in the same direction?--you don't move.1585

There is no net dipole: they cancel.1594

There is no net dipole: this is a non-polar species.1597

The species is non-polar; it has a charge--that is different--that is part of its structure; it is non-polar, though.1607

OK, all right; now, let's go to H2O (water).1614

All right, water: the Lewis structure for water is this.1626

Well, how many objects does it have around it?--number of objects: 1, 2, 3, 4--it has 4 objects.1635

How many lone pairs?1645

4 objects means it is tetrahedral (right?--the top row is tetrahedral).1646

How many lone pairs does it have?1651

Well, the lone pairs around the central atom are two lone pairs; so if we go to tetrahedral, and we go down to two lone pairs, that means that it is bent.1654

In other words, we have a bent molecule; O is like that; H...this is arranged in the shape (all of the objects are arranged in the shape) of a tetrahedron.1669

However, the molecule itself, based on just the atoms in space, not worrying about the lone pairs--it is bent.1684

A water molecule looks like a V; that is how we do this.1691

Well, let's check the dipoles on this: oxygen is more electronegative, so it is pulling that direction for this bond, and it is pulling...you know what, let me actually do this a little bit differently.1697

It is pulling in that direction for that bond, and it is pulling in that direction for that bond; well, the net dipole (because we are being pulled this way and pulled that way) is that way.1713

As it turns out, water acts like a bar magnet; it is as if this whole molecule, this whole thing, has a partial negative end; it has a negative end; and it has a positive end, just like a bar magnet.1729

When you put this thing in a magnetic field, all of the water molecules will align themselves with the magnetic field.1745

This has a net dipole moment; the net dipole moment is this thing right here.1752

It is when you add this and this (right?): somebody is pulling you this way; somebody is pulling you that way; the horizontal part of their pull cancels, but since they are both pulling you up that way, your net movement...you are going to be pulled up like that.1761

Right? Does that make sense?--if you are here and somebody pulls you that way and somebody pulls you this way, you are going to move in this direction.1775

That is what it is: that is the net dipole; this is a polar molecule--a highly polar molecule, in fact.1781

OK, let's do NH3: well, the Lewis structure for NH3 gives me 3 hydrogens arranged like that, with a lone pair.1788

So, how many objects do I have around there?--I have 4.1800

How many lone pairs do I have?--I have 1.1806

Well, if I go to the top, and I go to the objects, which is 4, which is tetrahedral, and I drop down to where it says 1 lone pair, both of these imply that the shape is pyramidal (or triagonal pyramidal, if you want; I think that is what I actually wrote).1809

Triagonal pyramidal: in other words, it is in the shape of a pyramid; that is it.1830

Nitrogen, hydrogen, hydrogen, hydrogen: so it looks like this--nitrogen...a little tripod, basically; it's exactly right--if you took an ammonia molecule and just put it on the table, it would be like a little tripod.1834

H, H, H, lone pair (right?): a triangular thing like that...1853

OK, now, let's do net dipole.1864

OK, I'm going to have to show you this one in space, using my hands.1868

Nitrogen is here; you have 1 hydrogen coming out this way; you have 1 going back this way; and you have 1 going back this way.1873

Nitrogen is more electronegative than hydrogen, so this bond is going to pull that direction; this is going to pull that direction; and this bond is going to be pulled--the electrons are going to be pulled in that direction.1881

Well, if something is pulling you that way, that way, and that way, which direction are you going to move?1892

You are going to move up; the horizontal components will cancel.1900

The vertical components will not cancel, because they are all pulling up; so this one definitely has a net dipole moment straight up.1904

In other words, if I took this thing, I could spin the nitrogen; the hydrogens--I could spin the hydrogens; it's like a bar magnet.1914

This end of the molecule is negative; this end of the molecule is positive.1924

So yes, this is polar: it has a net dipole moment; and the net dipole moment is straight up.1929

OK, we are getting there; let's do PCl5.1940

OK, PCl5: the Lewis structure for this one is going to be P; we have Cl, Cl, Cl, Cl, and Cl.1951

OK, what is the number of objects that we have around PCl5?--we have 5 of them.1964

How many lone pairs do we have around the central atom?--we have none; so, 5 and 0.1970

Well, if I go to 5, that gives me a triagonal bipyramidal, and 0 lone pairs--so the actual shape of the molecule is triagonal bipyramidal.1977

Here is what this thing looks like (oh, let me go ahead and write that: tri...bipyramidal): I'm going to have a phosphorus atom; 1 Cl on top; 1 Cl on the bottom; 1 Cl that direction; 1 Cl going behind the page; and 1 Cl in front of the page.1989

That is the shape.2013

Is this a polar molecule?--well, let's take a look at these two.2015

Chlorine is more electronegative than phosphorus, so that pulls that way; that pulls that way: those are the two respective dipole moments of each bond, but notice, they are in opposite directions, so they cancel.2019

Well, this thing--there is one pulling this way; there is one pulling that way, and one pulling that way, in the shape of a triangle.2031

I have drawn it like this; OK, if I turn it over, it is like this, like this, like that.2039

That is what I have done; so, the way I have drawn it, this chlorine and this chlorine are top and bottom; they cancel; this chlorine, this chlorine, and this chlorine are arranged like that.2048

Now, I'm going to turn it over: this chlorine, this chlorine, and this chlorine are here, here, and here.2058

Pole, pole, pole; they cancel--not polar.2065

Each individual bond is polar; the molecule as a whole is not polar--there is no net dipole.2071

OK, now let's try IF4+, the iodine tetrafluoride cation.2078

OK, this one...when you...let's actually do the Lewis structure for this one, because we hadn't seen this one before.2089

Iodine brings 8, and fluorine brings 7; 4 times 7 is 28, and then plus a charge means I have lost one.2095

Therefore, I have a total charge of 28 and 8 is 36, minus that...or actually, 4 times 7 is 28...oh, no, iodine doesn't bring 8; iodine brings 7; I'm sorry...2106

I thought that was a noble gas; iodine is a halogen.2119

We have 35-1; we have 34 electrons.2121

I have I; let me write F, F, F, F; 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26 ,28, 30, 32; and it looks like we have one pair, so when we have a lone pair, we stick it onto the central atom.2125

So now, that gives us a Lewis structure of F, F, F, F...we'll put the lone pair there; this is a cation, so we make sure we have that positive charge there; all of these little things are important.2151

What is the total number of objects?--we have 1, 2, 3, 4...we're including the lone pair, so we have 5 objects, and we have 1 lone pair.2166

I'm just going to write L/P for lone pair; I don't want to write the word anymore--I get very, very lazy.2176

5 objects, 1: well, 5 objects around a central atom means triagonal bipyramidal.2182

5 objects, going down the column to where it matches 1 lone pair--it implies that the actual shape of the molecule is in the shape of a seesaw.2191

Here is what it looks like: I have the I; I have an F on top (oops, stray lines again; they are going to get in the way...); we have F on the bottom; we have F going behind the page; we have F going in front of the page; and the lone pair is over here.2201

One on top, one on bottom, one behind, one in front: one on top, one on bottom, the way I have drawn it; one fluorine is back here; the other fluorine is out front; the lone pair is out here: 5 objects, 4 atoms, so the shape of this atom is actually that of a seesaw.2226

That is what it looks like: this is the shape of a seesaw.2246

Is it polar?--well, that bond has that dipole moment; this bond has that dipole moment; they cancel.2248

One is pulling this way; one is pulling this way; the vertical components cancel; the horizontal do not.2256

The net dipole moment of this molecule is that direction.2263

Yes, this is a polar molecule.2267

I hope this is starting to come together: it's not very difficult--just be systematic with it, and everything should be just fine.2271

OK, we have two more to go.2278

We have KrF4; OK, KrF4: well, krypton brings 8 electrons; F brings 7; 4 times 7 is 28; 28 plus 8 is 36 electrons.2279

I'm going to put Kr there...F there, F there, F there; that is 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36.2297

I end up with a structure which is krypton, single-bonded F, single-bonded F, single-bonded F...I'm going to leave off the lone pairs on the fluorine; I have 2 lone pairs.2314

How many objects do I have around krypton?--I have 6 of them: 4 fluorines and 2 lone pairs; that is 6.2328

And how many lone pairs do I have?--I have 2 lone pairs (right? 2 pairs).2335

That implies, if I look under objects (6 objects), I have to go to the octahedral column on the table.2341

Now, I go down to 2 lone pairs, and I end up with square planar.2351

Square planar: what this looks like is the following: I have krypton; 2 F's go behind the page; 2 F's come out of the page; lone pair on top; a lone pair on the bottom.2358

Krypton is in the center; fluorine here, fluorine here, fluorine here, fluorine here, in the shape of a square.2381

There is also a lone pair here and a lone pair here; I have drawn it this way.2387

Krypton is in the center; a lone pair is on the bottom, a lone pair on the top; fluorine, fluorine in front; fluorine, fluorine in back--the shape of a square.2393

Fluorine is more electronegative than krypton, so it's going to pull in that direction, that direction, that direction, that direction; all of the dipole moments of the individual bonds will cancel; this is no net dipole.2405

No net dipole--this is non-polar.2419

OK, so let's see what else we have here--what other molecules.2427

We have done linear; we have done square planar; we have done seesaw; we have done bent; we have done pyramidal; it looks like we have actually covered almost all of the table.2432

A couple of things that we didn't...but again, it's handled the same way.2439

Draw the Lewis structure; count the number of objects (which includes lone pairs and atoms); find the number of objects on the table (the column)--that is the general structure of it.2443

Go down the column, based on the number of lone pairs that are around the central atom, and that will give you the shape of the molecule.2455

The shape of the molecule represents the shape that the actual molecule has in space, not including the lone pairs--just the atoms.2461

This is the VSEPR model, and it is all handled the same way.2472

Thank you for joining us here at Educator.com. We'll see you next time; goodbye.2478