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AP Chemistry 2.4 Structure and Arrangement of Atoms 34 Views


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Transcript

00:03

Here’s your Shmoop du jour, brought to you by dipoles.

00:07

Because we're just dying to pole you in to this video. [Tablet showing a shmoop video]

00:10

Here’s the question we have to tackle:

00:13

Which of the following molecules has a dipole moment of zero?

00:17

And here are our potential answers…

00:23

There are two steps needed to figure out the dipole moment of a molecule. [Woman taking two steps on a piece of furniture]

00:26

Step one: figure out which bonds have dipoles, and in what direction.

00:30

And step two: cancel out dipoles pointing in opposite directions. [Hands removing dipoles]

00:34

Let’s jump right into step one. [Man jumps into swimming pool]

00:36

A dipole is just an unequal sharing of electrons in a bond.

00:40

Kind of like how our older brother “shares” things with us… [Younger brother trying to grab a teddy off older brother]

00:43

Dipoles occur in a bond between an atom with a high electronegativity and an atom with

00:48

a lower electronegativity, because atoms with high electronegativity are very selfish. [Atom with high electronegativity]

00:55

Just like our older brother, except he's less selfish over electrons, and more over basically everything else

01:01

in the world… [Brothers fighting over a toy car]

01:03

When we have an electronegative atom in a bond, we draw an arrow along the bond pointing

01:08

to the more electronegative atom, which calls them out for hogging the electrons.

01:13

Why be subtle, right?

01:14

Anyway, this arrow is the dipole of the bond. [Arrow showing dipole of a bond]

01:17

And how do we identify electronegative atoms in a bond?

01:20

We look at the handy-dandy periodic table, which is essentially Facebook for the elements. [Scientist studying the periodic table on a computer]

01:24

Electronegativity follows a regular trend on the periodic table, excluding the Noble

01:29

gases, because they can’t be bothered to follow the trends of the common folk.

01:34

As we move right or up along the periodic table, electronegativity increases, with Fluorine [Arrows point to Fluorine on a periodic table]

01:39

being the most electronegative atom.

01:42

And if we're comparing two atoms, the atom further to the right, or further up on the

01:46

periodic table is more electronegative.

01:50

This means the dipole of a bond between those two atoms will point to the atom further up

01:54

or further right on the periodic table.

01:56

Okay.

01:57

Phew.

01:58

So.

01:59

…What was the question asking again? Oh right.

02:01

Now let’s draw our dipole arrows on our molecules. [Girl drawing arrows on a molecule]

02:04

Our molecules without the dipole arrows will look something like this…

02:07

Let’s go through them one by one and draw in our dipoles.

02:10

First we have H2O. [H2O in chemical form]

02:13

The bonds in this molecule are between Hydrogen and Oxygen.

02:17

Oxygen is much further right on the periodic table than Hydrogen, so we know that it's

02:21

more electronegative.

02:22

And if we draw in our bond dipoles....

02:26

Any dipoles pointing in opposite directions will cancel. [Dipoles drawn on a H2O molecule]

02:30

The dipoles in this molecule point up left and up right.

02:33

The left and right cancel, but the up parts add, giving us a leftover dipole.

02:37

So answer A is off the table. [H2O answer crossed out]

02:40

What about answer B, CO2?

02:43

This molecule has bonds between Carbon and Oxygen.

02:46

Oxygen is much further right than Carbon on the periodic table so it's more electronegative [Periodic table showing oxygen and carbon]

02:50

and our dipoles will point towards oxygen.

02:52

One dipole points left and one points right, which means that no one wins this tug of war. [Dipoles pointing in opposite directions]

02:57

And if there's no leftover dipole, then yup, this looks like our answer…but let's check [Two boys in a tug of war]

03:01

out our other molecules, just for fun.

03:04

Because what could be more fun than looking at more dipole moments?

03:09

Applying the same logic to the next two molecules, we see that SOCl2 has dipoles pointing towards [SOCl2 molecule shown with dipoles pointing towards oxygen and chlorine]

03:15

oxygen and chlorine, like so:

03:17

The bond dipoles of this molecule all point in different directions, but nothing points

03:22

towards the lone pair.

03:24

This means there will be a net arrow pointing opposite that direction… [Finger points to dipole arrow]

03:27

And that means this definitely isn't our answer.

03:29

Which brings us to our last competitor, D, CHCl3. [Letter D in a boxing ring]

03:31

The molecule looks something like this:

03:36

Chlorine is much more electronegative than carbon, and hydrogen has a very similar electronegativity

03:43

to carbon, so our bond dipoles will look like this: [CHCl3 molecule]

03:46

Nice.

03:47

Our carbon-hydrogen bond will have no dipole, and our carbon-chlorine bonds will have dipoles

03:53

pointing to chlorine.

03:54

This gives us a net dipole that looks something like this: [CHCl3 molecule with dipoles pointing to chlorine]

03:58

And remember when we started this question approximately twenty years ago, that we were

04:02

looking for a molecule with NO dipole.

04:05

So this is not the answer.

04:06

So that means B is most definitely our answer.

04:09

But hey, what better way to spend your time than with molecules? [Scientist using a pipette to transfer blue substance into a flask]

04:12

….Point taken.

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