How can the shape of the molecule determine its polarity?

Answer 1

First, you must draw the Lewis dot structure of the molecule. If you don't know how, look at the Related Questions link to the left of this answer for step-by-step instructions on how to do that. Once you have a completed structure, you are ready to continue!

To determine the shape of a molecule, or its geometry, we use something called VSEPR Theory, which is short for Valence Shell Electron Pair Repulsion Theory. It's actually pretty simple! In short, it says that the electrons in bonds and lone-pairs around an atom will stay as far away from each other as possible! That's it! OK-- but how do we use it?

The first thing you have to figure out is something called the steric number, which I'll abbreviate SN. We are going to do this for each atom that has more than one atom bonded to it. To do this, we look at our Lewis dot structure and use this formula:

Steric number = SN = the number of bonds + the number of lone pairs NOTE: the number of bonds just counts each bond once, even if it is a double or triple bond. Don't worry about those yet... we'll get to that later!

Now, the steric number (or SN) will tell us the geometry of the molecule. I'm going to make a distinction between what I call the "geometry" and what I call the "shape." You'll see what I mean, but not all teachers will do this. The geometry is determined by how many things are attached to the atom, including both bonded atom, and lone pairs. The geometry determines the angles between the bonded atoms and the lone pairs. However, the shape is different. When you look at a molecule, you see where the atoms are, not the lone pairs. So the shape is given by where the atoms are located. But the lone pairs often determine where the atoms are located, so first we have to find the geometry, and then we use that to get the shape! Don't worry... I'll explain this more.

As I said, the steric number (or SN) determines the geometry of the molecule. So, here they are (if something has SN = 1, it's easy! It has to be linear!):

SN = 2 ---> geometry is linear (with an angle of 180°)

SN = 3 ---> geometry is trigonal planar (with all angles equal to 120°)

SN = 4 ---> geometry is tetrahedral (with all angles equal to 109.5°)

SN = 5 ---> geometry is trigonal bipyramidal (angles of 90° and 120°)

SN = 6 ---> geometry is octahedral (all angles equal 90°)

OK, now we'll use the geometry to determine the shape. It gets a bit tricker now. We now have to count how many bonds and how many lone pairs there are around each atom (again double and triple bonds only count as one bond). For each value of SN above, the can be several possibilities. I will go through them one by one.

To help you, look at the Web Links to the left of this answer for pictures.

The table at the bottom of the linked Wikipedia page will be especially useful here. Notice the steric number is listed on the left. They use the same idea of geometry versus shape, but the name of the shape is listed directly under each molecule in the chart.

The Ausetute page is also good, except note that they don't call the steric number the "Total number of electron pairs," and they call the geometry the "Arrangement of electron pairs."

SN = 2

-- 1 bond, 1 lone pair: The shape is linear! There is only one thing attached!

-- 2 bonds: The shape is linear. There is one atom in the middle, with two atoms on either side, all in a straight line. The angle formed is 180°.

SN = 3

-- 2 bonds, 1 lone pair: The shape is bent. There is one atom with two atoms attached to it, coming out in a V-shape. The angles formed are approximately 120° and 240°

-- 3 bonds: The shape is trigonal planar. There is one atom with three atoms around it, all in the same plane. The angle between each atom is 120°.

SN = 4

-- 2 bonds, 2 lone pairs: The shape is bent. There is one atom with two atoms attached to it, coming out in a V-shape. The angles formed are approximately 109° and 251°

-- 3 bonds, 1 lone pair: The shape is trigonal pyramidal. There is one atom with three atoms bonded to it, but they are all lower than it (like a camera on a tripod), and there is a lone pair sticking straight up. The angles between the bonded atoms are all approximately 109°

-- 4 bonds: The shape is tetrahedral. There is one atom in the middle, with four atoms around it. The angles between each atom is 109.5°

SN = 5

There are many possibilities here, but I will only do one:

-- 5 bonds: The shape is trigonal bipyramidal.

Other shapes possible for SN=5 are T-shaped, see-saw, linear, and bent. SN = 6

Again, there are many possibilities here, but these two are most common:

-- 4 bonds and 2 lone pairs: The shape is square planar, with 90° and 180° angles between each of the bonded atoms, all in the same plane

-- 6 bonds: The shape is octahedral, with 90° between all bonds.

You can also have these shapes for SN = 6: square pyramidal, T-shape, and linear. Again, LOOK AT THE WEB LINKS FOR PICTURES! Use the pictures along with what I've written here and it will make much more sense!

There are some other things that affect the shape of a molecule. Double bonds and triple bonds take up more space than single bonds, and so they tend to push the other bonds away from each other distorting the shape. Lone pairs do that also. To remember this rule, I say "LONE PAIRS ARE FAT"!

Often times, a follow-up question will ask to state overall polarity after determining shape. See the related question below.

Answer 2

a symmetrical molecule cancels out the effects of polar bonds