POLAR BONDS AND MOLECULES

Ms. WithrowNovember 10, 2008

Polar Bonds

When involved in a bond, atoms of some elements attract the shared electrons to a greater extent than atoms of other elements – This property is called Electronegativity (EN)

The following chart is used to determine the electronegativities of each atom

Based on the difference in electronegativities of atoms we can predict the type of bond that will form

Formula: ∆EN = ENA – ENB

Chart:

Examples

Potassium Fluoride KF ∆EN = ENF – ENK = 3.98 – 0.82 = 3.16 IONIC BOND

Two Oxygen Atoms O2 ∆EN = ENO – ENO = 3.44 – 3.44 = 0 NON-POLAR COVALENT

Carbon Tetrachloride CCl4 ∆EN = ENCl – ENC = 3.16 – 2.55 = 0.61 POLAR COVALENT

With respect to polar covalent bonds, the differences in electronegativity tell us about the sharing of electrons

Example: Carbon Tetrachloride (CCl4) Cl has EN = 3.16 C has EN = 2.55 From this, we say that chlorine has stronger

attraction for electrons than carbon Thus, electrons will spend more time

around the Cl than C

This results in a slight separation of positive and negative charges which we call “partial charges” and represent them as δ+ or δ-

Example: CCl4 Chlorine with greater EN will have greater

attraction of e- and thus will have partial negative charge δ-

Carbon with lower EN will have less attraction of e- and thus will have partial positive charge δ+

Shown as δ+C-Cl δ-

When the bond is separated into partial positive and negative charges we call this bond a dipole bond

We represent dipole bonds with a vector arrow that points to the more electronegative atom

Example CCl4

δ+C-Cl δ-

Examples

Remember to Determine the bond type (by finding ∆EN) Assign the partial charges Place the dipole moment

Copper and Oxygenδ+C-O δ-

Carbon and Fluorine

δ+C-F δ-

Polar Molecules

We use our information on polar bonds to predict whether molecules will be polar or non-polar

We also must know our VSEPR shapes in order to do this!!

Water H20

Determine bond type ∆EN = ENO – ENH = 3.44 – 2.20 = 1.24 Thus is POLAR COVALENT

Determine partial chargesO has higher EN and H has lower ENOur partial charges are:

If we include the dipoles

Bent shape according to VSEPR

This is where VSEPR is important! -- You must know the shape of the molecule in order to determine it’s polarity

Water has two partially positive ends and one partially negative end The two dipole arrows point in the same direction. If we add these together we can see the molecule will have an overall net dipole

Because the dipoles do not cancel each other a net dipole is produced and we say that the molecule is POLAR

Carbon Dioxide CO2

Determine bond type ∆EN = ENO – ENC = 3.44 – 2.55 = 0.89 Thus is POLAR COVALENT

Determine partial chargesO has greater EN than COur partial charges are:

If we include the dipoles

Linear shape according to VSEPR

The dipoles created in this molecule are pointing in opposite directions and thus will cancel each other

This molecule has no net dipole and therefore is said to be NON-POLAR

Determine bond type ∆EN = ENN – ENC = 3.04 – 2.55 = 0.49 Thus is slightly POLAR COVALENT ∆EN = ENC – ENH = 2.55 – 2.20 = 0.35 Is also slightly POLAR COVALENT

Determine partial chargesN has greater EN than C – N will have δ-

C has greater EN than H – C will have δ-

Hydrogen Cyanide HCN

When we assign the dipoles

We see that they are both pointing the same direction

Thus they will not cancel, but will result in an overall net dipole

This molecule is said to be POLAR

Note the Difference!

When we had a linear molecule with the same atoms attached to the central atom the molecule was non-polar ex. CO2

When we had a linear molecule with two different atoms attached to the central atom, the molecule was polar Ex. HCN

It is very important to look at the electronegativities associated with the atoms and not just the VSEPR shape

Sulfur Trioxide SO3

Determine bond type ∆EN = ENO – ENS = 3.44 – 2.58 = 0.86 Thus is POLAR COVALENT

Determine partial chargesO has greater EN than SOur partial charges are:

Trigonal Planar shape according to VSEPR

When we assign dipole arrows All the dipoles are pulling

away from the central atom

You may think that because there are three dipoles they will not cancel and will result in a polar molecule

This is not correct however!!

Look at the horizontal and vertical components of the vectors (red and green arrows)

The red arrows will cancel The green arrows can add

together This green arrow will cancel

with the blue vector created by the top O

Therefore all dipole vectors will cancel in this molecule creating no net dipole and therefore the molecule is NON-POLAR

Similar to our linear molecule, difference will occur when the atoms attached to the central atom are different

We must be sure to look at the electronegativities of each atom when comparing the dipole vectors

Ex. CCl2O O has higher EN than Cl and will therefore have a greater dipole

The two dipoles from Cl will add together but they will still be less than that of O

Overall net dipole will result and thus molecule is POLAR

Ammonia NH3

Determine bond type ∆EN = ENN – ENH = 304 – 2.20 = 0.84 Thus is POLAR COVALENT

Determine partial chargesN has greater EN than HOur partial charges are:

Pyramidal shape according to VSEPR

Assign dipole vectors

The three vectors will add together to create an overall net dipole

This will result in a POLAR molecule

Carbon Tetrachloride CCl4

Determine bond type ∆EN = ENCl – ENC = 3.16 – 2.55 = 0.61 Thus is POLAR COVALENT

Determine partial chargesCl has greater EN than COur partial charges are:

Tetrahedral shape according to VSEPR

When we assign dipoles We can see that all the dipoles are

of the same magnitude because the EN differences are all the same

There are equal amounts of dipoles in opposite directions and thus they will all cancel

This results in no net dipole and therefore the molecule is NON-POLAR

Chloroform CHCl3

Determine bond type ∆EN = ENCl – ENC = 3.16 – 2.55 = 0.61 Thus is POLAR COVALENT ∆EN = ENC – ENH = 2.55 – 2.20 = 0.35 Thus is slightly POLAR COVALENT

Determine partial charges Cl has greater EN than C C has greater EN than H Our partial charges are:

Tetrahedral shape according to VSEPR

Assign dipoles (blue arrows) We can see that the dipoles to

Cl will all add up to create the larger green dipole vector

This is opposite to the dipole vector created by H-C but does not have the same magnitude

Thus, it will not cancel and result in a net dipole

This molecule is POLAR

Summary of Polarity of Molecules

Linear: When the two atoms attached to central

atom are the same the dipoles will cancel, leaving no net dipole, and the molecule will be Non-Polar

When the two atoms are different the dipoles will not cancel, resulting in a net dipole, and the molecule will be Polar

Bent: The dipoles created from this

molecule will not cancel creating a net dipole and the molecule will be Polar

Pyramidal: The dipoles created from this

molecule will not cancel creating a net dipole and the molecule will be Polar

Summary of Polarity of Molecules

Trigonal Planar: When the three atoms attached

to central atom are the same the dipoles will cancel, leaving no net dipole, and the molecule will be Non-Polar

When the three atoms are different the dipoles will not cancel, resulting in a net dipole, and the molecule will be Polar

Summary of Polarity of Molecules

Tetrahedral: When the four atoms attached to the

central atom are the same, the dipoles will cancel, leaving no net dipole, and the molecule will be Non-Polar

When the four atoms are different, the dipoles will not cancel, resulting in a net dipole, and the molecule will be Polar

Summary of Polarity of Molecules

Examples to Try

Determine whether the following molecules will be polar or non-polar SI2 CH3F AsI3 H2O2