1. Section 5.4—Polarity of Molecules
Objectives
examine the trend of electronegativity on the periodic table
use electronegativity to determine the polarity of covalent bonds

2. Electronegativity
Electronegativity is the attraction an atom has for the electrons it shares with another atom in a bond.
Electronegativity has a periodic and group trend

3. Periodic Table with Electronegativies
increases
decreases

4. Polar Bonds
Recall from Sec. 5.1, that covalent bonds (where electrons are shared between 2 atoms) can be nonpolar or polar.
In nonpolar covalent bond, electrons are shared equally but in a polar covalent bond, they are not shared equally.
The electronegativity difference between the 2 atoms in the bond determines the type of covalent bond that forms.

5. Nonpolar: Two atoms sharing equally
Each nitrogen atom has an electronegativity of 3.0.
There is no electronegativity difference.
They pull evenly on the shared electrons so, the
electrons are not closer to one or the other of the atoms.
This results in a non-polar covalent bond.

6. Nonpolar: Atoms sharing almost equallyC H
Electronegativities: H = C = 2.5
Electronegativity difference of each bond: 0.4
The carbon pulls on the electrons slightly more (having a higher electronegativity), but the difference isn’t enough to create a polar bond.
This is a non-polar covalent bond.

7. Polar: Two atoms share unequallyC O H
Electronegativities: H = C = O = 3.5
The C-H difference is 0.4; the C-O difference is 1.0.
The C-H bonds are nonpolar.
But, the oxygen atoms pulls significantly harder on the electrons than the carbon does. This is a polar covalent bond!

8. A polar covalent bond results in one end of the bond (and molecule) having a slight negative charge and one end having a slight positive charge.
The atom having a higher electronegativity pulls the electrons closer to itself and, so, has a partial negative charge.
The atom with the lower electronegativity has the electrons farther away and, so, has a partial positive charge.

9. Showing Partial Charges
There are two ways to show the partial separation of charges
Use of “” for “partial”
Use of an arrow pointing towards the partial negative atom with a “plus” tail at the partial positive atom C O H + - C O H

10. Ionic Bonds
Ionic bonds occur when the electronegativies of two atoms are so different that they can’t even share unevenly…one atom just takes them from the other

11. How to determine bond type
Find the electronegativies of the two atoms in the bond
Find the absolute value of the difference of their values
If the difference is 0.4 or less, it’s a non-polar covalent bond
If the difference is greater than 0.4 but less than 1.4, it’s a polar covalent bond
If the difference is greater than 1.4, it’s an ionic bond

12. If the bond is polar, draw the polarity arrow
Let’s Practice
C – H
O—F
F—F
C—Cl
Example:
If the bond is polar, draw the polarity arrow

13. If the bond is polar, draw the polarity arrow
Let’s Practice
C – H
O—F
F—F
C—Cl
2.55 – 2.2 = nonpolar
3.44 – 3.98 = polar
3.98 – 3.98 = non-polar
2.55 – 3.16 = polar
Example:
If the bond is polar, draw the polarity arrow