I. “Molecular Polarity” – A term that is used to distinguish two types of molecules…. Based on the
presence or absence of a “molecular ________________________”.
A. Some molecules show characteristics indicating that they have ______________________-charged ends
(a _______________________end and a________________________ end). These molecules are said to
be ___________________________.
B. Other molecules show characteristics indicating that their structure doesn’t have oppositely–charged ends.
These molecules are said to be __________________________.
II. How to determine whether a molecule is polar
A. IMPT: You MUST first make a “sphere and stick” sketch of the molecule; then, follow these steps:
1. Calculate each individual bond’s polarity.
a. First identify the “_____________________________ value” (referred to as “EN”) of each atom.
(1) The EN is a number from 0 to 4, given on your periodic table, side 2, within each element’s
square…..upper right corner of the square, in black print.
(2) Atoms whose EN is close to “4” attract electrons ____________________; atoms whose EN
is close to “0”, attract electrons ______________________.
2. Subtract the two EN values. Take the absolute value of the answer (make the answer positive).
3. Look at the chart below to see where the│EN – EN│answer falls .
Subtraction Answer ( │EN – EN│ )
Category of Bond
0
0.4 (including 0.4)
Non-Polar Covalent Bond
0.4
2.0 (over 0.4; including 2)
Polar Covalent Bond :
Indicated by writing δ +
and δ – symbols
> 2.0 (over 2)
Ionic Bond
δH+
δ
O
3.44
2.20
δ
δH+
2.20
│3.44 – 2.20│ =
1.24 = Polar Covalent Bond
+
a. When │EN – EN│ falls in the ____________________ category, insert a δ symbol next to the atom
–
with the lower EN and a δ symbol next to the atom with the higher EN. [This might be the opposite
from what you would have expected…..Remember, it is the atom with the highest the EN, which is the
most able to attract electrons (which are negatively-charged).]
(1) A polar bond occurs when one atom attracts the electrons much more strongly than the atom it is
connected to, giving the bond a “partial positive end” and a “partial negative end”.
b. Also, draw an arrow _____________________ to the polar bond and the same length as the polar bond;
–
additionally, the arrow “point” must be pointed toward the δ symbol. Last, place a “+” over the nonpointy end of the arrow. Ex:
+
c. When │EN – EN│ falls in the _____________________ category, don’t insert those two symbols or the
arrow. In nonpolar bonds, atoms share the bonding electrons more or less equally.
B. Practice bonds: For each bond below, find the EN values, then calculate the │EN – EN│ answer. Then
form your conclusion: polar covalent bond
or
nonpolar covalent bond .
│EN – EN│= _______
( │EN – EN│ )
Category of Bond
0
0.4
(including 0.4)
0.4
2.0
(over 0.4; including 2)
> 2.0 (over 2)
Non-Polar
Covalent Bond
Polar
Covalent Bond
C–C
C–H
Na – Cl
H–F
F–F
│2.55 – 2.55│ = 0.00
Category:
nonpolar covalent
Ionic Bond
III. The second part of determining “molecular polarity” (the whole molecule’s polarity) involves shape.
A. A molecule that has all nonpolar bonds must be ______________________.
B. But, a molecule that has all polar bonds may or may not be polar! Sometimes bond polarities “cancel”,
giving a nonpolar molecule. Sometimes bond polarities “add”, giving a polar molecule.
C. As long as every bond in a given molecule is identical and polar, then the 3 shapes drawn below to the
left represent shapes where polar bonds are canceled.
D. The structures drawn below to the right represent instances where the polar bonds do not cancel.
Polar bonds NonPolar molecule
-
+
-
Polar bonds  Polar molecule.
+
Bent
+
+
-
Linear triatomic
-
+
+
-
-
Trigonal planar
(flat triangle)
- - -
+
+
+
-
-
+
+
Linear diatomic
Tetrahedral
+
Notice I’ve drawn the tetrahedral structure
To look purposely like an “X”
Tetrahedral
Pyramidal
+
+
Linear triatomic
+
+
+
-
OMG, what if I forget in which shapes the polarity gets canceled?
Then you use the arrows that I showed you earlier.
Combine them together, head to tail, and see if they cancel, or if they add.
If you end anywhere different from where the first arrow started, then the arrows “add”, which means that the
polarity combines (and the molecule stays polar).
If your arrows form a “closed loop” (the last arrow ends at the same spot as where the first arrow started), then
the arrows “cancel” (and the bond polarity cancels).
Observe the vector addition for linear triatomic, trigonal planar, and tetrahedral molecules: All these molecules
are non-polar, even though they all had individually polar bonds.
δ-
δ+
δ-
δ+
#2
#1
#1
Notice how the dotted arrow ended where
the first arrow began… This means the
two polar bonds cancel, and the molecule
is nonpolar as a whole.
#2
+
+
-
-
Notice how the 3rd arrow ended where
the first arrow began… This means the 3
polar bonds cancel, and the molecule is
nonpolar as a whole.
Trigonal planar
tetrahedral
+
+
- - -
+
+
Notice how the 4th arrow ended where
the first arrow began… This means the 4
polar bonds cancel, and the molecule is
nonpolar as a whole.
Last, when there is only 1 polar bond in the molecule, there’s nothing to cancel it with…. So the molecule is
automatically polar.
The following are shapes in which polar bonds do NOT cancel.
Bent
+
+
-
-
-
Notice how the dotted arrow in each example does NOT end
where the first arrow began… This means the polar bonds do
NOT cancel, and the molecule stays polar as a whole.
Same idea:
Polar molecule
Pyramidal
+
+
+
IV. Polar molecules respond to an electric field:
A. When placed between a positively charged metal plate and a
negatively charged metal plate, the water molecules line up, with their
_____________________-charged region attracted toward the negative
δH+
δ
O
3.44
δ
2.20
δH+
2.20
plate and their ______________________ charged region attracted toward the positive plate.
V. _________- molecular “Bonds” (forces of attraction).
A. Inter-molecular bonds (forces) are the attractions _________________2 molecules. They hold
molecules together. They are much __________________ than polar and non-polar covalent bonds.
B. Breaking an inter-molecular bond is a _____________________ change; whereas, breaking an intramolecular bond is a chemical change. When you boil water, all you really do is break the intermolecular
bonds between water molecules
C. Important category of intermolecular bond: The ___________________ bond.
1. Hydrogen bonding occurs when there is a hydrogen atom covalently bonded to one of these three
atoms: _________, ___________, _______________. Ex: H2O, HF, NH3
Life would not occur as we know it were it not for hydrogen bonding: Hydrogen bonds hold the DNA double
helix together, cause water to expand as it freezes, causes water to have a “greater than expected” boiling
temperature, and play a part in maintaining the structure of proteins.