BOND POLARITY AND DIPOLE MOMENTS

giantsneckspiffyElectronics - Devices

Oct 13, 2013 (3 years and 10 months ago)

72 views

CHEM. 2AP UNIT 7 CH. 8 Bonding: General Concepts
22
BOND POLARITY AND DIPOLE MOMENTS


• You will recall that the polarity of a bond is determined by determining the difference in the electronegativities. If a molecule
is diatomic (2 atoms) there is often only one bond and that will determine whether the molecule is polar.
• For ex. the H−F bond is polar with fluorine being the more electronegative. A partial negative charge resides on the fluorine
atom and partial positive charge.


H−−−−−−F

δ
+
δ

• The arrow points to the center of negative charge while the tail is at the center of positive charge. a dipole moment means that
the molecule has two poles.
• The situation is clear-cut with HF. It becomes more difficult with 3 or more atoms in a molecule because the individual dipoles
can cancel each other out.
• Here are some basic guidelines to determine if a molecule is polar or non-polar. Later you will be shown how to determine the
dipole moment with the arrow shown above.

HOW TO DETERMINE MOLECULAR POLARITY (EXCLUDING DIRECTION OF DIPOLE MOMENT



Molecules that are not totally symmetrical are polar molecules. In a polar molecule, electron density accumulates
toward one side of the molecule giving that side a slight negative charge δ-, and the other side a slight positive charge of
equal value δ+. Polar molecules are said to possess a dipole moment which means that it has 2 poles (+ and -). A polar
molecule is a dipole.
• Polarity is due to the polarity of the bonds and the lone pairs on the central atom. One lone pair on the central atom makes
the molecule polar. This only works for one lone pair, not 2, 3, 4, etc. If more than one lone pair, determine the polarity from
the bonded atoms.

To determine if a molecule is polar (has a dipole moment):
1. Draw an acceptable Lewis dot structure,
2. Predict the electron-pair geometry
3. Determine whether the molecule is totally symmetrical.
4. An analogy for polarity is to imagine that an object is being pulled in directions determined by the electronegativities of the
atoms. If the forces are equal, the object will not move (nonpolar).

Criteria for polarity:
• If a molecule is diatomic (2 atoms) and the atoms are different, it is polar.
• A molecule having just one lone pair of electrons is polar.
• If all of the terminal atoms are the same and there are no lone pairs of electrons around the central atom, the molecule is
totally symmetrical and nonpolar.
• If the molecule is not symmetrical, it is polar. The terminal atoms are different and the dipole moments do not cancel each
other out. (Pulling moves the object).
11/16/2002
Examples:Predict whether the following molecules are polar or nonpolar:
(a) CO
2

(b) CH
2
O
(c) CCl
4

(d) CCl
2
F
2














CHEM. 2AP UNIT 7 CH. 8 Bonding: General Concepts
23
HOW TO DETERMINE THE DIRECTION OF THE DIPOLE MOMENT














































ASSIGNMENT: Determine if the following molecules or ions are polar (possess a dipole moment) or not polar.
Determine the direction of the molecule dipole moment (if any).
1. BCl
3

2. SO
3

3. PF
3

4. SF
6

5. BeH
2

6. CO
2

7. SO
2

8. NO
2
-
9. NH
4
+

10. SnCl
5
-

11. AlH
3


DO ASSIGNMENT #3 ON ASSIGNMENT SHEET
P. 409-410 #80-90 even, 91, 92, 94, 96

11/16/2002