Polarity of Molecules

1
Polarity of Molecules

• When does a molecule have polar bonds?
• When the elements bonded together have a
  difference in their electronegativites.
• What side of the molecule is said to be positive?
• What side of the molecule is said to be negative?

2
Using an electric field

• Can use an electric field to determine if the
  molecules that you are looking at are polar or
  nonpolar
• Polar molecules will orient themselves to have
  the polar end pointing towards the negative plate
  and the negative end will point towards the
  positive plate
• Nonpolar molecules will not be affected by the
  electric field

3
How to tell if a molecule is polar

• Step 1 Draw the Lewis Dot structure for the
  molecule
• Step 2 Determine what electron-group, then what
  molecular shape the molecule has
• Step 3 Identify where the positive and negative
  ends are for each bond
• Step 4 Draw in the arrows for the polarity and
  see if the polar bonds cancel in the molecule

4
Can a molecule have polar bonds, but not be polar?

• Yes or No
• Lets look at C Cl
• This molecule has polar bonds, but because of the
  geometry, it is not polar

5
London Dispersion

• Present between polar and nonpolar moelcules
• Picture

6
Why do you suppose that Cl2 is a gas, Br2 is a
liquid, and I2 is a solid?

• Cl has less electrons than Br, and Br has less
  electrons than I
• More electrons cause more electron shifting
  greater LD forces
• The greater the number of electrons, the greater
  the LD force
• As you increase the size of the atom, you
  increase the LD force

7
Which member of each pair has the stronger LD
forces?

• Ne and Kr
• F2 and Cl2
• CH4 and SiCl4
• N2 and O2
• N2 has a boiling point of 77.4 K
• O2 has a boiling point of 90.2 K
• Larger BP means larger LD

8
Dipole-Dipole Forces

• Stronger than London Forces
• Definition The permanent dipoles of polar
  molecules attract one another
• Strength of these forces depend on the dipole
  moment of the moelcules
• So, the greater the difference in EN, the larger
  the dipole moment, the strong the dipole-dipole
  forces

9
Hydrogen-bonding

• The H bond is an especially strong dipole-dipole
  force
• Hydrogen can hydrogen bond with F, O, or N
• Conditions for occurrence
• H must be covalently bonded to a small, highly
  electronegative element in one molecule
• The H is then positively charged, so it is
  attracted to the negative end of another molecule

10
What types of intermolecular forces are observed
in each of the following?

• H2O HF
• HBr NH3
• PF3 CH3OH
• F2 N2
• CO2

11
Hydrogen-bonding

• H-bonding is critical for life
• Used in the structure of DNA
• Stabilizes the shapes of proteins and nucleic
  acids, protecting their biological roles