Forces

1
Forces Polarity
2
Bond Polarity

• If two atoms that share electrons have different
  electronegativities (ability to attract that
  electron) then they share the electron UNEQUALLY.
• Depending on how unequally the electrons are
  shared the bond can have different
  classifications.

3
Bond Polarity and Electronegativity

• If the difference in electronegativities is
  greater than 1.7 then the electron is NOT shared
  it is completely transferred and the bond is
  IONIC.

4
Bond Polarity and Electronegativity

• If the difference in electronegativities is less
  than 1.7 then the electron is shared but not
  equally and the bond is POLAR COVALENT.

5
Bond Polarity and Electronegativity

• If the difference in electronegativities is zero
  then the electron is shared equally and bond is
  NON-POLAR COVALENT.

6
Bond Polarity and Electronegativity
7
Examples
NaCl ? Na0.9, Cl3.0 3.0-0.92.1 gt 1.7 ? IONIC
8
Examples
SiBr ? Si1.8, Br2.8 ? 2.8-1.81.0 lt 1.7 ?
Polar Covalent
9
Examples
Se2 ? Se2.4, Se2.4 ? 2.4-2.40 ? Non-Polar
Covalent
10
Molecular Forces Polarity
11
Molecular Forces

• Intramolecular forces are the forces that keep
  the atoms in a compound stuck to each other - in
  other words, they're just chemical bonds. 
•  H2O H to O
• NaCl Na to Cl

12
Molecular Forces

• Intermolecular forces, on the other hand, are the
  forces that hold two covalent molecules to one
  another. 
• H2O to H2O
• These aren't chemical bonds - they're all based
  on attractions between different molecules. 

13
Intermolecular Forces

• There are three main types of intermolecular
  forces. 
• 1) hydrogen bonds
• 2) dipole-dipole forces
• 3) London dispersion(van der Waals)

14
Hydrogen Bonds

• Hydrogen Bonds are the strongest intermolecular
  force.
• They are not really a bond
• They are an attraction between a N, O, or F of
  one molecule and the hydrogen of another molecule.

15
Dipole-Dipole Forces

• A dipole is when one substance has a positive and
  negative end. (Like a magnet)
• Dipole-dipole forces take place when the positive
  side of one molecule is attracted to the negative
  side of another.

16
Dipole-Dipole Forces

• A substance that has dipole-dipole forces is a
  POLAR substance
• All substances that have hydrogen bonding also
  have dipole-dipole forces.
• Dipole-dipole are the 2nd strongest
  intermolecular force.

17
London Dispersion/van der Waals

• The weakest intermolecular force is the London
  dispersion/van der Waals force.
• This is a force that occurs between non-polar
  molecules.
• It happens in ALL molecules when electrons in a
  bond are attracted to the nucleus of an atom in
  another molecule.

18
How to Determine Intermolecular Forces in a
Molecule?

• STEP 1 DRAW THE LEWIS STRUCTURE
• STEP 2 Look for H bonded to N, O, or F
• If it has it, it has all 3 types of
  forces
• STEP 3 Look to see if there is a lone pair or
  different atoms bonded around the outside of
  the structure
• If you find them then it has
  dipole-dipole and LD/vdW
• STEP 4 If none of the above then only has
  LD/vdW

19
DRAW THE LEWIS STRUCTURE
Is there an H-N, H-O, or H-F
Yes HB, D-D, LD/vdW
No, Is there a Lone Pair
No, Are all outside atoms the same?
Yes D-D LD/vdW
No D-D LD/vdW
Yes LD/vdW
EX. H2O
20
DRAW THE LEWIS STRUCTURE
Is there an H-N, H-O, or H-F
Yes HB, D-D, LD/vdW
No, Is there a Lone Pair
No, Are all outside atoms the same?
Yes D-D LD/vdW
No D-D LD/vdW
Yes LD/vdW
EX. SiO2
21
DRAW THE LEWIS STRUCTURE
Is there an H-N, H-O, or H-F
Yes HB, D-D, LD/vdW
No, Is there a Lone Pair
No, Are all outside atoms the same?
Yes D-D LD/vdW
No D-D LD/vdW
Yes LD/vdW
EX. HPO
22
DRAW THE LEWIS STRUCTURE
Is there an H-N, H-O, or H-F
Yes HB, D-D, LD/vdW
No, Is there a Lone Pair
No, Are all outside atoms the same?
Yes D-D LD/vdW
No D-D LD/vdW
Yes LD/vdW
EX. SiFCl3