Chapter 8 Molecular Shape

1
Chapter 8 Molecular Shape

• The shape of a molecule can be important in
  determining its chemical reactions
• Molecular shape is often very important in the
  chemistry of living things.
• Molecular geometry also helps determine the
  polarity of molecules

2

• High polarity higher boiling and melting points
  
• CO2 gas is a nonpolar molecule (low boiling
  point)
• H2O is a polar molecule (high melting point)

3
Valence-shell electron pair repulsion theory
(VSEPR theory)

• Atoms and electron pair groups around an atom
  repel each other
• A molecule will have a geometry that will
  keep these groups as far apart as possible.

4

• To determine geometry draw an electron dot
  structure and count the groups around the central
  atom. (note a nonbonding electron pair counts
  as a group.)

5
Two Groups

• Example CO2 (see electron-dot)
• two groups around the central C atom
• 180o allows these groups to be as far apart as
  possible.
• Will be linear (180o bond angle).

6
Three Groups

• Example CH2O see electron-dot structure
• Three groups form a triangle around the central C
  atom.
• Called trigonal planer geometry (120o bond
  angle)

7
4 groups around central atom

• Example CH4 (see electron-dot structure)
• Form a tetrahedron around the central C atom
  (109.5o bond angle)
• If all 4 groups are atom groups, the geometry is
  called tetrahedral.
• If 3 atom groups and one electron pair, the
  geometry is called pyramidal. (NH3)
• If 2 atom groups and 2 electron pair groups, the
  geometry is called bent (H2O)

8
Bond Hybridization

• The valence electrons are the s and p electrons.
• There is one s orbital and three p orbitals on
  the outer energy level.
• When bonding occurs the orbitals rearrange
  (hybridize) to create the most stable geometry.

9

• 2 groups around atom sp hybridization
• 1 s orbital combines with 1 p orbital to make 2
  sp hybrid orbitals.
• 3 groups around atom sp2 hybridization
• 1 s orbital combines with 2 p orbitals to make 3
  sp2 hybrid orbitals.
• 4 groups around atom sp3 hybridization
• 1 s orbital combines with 3 p orbitals to make 4
  sp3 hybrid orbitals.

10

• http//www.mhhe.com/physsci/chemistry/animations/c
  hang_7e_esp/bom5s2_6.swf

11
Molecular Polarity

• There are two requirements for a molecule to be
  polar.
• 1. The molecule must have polar bonds. (check
  the electronegativity)
• 2. The geometry must allow for a net dipole.
• Example CO2
• Example H2O