Bond angles

1
Bond angles

• The standard angles are as follows
• Linear 180
• Triangular planar 120
• Tetrahedral 109.5
• Trigonal bipyramidal 90 (between vertical and
  horizontal planes) 120 (within the horizontal
  plane)
• Octahedral all 90

2
Why?

• The major thing that effect bond angles is
• The number of areas of electron density around
  the central atom 2180 , 3 120 , 4 109.5 ,
  5 120 and 90 and 690

3
Minor changes due to non-bonding electrons

• When there are non-bonding electrons, they are
  held closer to the central atom as there is no
  other atom pulling on the electrons. Therefore
  repulsion is increased, and the bond angle
  between the bonded atoms decreases.
• For example, water, methane and ammonia all have
  4 areas of electron density and so are basically
  tetrahedral. However, methane has no non-bonding
  electrons, ammonia has one pair and water has
  two.
• The bond angles are methane 109.5 , ammonia
  - 107 , and water 105 .

4
Question

• Rank the following compounds in terms of the bond
  angle (highest to lowest)
• NO2, NO2-, NO2

5
Answer

• Highest to lowest
• NO2 gt NO2 gt NO2-
• Reasons NO2 has a single non-bonding electron
  which repels less than a pair of non-bonding
  electrons in NO2- . Both have 3 areas of electron
  density and so the bond angle is approximately
  12o degrees. NO2 has only 2 areas of electron
  density and so its bond angle is 180 degrees.

6
Question

• Draw the structural formula for methyl ethanoate.
  Label the following bond angles H-C-H, O-CO,
  and C-O-C.

7
Answer

• H-C-H - tetrahedral 109.5
• O-CO - triangular planar 120
• C-O-C - tetrahedral with non-bonding pairs
  approx 105 .

8
Bond lengths

• Rank the following bonds in order of length,
  longest to shortest
• C-C, CC and CC
• C-C, H-H and Cl-Cl

9
Answers

• C-C gt CC gt CC
• Reason More electrons in a multiple bond means
  a stronger attraction for both nuclei and
  therefore a shorter bond.
• Cl-Cl gt C-C gt H-H
• Reason Bond length is the distance between the
  centre of the 2 nuclei. As Cl has more electron
  shells, the distance between the 2 Cl nuclei is
  greater than that of C and H.