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 °
Why?
► The
major thing that effect bond angles is
The number of areas of electron density
around the central atom – 2=180 °, 3=
120 °, 4= 109.5 °, 5= 120 and 90 ° and
6=90 °
Minor changes due to nonbonding 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 °.
Question
► Rank
the following compounds in terms of
the bond angle (highest to lowest)
NO2, NO2-, NO2+
Answer
►
Highest to lowest:
NO2+ > NO2 > 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.
Question
► Draw
the structural formula for methyl
ethanoate. Label the following bond
angles: H-C-H, O-C=O, and C-O-C.
Answer
► H-C-H
- tetrahedral 109.5 °
► O-C=O - triangular planar 120 °
► C-O-C - tetrahedral with non-bonding pairs
approx 105 °.
Bond lengths
►
1.
2.
Rank the following bonds in order of
length, longest to shortest:
C-C, C=C and C=C
C-C, H-H and Cl-Cl
Answers
C-C > C=C > C=C
Reason: More electrons in a multiple bond
means a stronger attraction for both nuclei
and therefore a shorter bond.
2. Cl-Cl > C-C > 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.
1.