Molecular Shapes
Lewis Diagram Allow You to
Determine the Three-dimensional
Shape of a Molecule
Molecular Shape is Important!

Shape influences physical properties
– Affects features such as boiling point,
physical state, hardness, etc
– Water, for example, would surely be a gas
at room temperature (and well below) if
its shape were linear
– Instead water’s bent shape makes it a
liquid at ordinary temperatures
Water
H O H
If water had a linear
shape, it would be a
gas!
Water
H
H O
H
If water had a linear
shape, it would be a
gas!
O
H
Bent water is, of
course, a liquid;
shape matters!
Molecular Shape is Important

Shape influences chemical properties
– Affects rate of reaction
– Affects site within a molecule where reaction
might occur
– Affects type of catalyst or enzyme which might be
operative with reaction
Determining a Molecule’s
Shape:
It’s as Easy as 1-2-3
 Actually,
not quite...
 It’s as easy as 2-3-4, instead!
 We only need to count GROUPS of
electrons around certain atoms to determine
shape in the vicinity of that atom
Electron Groups
 An
electron group is a set of electrons that are
in the same area of the molecule
–
–
–
–
The two electrons of a lone pair
The two electrons of a single bond
The four electrons of a double bond
The six electrons of a triple bond
 Each
of the above counts as a single electron
group
Electron Groups
Electron Groups
Electron Groups
Electron Groups
Electron Groups
Four groups of Electrons
Electron Groups
H
H
N:
H
Electron Groups
H
one group
H
N:
H
Electron Groups
one group
H
one group
H
N:
H
Electron Groups
one group
H
one group
H
N:
one group
H
Electron Groups
one group
H
one group
H
one group
N:
one group
H
Four groups of electrons.
Lone pairs count!
Electron Groups
H
..
O:
H
Electron Groups
one group
H
..
O:
H
Electron Groups
one group
H
one group
..
O:
H
Electron Groups
one group
H
one group
one group
..
O:
H
one group
Again, lone pairs count.
Electron Groups
There are three electron groups around
carbon. Double bond counts as one group.
Electron Groups
one group
H C N:
one group
Around carbon, there are only two electron
groups; the triple bond is one group.
Determining Shapes
 Use
the Lewis diagram to establish shape
 The shape is described at each central
(pivot, “hinge”) atom in the molecule
 Some molecules may have different shapes
at different central atoms
 Don’t worry about shape at outside atoms
 Our molecules will usually have only one
central atom
Determining Shapes
H
..
O:
H
outside atom (no shape)
central atom (shape important)
Determining Shapes

 . .
CN 

H

shape not important
(outside atoms)
shape important (central, pivot atoms)
Determining Shapes
 The
number of electron groups around a
central atom determines its shape
 The shape will correspond to a 3-D
arrangement which allows maximum
separation of each electron group
 Remember, an electron group is:
– a lone pair
– a bond group (single, double, triple bond)
Two electron groups
 For
octet-rule atoms (ours are!), this will be
either
– one single bond + one triple bond
HCN:
2 electron groups
Two electron groups
 For
octet-rule atoms (ours are!), this will be
either
– one single bond + one triple bond
– two double bonds
HCN:
2 electron groups
..
..
:O=C=O:
2 electron groups
Two electron groups
 Two
electron groups generate a LINEAR
(straight-line) geometry (180°) separation
 Atoms connected to the linear electron
groups lie in their same direction
..
..
:O=C=O:
HCN:
180° separation
180° separation = linear
Three Electron Groups
 For
octet-rule atoms this combination will
be:
– two single bonds + a double bond
– one single bond + one double bond + one lone
pair
one group
one group
one group
(lone pairs at O, Cl omitted,
for clarity)
Three Electron Groups
 Three
electron groups generate a
TRIGONAL (triangle-like) geometry
– electron groups and their atoms are about 120°
apart
Four Electron Groups
 Result
–
–
–
–
from:
four single bonds
three single bonds + a lone pair
two single bonds and two lone pairs
never multiple bonds
Four Electron Groups
 Result
from:
– four single bonds
Four Electron Groups
 Result
from:
– four single bonds
– three single bonds + a lone pair





4 groups,
including lone pair
Four Electron Groups
 Result
from:
– four single bonds
– three single bonds + a lone pair
– two single bonds and two lone pairs

..

H





4 groups:
4 groups,
including lone pair count ‘em...
Four Electron Groups
4
Electron groups generate a
TETRAHEDRAL geometry, named after
tetrahedron, a figure from 3-D geometry
109°
X
It is as if the tetrahedral atom were “buried” in the
tetrahedron. An angle of about 109° exists between
corners and the central atom
Four groups of Electrons
Four groups of Electrons
N
Four groups of Electrons
O
H
H