MOLECULAR GEOMETRY
AND VSEPR THEORY
VSEPR THEORY
• To understand the shapes of molecules, we use a theory called the
Valence Shell Electron Pair Repulsion Theory
• This theory tells us how we can understand the geometry of a molecule by
looking at the electron groups that surround it
• Electron groups can be:
•
•
•
•
Single Bonds
Double Bonds (1 e- group)
Triple Bonds (1 e- group)
Lone Pairs (still 1 e- group)
VSEPR THEORY
• VSEPR Theory tells us that electron groups will repel
each other
• This is because of negative charges. A negative
and a negative from the electrons will repel
• The electron groups then move as far away from
each other as possible
• This will determine the shape of the molecules
ELECTRON GROUP GEOMETRIES
• Electron Group Geometries (EGG) are the shapes made
when the electron groups repel
• There are 5 basic electron group geometries
• Linear (2 electron groups)
• Trigonal Planar (3 electron groups)
• Tetrahedral (4 electron groups)
• Trigonal Bipyramidal (5 electron groups)
• Octahedral (6 electron groups)
LINEAR GEOMETRY
• When 2 electron groups are around the central atom, the
farthest away is 1800
• This creates a linear shape
• Example
• Draw a molecule of CO2 and determine the EGG
TRIGONAL PLANAR
• With 3 electron groups around the central atom, the
farthest away for all electrons to be is 1200 in the same
plane
• Because they are in the same plane, the is called Trigonal
Planar (3 in same plane)
• Example
• Draw a molecule of COCl2 and determine the EGG
TETRAHEDRAL
• With 4 Electron groups, the furthest point for electron groups is 109.50
• This may seem weird since on paper is looks like 900 but
remember, molecules are 3-dimensional and can make complex
shapes
• This shape forms a tetrahedron and is called Tetrahedral
• Example
• Draw a molecule of NH3 and determine the EGG
TRIGONAL BIPYRAMIDAL
• 5 electron groups around the central atom
makes a Trigonal Bipyramidal structure
• This is like 2 pyramids (bipyramidal) joined
together
• It also has 3 atoms atoms making a trigonal
structure in it
• These have 2 angles, 900 between the xplane and the y-plane and 1200 in the xplane
• Example
• Draw a molecule of PCl5 and determine
the EGG
OCTAHEDRAL
• With 6 electron groups an
octahedral structure is formed
• All angles are 900
• Example
• Draw a molecule of SF6 and
determine the EGG
MOLECULAR GEOMETRY
• The EGG tells us the shape of ALL ELECTRON GROUPS
• The molecular geometry is the geometry of the atoms
• This is determined by the number of electron groups AND the number of
bonds around a central atom
MOLECULAR GEOMETRIES
MOLECULAR GEOMETRY EXAMPLE
• Draw a molecule of H2O
• Determine the EGG
• Use the chart provided to find the Molecular Geometry