Covalent Molecules
Molecular Shapes
Covalent molecules form definite shapes.
The shape depends upon the number of
bonds formed and the number of unshared
clouds of electrons around an atom.
These covalent bonds also create definite
and measurable angles.
Shapes always refer to the central atom.
The VSEPR theory is used to predict the bond
angles.
VSEPR stands for Valence Shell Electron Pair
Repulsion.
According to the VSEPR theory, electron
clouds want to arrange themselves in space
as far apart as possible.
Also, the unshared pairs of electrons take up
more space than the bonded electrons and
reduce the bond angles in these molecules.
Shapes of Molecules
When naming the shape,
• refer to the central atom
• count the number of shared and unshared
clouds around it.
• To determine the shape, you must first draw a
Lewis Structure
2 Atom Molecules
Only one shape may exist because it is just
two atoms.
Drawing of Lewis Structure for H2
1 shared cloud between, no central atom
The 3-dimentional structure of a hydrogen
molecule
3 Atom Molecular Shape
Number of shared clouds: 2
Number of unshared clouds: 0
Shape Name: linear
Bond angle: 180°
An example is BeF2
Lewis structure:
Number of shared clouds: 2
Number of unshared clouds: 0
Molecular Structure: 3 dimensional shape
Shape name: linear
Bond angle: 180°
4 Atom Molecular Shape - A
Number of shared clouds: 3
Number of unshared clouds: 0
Shape name: trigonal planar
Bond angle: 120°
Example: BF3
Draw Lewis Structure
Number of shared clouds: 3
Number of unshared clouds: 0
Molecular Structure:
Trigonal Planar – the molecule flat and if the
points were connected it would be a large
triangle cut into three smaller triangles.
Bond angle: 120°
4 Atom Molecular Shape- B
Number of shared clouds: 2
Number of unshared clouds: 1
Shape name: bent (or angular or V-Shaped)
Bond angle: 117°
The bond angle is being compressed by the 1
pair of unshared electrons.
Example is O3
Draw Lewis Structure:
Number of shared clouds: 2
Number of unshared clouds: 1
Molecular Structure
Shape is bent, please note that the lone pair
of electrons is not shown in the molecular
geometry.
Bond Angle
117°
5 Atom Molecular Shape- A
Number of shared clouds: 4
Number of unshared clouds: 0
Shape name: tetrahedral
Bond angle: 109.5°
Example CH4
Draw Lewis Structure:
Number of Shared Clouds: 4
Number of Unshared Clouds: 0
Molecular Geometry
Shape: tetrahedral – note the molecule is not
flat
Bond angle: 109.5°
A tetrahedral is not a flat molecule,
but it needs to be drawn on flat paper.
To show a 3-d molecule on flat paper
you use a wedge and hash marks.
The hash marks: bond goes into the
board.
The wedge: bond is coming out of the
board.
Both hash and wedge are in the same
region of the molecule drawing.
5 Atom Molecular Shape- B
Number of shared clouds: 3
Number of unshared clouds: 1
Shape name: trigonal pyramidal (also not
flat)
Bond angle: 107.3°
Example NH3
Draw Lewis Structure:
Shared clouds: 3
Unshared clouds: 1
Molecular Geometry
Shape: trigonal pyramidal
Bond angle:
5 Atom Molecular Shape - C
Number of shared clouds: 2
Number of unshared clouds: 2
Shape: bent
Bond angle: 104.5°
Example H2O
Draw Lewis Structure:
Shared Clouds: 2
Unshared Clouds: 2
Molecular Structure
Shape: bent
Bond angle: 104.5°
The bond angle is being compressed by 2
lone pairs of electrons instead of just one.
Shapes Outside the Octet Rule
Octet Rule Violation
After period 2 on the periodic table, the dorbitals are available for use.
The d-orbitals are not occupied so they may
be used to form bonds.
This only occurs for central atoms in period 3
or higher!
6 Atom Molecule- Expanded Octet
Number of shared clouds: 5
Number of unshared clouds: 0
Shape name: trigonal bipyramidal
Bond Angle: varies depending upon the 3
atoms used to form the angle
Example PCl5
Draw Lewis Structure
Does not follow the octet rule, expanded octet
Shared Clouds: 5
Unshared Clouds: 0
Molecular Shape
Shared Clouds: 5
Unshared Clouds: 0
7 Atom Molecule (expanded octet
used)
Number of shared clouds: 6
Number of unshared clouds: 0
Shape name: octahedral
Bond angle(s): varies depending upon which
three atoms form the angle
Example SF6
Draw Lewis Structure:
Does not follow the octet rule, expanded octet
Shared Clouds: 6
Unshared Clouds: 0
Molecular Shape
Shape name: octahedral
Angle: varies
Use hashes and wedges in 2-d
drawing of 3-d shape