Shapes of Molecules
Learning objectives
• Explain the shape of simple molecules based on
repulsion of electron pairs around a central atom.
• Understand that lone pairs repel more than
bonded pairs
• Explain the shapes and bond angles in trigonal
planar, tetrahedral, octahedral, pyramidal, nonlinear and linear.
Covalent bonds
Unlike Group 1 and group 7 elements,
most elements need to gain, lose or share
more than one electron
Draw a dot cross diagram for oxygen
What holds covalent bonds together?
What force exists between two atoms?
The Electrostatic force
Draw a diagram to show these forces
The atomic separation of particles in a
nucleus is determined by the balance
of these forces
Lewis Diagrams
A Lewis diagram consists of the element
symbol surrounded by "dots" to represent
the number of electrons in the outer
energy level
Draw Lewis diagrams for every element in
the second period
Shapes of molecules
Electrons go in pairs (the pairs are in the same orbital). The electrons in the
pairs aren’t repelling each other because of their spin.
The shape of a molecule can
be predicted based on the
number and arrangement of
electron pairs around a central atom
Each electron pair will
repel another electron as far
away as it can
This is sometimes known as VSEPR (Valency shell electron pair
Repulsion) theory
VSEPR
Principle: Electron pairs around a central atom arrange themselves so that
they can be as far apart as possible from each other.
It is important to remember that
Atoms are three dimensional objects
In the diagrams the central atom
is denoted by X and attached
surrounding bonded atoms by Q.
The bond angle is therefore based
on angle between the atoms Q-X-Q.
Therefore the total bond angle
for a molecule in 2
Dimensions will be 360˚
Linear Molecules
In a linear molecule, the atoms lie in a straight line. The bond angle formed is therefore 180°.
Trigonal Planar molecule
All four atoms lie on the same plane and the bond angles are all 120°
Tetrahedral Molecules
A tetrahedral molecule is a molecule which has four atoms extending
out from a central atom. The four atoms are located at the
vertices of a tetrahedron. All bond angles are 109.5°.
Drawing molecules
A simple straight line represents a
bond lying approximately in the surface plane.
A wedge shaped bond is directed in front
of this plane (thick end toward the viewer),
and a hatched bond is directed
in back of the plane (away from the viewer),
You may see a dashed bond used in place of
A hatched bond
Octagonal molecule
It has six vertices with atoms all extending from a central atom.
All bond angles are 90°
Lone pairs
What is a lone pair?
Lone pairs occur in elements from group
5, 6 and 7
Lone pair
How many lone pairs
does Oxygen have?
Lone pairs
Lone pairs affect
the shape of the
molecule
Lone pairs
Any lone pairs of non-bonding
electrons on the central atom X, are
closer to X than bond pairs because
there is no Q atom
attracting/sharing the lone pair
electron charge.
This will increase the repulsion between a lone pair
of electrons on X and any other bonding/non-bonding on X
Pyramidal molecule
There is a lone pair on the phosphorus atom
So there is more repulsion from this lone pair, this means that the angles between
Chlorine atoms will be slightly smaller
Lone pairs
Bent molecules
Water has two lone pairs
This means the two hydrogen atoms
are pulled even closer together