Shapes of molecules
and ion
1.
2.
Recall expanded
octets
Determine the shape
of molecules
Author: J R Reid
Starter
Ncea question 06,07,08 Lewis
diagrams
Recall
Incomplete octet
 Due
to insufficient pairs of electrons
 Occurs when central atom is
 Examples so far BF3,AlCl3,BeCl2
 examples
BH3, NO,
Recall
Expanded octet
 Atoms
belonging to th 3rd period or lower
 Use the empty 3d ( level 3 can hold up to
18 electron using 3s, 3p and 3d as the
valence shell) this is the case with
sulphur and phosphorus
 Examples ozone, PF5 PCl5 SCl6, SF6, BrF3
and BrF5
Ions
Monatomic ion – single atom
Polyatomic – contain more than one
atom eg ammonium ion, nitrate ion,
carbonate and sulphate ion.
Drawing Lewis structures adjust the
valence number of electrons according
to the charge on the ion
Square brackets are place around the
structure and the charge place on the
outside.
Note: Dative Bond
This is when both electrons in a covalent bond are
supplied by one atom
An arrow can drawn to show the direction that the
electrons have been donated (donor > acceptor)
If the central atom doesn't have an octet of
electrons an outer atom may provide both electrons
to be shared between them.
If the central atom has a lone pair of electrons it
may provide both for the shared bond
Example ammonium ion, nitrogen dioxide.
Limitations of Lewis
structures
Sometimes there is more than one
structure for a molecule or polyatomic
ion other than the expected one
These have been found through X-ray
diffraction work
Examples sulphate ion, phosphate ion
and the chlorate ion
Shapes
Lewis structure predict shapes of
molecules and ions as they show sets
of electrons bonding and non bonding
around the central atom
Each bond – area of electron density
that arrange themselves due to
electron repulsion VSEPR valance
shell electron pair repulsion theory
Shapes of Molecules
Molecules (covalent chemicals) form certain
shapes depending on how many lone and
bonding pairs of electrons it has.
H
H
Bonding pair – these can also be
drawn as straight lines
O
Lone pair
Because the electron pairs repel each other
we get certain shapes being formed. These
are due to a certain rule called VSEPR
(Valence Shell Electron Pair Repulsion)
Shape
Depends on number of atoms linked
to the central atom
Number of regions of electron density
Lone pairs are not drawn in the final
shape
The name of the shape is determined
by the bonded atoms only
We can also find bond angle using
Lewis diagram and shape
VSEPR – base shapes
2
Electron
Repulsion
Zones
3
Electron
Repulsion
Zones
4
Electron
Repulsion
Zones
5
Electron
Repulsion
Zones
6
Electron
Repulsion
Zones
Shape:
Shape:
Shape:
Shape:
Shape:
Linear
Trigonal
Planar (Flat)
Tetrahedral
Trigonal
Bipyramid
Octahedral
3D Shapes
Shape
Tetrahedral
Formula
CH4
Examples
Lewis
H
H
C
H
H
Trigonal
Pyramid
NH3
H
H
N
H
Bent/Angular
H 2O
H
O
H
Diagram
“Flat” Shapes
Shape
Linear
Trigonal
Planar
Examples
Formula
CO2
CH2O
Lewis
O
C
O
H
C
H
O
Diagram
Level 3 - Shapes
Shape
Formula
Octahedral
SCl6
Trigonal
Bipyramid
PCl5
Square Planar
XeF4
Examples
Lewis
Diagram
Worksheet and models
Complete worksheet with example
Make straw models