Transcript Formulae, Diagrams and Shapes

Formulae, Diagrams
and Shapes
Type of Formulae
Lewis Diagrams
Shapes of Molecules
Author: J R Reid
Types of Chemical Formulae
Molecular/Chemical Formula
This is the ‘normal’ formula showing the elements
and their numbers in a chemical i.e. H2O, HCl,
Zn(NO3)2
Empirical Formula
This shows the proportion of each element in a
compound – like a ratio. For example C2H4 becomes
CH2 and C6H12O6 becomes CH2O
Structural Formula
This type shows the linking inside covalent
molecules. See the next page for details
Structural Formulae - Details
There are actually two types of
structural formulae:
Condensed – this does not show all of the
bonds but it does show the structure. For
example CH3CH2CH2CH3
Expanded – this has all the sticks drawn
in. We will learn how to create these in
the next section. For example:
H
H
H
H
H
C
C
C
C
H
H
H
H
H
Lewis Diagrams (Electron
Dot Diagrams)
Lewis diagrams (also called electron dot
diagrams) show the valence electrons of
covalently bonded atoms. Here is how you
draw them:
Work out how many valence electrons are
present in the atoms you are working with,
arrange them in pairs
e.g. CH4
C has 4 electrons (in its outer shell)
H has 1 electron
Arrange them so that all the valence shells are
full (normally 8 except for hydrogen)
The diagram shows how the carbon is sharing
the electrons with the hydrogens to fill the
shells of the carbon and the hydrogens
H
H
C
H
H
Drawing Lewis Diagrams
Chemical
HCl
H 2O
NH3
Cl2
C2H6
Lewis Diagram
H
Cl
These are called lone
pairs
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)
VSEPR – The Results
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
Exam Practice - 2005
Have a go at Questions:
• One
•
Can’t see the exam paper below?
Go to the NCEA website and
search for 90308
Exam Practice - 2006
Have a go at Questions:
• One (a) (b) and (c)
• Four
Can’t see the exam paper below?
Go to the NCEA website and
search for 90308
Exam Practice - 2007
Have a go at Questions:
• One (a)
• One (b)
Can’t see the exam paper below?
Go to the NCEA website and
search for 90308