A summary highlighting all important notes with regards to shapes of molecules, bonding, groups - eg, bipyramidal, trigonal, planar etc. OCR A Chemistry AS/A Level. Hits all points covered in specification. For new exams.
The shape of simple covalent molecules is determined by the number of bonding pairs of electrons
and the number of lone pair of electrons.
In determining shape the electron pairs repel away from each other, and will move as far away as possible.
There are five basic shapes and then variations on each shape where lone pairs replace bond pairs
Name No Diagram
bonding
pairs
linear 2
Cl Be Cl
Trigonal 3
planar Cl Cl
B
Cl
Tetrahedral 4
H
C
H H
H
Trigonal 5
Bipyramidal
F
F
P F
F
F
Octahedral 6 F
F F
S
F F
F
N Goalby chemrevise.org 1
, We need to be able to work out for any simple molecule how many bond pairs and lone pairs there will
be. There is a simple method for compounds with single covalent bonds below that I will set out below in
a tabular form
This is the same as the Periodic Table group
number, except in the case of the noble gases Number of Electrons on central atom
which form compounds, when it will be 8.
Add one electron from each atom being bonded in
if the ion has a 1- charge, add one Add or subtract electron if the molecule has a charge
more electron. For a 1+ charge,
deduct an electron. Total electrons
Add first three rows together
Total pairs of electrons
Divide total electrons by 2
Number of bonding pairs
The bonding pairs will be the number of Number of lone pairs
other atoms that are joined to the
central atom.
The lone pairs will be the difference between the total
pairs and bonding pairs
Example: SiF4 and SF4 Are they the same shape?
SiF4 Si in group 4 SF4 S in group 6
Number of Electrons on central atom 4 Number of Electrons on central atom- 6
Add one electron from each atom being bonded in 4 Add one electron from each atom being bonded in 4
Add or subtract electron if the molecule has a charge - Add or subtract electron if the molecule has a charge -
Total electrons 8 Total electrons 10
Total pairs of electrons 4 Total pairs of electrons 5
Number of bonding pairs 4 Number of bonding pairs 4
Number of lone pairs Number of lone pairs
0 1
So SiF4 has a shape based on 4 bond SF4 has a shape based on 5 pairs of
pairs which is tetrahedral electrons which is trigonal bipyrimidal
where one pair is lone pair
F F
F
Si S :
F F F
F F
Let us look in more detail at each shape.
N Goalby chemrevise.org 2
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller eduan1. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £5.49. You're not tied to anything after your purchase.
