Summary OCR AS Chemistry Textbook, ISBN: 9781844894345 AS Unit F321 - Atoms, Bonds and Groups
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Course
AS Unit F321 - Atoms, Bonds and Groups
Institution
OCR
Book
OCR AS Chemistry Textbook
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
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