SHAPES OF MOLECULES NOTES MISS CHOHAN
12/19/2015
1.3 SHAPES OF MOLECULES
The shape of simple covalent molecules is determined by the number of bonding pairs of
electrons and the number of lone pair of electrons.
These are lone electron
pairs as they are NOT
i.e.
involved in bonding
These are known as bond
pairs because they are
involved in bonding
VALENCE SHELL ELECTRON PAIR REPULSION THEORY (VSEPR)
STATES THAT:-
1. ALL ELECTRON PAIRS REPEL AS FAR APART FROM EACH OTHER AS
POSSIBLE
2. LONE PAIR OF ELECTRONS REPEL MORE THAN BOND PAIR OF ELECTRONS
(i.e. LpLp > LpBp > BpBp)
REMEMBER:
Also each lone pair reduces a bond reduces a bond angle by ~2.5o this rule
can work for about 80% of molecules BUT NOT ALL
There are five basic shapes and then variations on each shape where lone pairs replace bond
pairs
SHAPES OF MOLECULES NOTES MISS CHOHAN
12/19/2015
2 ELECTRON PAIRS (OR 2 BOND PAIRS)
Linear shape : 2 bonding pairs
EXAMPLE: BeCl2
2 bond pairs around
central atom
2Bp & 0Lp = Linear Shape, Bond Angle 1800
3 ELECTRON PAIRS (OR 3 BOND PAIRS)
Trigonal planar: 3 bonding pairs of electrons
Example: BCl3
3Bp & 0Lp = Trigonal planar Shape, Bond Angle 1200
SHAPES OF MOLECULES NOTES MISS CHOHAN
12/19/2015
4 ELECTRON PAIRS (OR 4 BOND PAIRS)
Tetrahedral: 4 bonding pairs of electrons
EXAMPLE: CH4
4Bp & 0Lp = Tetrahedral Shape, Bond Angle 109.50
5 ELECTRON PAIRS (OR 5 BOND PAIRS)
Trigonal Bipyramidal: 5 bonding pairs of electrons
EXAMPLE: PF5
5Bp & 0Lp = Trigonal bipyramidal Shape. Bond Angle 1200 & 900
SHAPES OF MOLECULES NOTES MISS CHOHAN
12/19/2015
6 ELECTRON PAIRS (OR 6 BOND PAIRS)
Octahedral: 6 bonding pairs of electrons
EXAMPLE: SF6
6Bp & 0Lp = Octahedral Shape. Bond Angle 900
REMEMBER: VALENCE SHELL ELECTRON PAIR REPULSION THEORY
(VSEPR)
ELECTRON PAIR REPULSION THEORY
This theory states that:


Electron pairs repel as far apart as possible
And that lone pairs repel more than bond pairs i.e.
LpLp>LpBp>BpBp
( > is more than)
REMEMBER THE RULES
When deciding the shape of a molecule always:
1. Draw a dot and cross diagram first
2. Work out the TOTAL number of electron PAIRS around the central atom
3. Then work out how many of these electron pairs are bond pairs and lone pairs
SHAPES OF MOLECULES NOTES MISS CHOHAN
12/19/2015
EXAMPLES
3 BOND PAIRS & 1 LONE PAIR (4 ELECTRON PAIRS IN TOTAL)
Bp
Lp
EXAMPLE: AMMONIA (NH3)
1 LONE PAIR
3 BOND PAIRS
Here there are 4 pairs of electrons in
total BUT one is a lone pair so ∴ here
the shape is based on a tetrahedral
with a bond angle of 109.50 BUT the
lone pair reduced the bond angle by
2.50 so the bond angle in ammonia is
1070.
3BP + 1LP = PYRAMIDAL
SHAPE BOND ANGLE 1070
2 BOND PAIRS & 2 LONE PAIRS (4 ELECTRON PAIRS IN TOTAL)
Bp
Lp
2 LONE PAIRS
2 BOND PAIRS
Here there are 4 pairs of electrons
in total BUT two are lone pairs so ∴
here the shape is based on a
tetrahedral with a bond angle of
109.50 BUT the lone pairs reduced
the bond angle by 50 so the bond
angle in water is 104.50.
SHAPES OF MOLECULES NOTES MISS CHOHAN
12/19/2015
2BP + 2LP = BENT NON-LINEAR SHAPE
BOND ANGLE 104.50
OTHER SHAPES WHICH MUST BE LEARNT BY HEART
1.
Bp
Lp
Examples
SO2 SnCl2 NO2
2BP + 2LP = BENT NON-LINEAR
SHAPE BOND ANGLE 104.50
2.
EXAMPLE
SF4
4BP + 1LP = SEA SAW SHAPE
BOND ANGLES 1190 & 890
SHAPES OF MOLECULES NOTES MISS CHOHAN
12/19/2015
3.
EXAMPLE
ICl3
3BP + 2LP = T- SHAPE BOND ANGLE 890
4.
Example
IF5
5BP + 1LP = SQUARE PYRAMID
SHAPE BOND ANGLE 890
5.
SHAPES OF MOLECULES NOTES MISS CHOHAN
EXAMPLE
XeF4
4BP + 2LP = SQUARE PLANAR
SHAPE BOND ANGLE 900
SHAPES OF MOLECULAR IONS
For a negative ion add extra electrons
and
for a positive ion take electrons away
EXAMPLE: IF4+
This time there will only be 6 electrons in
the outer shell of the iodine atom as one is
lost due to the positive charge.
∴ here there are 4 Bp`s and 1 Lp around
the central atom
DISTORTED TETRAHEDRON
SHAPE
EXAMPLE: I3This time there will be 8 electrons in
the outer shell of an iodine atom as one
is gained due to the negative charge.
∴ here there are 2 Bp`s and 3 Lp`s
around the central atom
12/19/2015
SHAPES OF MOLECULES NOTES MISS CHOHAN
12/19/2015
MULTIPLE BONDED COMPOUNDS
NOTE:
For double and triple bonded compounds we just ignore the multiple bond and take
them as single covalent bonds when deciding the shape of a molecule
EXAMPLE CO2
O=C=O
LINEAR SHAPE
So, here we can say that there
are just 2 bond pairs around
the central carbon atom.
Hence CO2 has a linear shape
with a bond angle of 1800
BOND ANGLE = 1800C
EXAMPLE: SO422 electrons added to
each oxygen atom
here
SO42- has a -2 charge which is
held on two of the oxygen
atoms. The sulphate ion has
two double bonds, and two
single bonds. So, again
ignoring the double bonds we
can say that there are 4
bond pairs of electrons
around the sulphur so the
sulphate ion has a
tetrahedral shape with a
bond angle of 109.50.
TETRAHEDRAL SHAPE
BOND ANGLE = 109.5
SHAPES OF MOLECULES NOTES MISS CHOHAN
12/19/2015
EXAMPLE NO3-
So this time ignoring the double bond
we have 3 bond pairs around the
central nitrogen atom
Trigonal planar shape
Bond angle 1200
SHAPES OF MOLECULES NOTES MISS CHOHAN
SHAPES OF MOLECULES
12/19/2015
SHAPES OF MOLECULES NOTES MISS CHOHAN
12/19/2015