Molecular Shapes
Download
Report
Transcript Molecular Shapes
Title: Lesson 4 Molecular Shapes
Learning Objectives:
• Model, draw, and explain the shapes of molecules with 2-4 charge centres around
a central atom
• HL Only: Model, draw, and explain the shapes of molecules with 5 and 6 charge
centres around a central atom
VSEPR – a brief introduction
Valence Shell Electron Pair Repulsion (aka ‘vsepr’)
Pairs of electrons around an atom repel each other – this determines a molecule’s
shape
Pairs of electrons are known as ‘charge centres’ and include both:
The electrons in a covalent bond
a double/triple bond only counts as one charge centre!
Lone pairs / non-bonding pairs
Example: ethyne
The carbon has two charge centres (the C-H bond and the CC bond)
They push as far away from each other as possible making a 180o bond angle
Main Menu
Drawing shapes in three dimensions:
Draw as many atoms as you
can in the same plane ‘flat’ on
the paper
Use solid wedges to show
atoms coming out of the
plane of the paper towards
you
Use dashed wedges to show
atoms going back into the
plane of the paper away from
you
Main Menu
3D-SHAPES OF MOLECULES
Explained and predicted using V.S.E.P.R. theory.
(VALENCE SHELL ELECTRON PAIR REPULSION)
Electron pairs repel each other
and GET AS FAR APART AS POSSIBLE
to minimise repulsions.
If bond pairs (BP) ONLY present, [no lone pairs, (LP)]
totally symmetrical shape
If LP also present
slight distortion of symmetrical shape
VSEPR Summary
Repulsion applies to electron domains
Number of domains around central atom determines the
geometrical arrangement.
Shape determined by angles between bonded atoms
Lone pairs have a higher concentration of charge (not shared
between 2 atoms) so have more repulsion.
lone pair – lone pair > lone pair- bonded pair > bonded par – bonded pair
Shapes of molecules
6 of 33
© Boardworks Ltd 2009
Working out basic shapes of molecules
7 of 33
© Boardworks Ltd 2009
Molecular shape calculations
8 of 33
© Boardworks Ltd 2009
SYMMETRICAL MOLECULES
Have at least THREE planes of symmetry at 90.
In the following slides:
BP
= NUMBER OF BOND PAIRS ON CENTRAL ATOM
LP
= NUMBER OF LONE PAIRS ON CENTRAL ATOM
CN
= COORDINATION NUMBER of central atom
= number of atoms bonded to central atom
MOLECULE AND
FORMULA
DOT-AND-CROSS
DIAGRAM
.
.
F . Be . F
Beryllium Fluoride
BeF2
BP
2
180º
LP
NB Be does NOT
have an octet
of e-
SHAPE and BOND
ANGLE(S)
CN
0
2
F
Be
LINEAR
F
ADDING ANOTHER ATOM - ANIMATION
Stopwatch Graph Home
MOLECULE AND
FORMULA
DOT-AND-CROSS
DIAGRAM
F
..
.
F . B
..
F
NB B does NOT
have an
octet of e-
Boron Trifluoride
BF3
BP
SHAPE and BOND
ANGLE(S)
F
3
B
LP
CN
0
3
F
120º
F
TRIGONAL (or
TRIANGULAR)
PLANAR
ADDING ANOTHER ATOM - ANIMATION
Stopwatch Graph Home
MOLECULE AND
FORMULA
DOT-AND-CROSS
DIAGRAM
Tetrafluoromethane
CF4
BP
F
..
.
.
F . C . F
..
F
SHAPE and BOND
ANGLE(S)
F
4
109.5º
C
LP
0
F
F
F
CN
4
TETRAHEDRAL
MOLECULE AND
FORMULA
DOT-AND-CROSS
DIAGRAM
F
F
F
..
P
Phosphorus
Pentafluoride
PF5
BP
NB P does NOT
have an
octet of e-
F
5
F
F
SHAPE and BOND
ANGLE(S)
F
LP
CN
0
5
P
F
90º
F
120º
F
TRIGONAL
BIPYRAMID
MOLECULE AND
FORMULA
DOT-AND-CROSS
DIAGRAM
F
F
F
S
F
F
F
NB S does NOT
have an
octet of e-
Sulphur Hexafluoride
SF6
BP
SHAPE and BOND
ANGLE(S)
F
6
F
LP
CN
0
6
F
S
F
90º
F
F
OCTAHEDRAL
SUMMARY :
Symmetrical molecules
NO LP on central atom,
only BP belonging to single bonds.
BP are identical
REPEL each other equally.
SYMMETRICAL shape
180o, 120o, 109.5o or 90o
REGULAR SHAPES
Molecules, or ions, possessing ONLY BOND PAIRS of
electrons fit into a set of standard shapes. All the bond
pair-bond pair repulsions are equal.
All you need to do is to count up the number of bond
pairs and chose one of the following examples...
BOND
PAIRS
SHAPE
C
A covalent bond will repel
another covalent bond
BOND
ANGLE(S)
EXAMPLE
2
LINEAR
180º
BeCl2
3
TRIGONAL PLANAR
120º
AlCl3
4
TETRAHEDRAL
109.5º
CH4
5
TRIGONAL BIPYRAMIDAL
90º & 120º
PCl5
6
OCTAHEDRAL
90º
SF6
What happens if LP are present?
LP are NEARER to the nucleus than BP
LP are closer to BP than BP’s are to eachother
and LP are even closer to each other
LP by BP repulsions
BP by BP repulsions
LP by LP repulsions
INCREASING strength
of repulsions
BP pushed closer to each other by stronger
LP repulsions
“normal” (symmetrical) angles reduced
by about 2 per LP.
modifications of trigonal, tetrahedral, trigonal
bipyramid and octahedral shapes
Effect of lone pairs on shape
The number of lone pairs in a molecule is calculated by
subtracting the number of bonding pairs from the total
number of electron pairs in the outer principal energy level.
The shape of a molecule with lone pairs is based on the
basic shape for the total number of outer electron pairs,
but with a lone pair replacing one of the bonds.
tetrahedral
pyramidal
replacing one
bonding pair
with a lone pair
20 of 33
V-shaped
replacing another
bonding pair with
a lone pair
© Boardworks Ltd 2009
Effect of lone pairs on bond angles
21 of 33
© Boardworks Ltd 2009
MOLECULE AND
FORMULA
DOT-AND-CROSS
DIAGRAM
Ammonia
NH3
SHAPE and BOND
ANGLE(S)
modified tetrahedron
BP
H
..
.
.
H . N .
..
H
LP
CN
3
1
3
..
N
H
H
H
107º
TRIGONAL
PYRAMID
MOLECULE AND
FORMULA
DOT-AND-CROSS
DIAGRAM
Water
H 2O
SHAPE and BOND
ANGLE(S)
modified tetrahedron
BP
..
.
.
H . O. H
..
2
.. ..
O
LP
CN
2
2
H
H
105º
V-SHAPE
(BENT)
Modifications of TRIGONAL PLANAR shape
3 BP ; 0 LP
2 BP ; 1 LP
V-shape
120º
LP pushes
BP closer
118º
Modifications of TETRAHEDRAL shape
4 BP ; 0 LP
109.5º
3 BP ; 1 LP
2 BP ; 2 LP
Trigonal pyramid
like AMMONIA
V - shape
like WATER
107º
105º
LP pushes
BP closer
2LP push BP
even closer
Modifications of TRIGONAL BIPYRAMID shape
5 BP ; 0 LP
4 BP ; 1 LP
3 BP ; 2 LP
2 BP ; 3 LP
90º and 120º
88º and 118º
120º
180º
See-saw
Trigonal
Linear
Modifications of OCTAHEDRAL shape
6 BP ; 0 LP
90º
5 BP ; 1 LP
4 BP ; 2 LP
88º
Umbrella
90º
Square Planar
3 BP ; 3 LP
2 BP ; 4 LP
88º
T-shape
180º
Linear
Shape of CF3+ ?
C
Gp 4
4 e-
3F
3 bonds
3 e-
Charge
1+
- 1 e-
Total number e- around C =
6
+
3 e- pairs 3 BP + 0 LP
symmetrical
trigonal planar
F
120º
C
F
F
Shape of SCl4 ?
S
Gp 6
6 e-
4Cl
4 bonds
4 e-
Charge
0
0 e-
Total number e- around S =
5 e- pairs
4 BP + 1 LP
modified trigonal bipyramid
see-saw shape
10
Cl88º
Cl
:S
118º
Cl Cl
Shape of SeF42- ?
Se
Gp 6
6 e-
4F
4 bonds
4 e-
Charge
2-
2 e-
Total number e- around Se = 12
6 e- pairs 4 BP + 2 LP
modified octahedron
square shape
2-
F
F
..
S
..
F
F
90º
Draw the dot-and-cross diagram for each of the following
molecules. Then predict and DRAW the 3D shapes. State
the approximate bond angles in each case.
(a)
(c)
(e)
(g)
(i)
SiCl4
XeF4
CH3+
PCl6PCl4+
(b)
(d)
(f)
(h)
(j)
H2S
PCl3
XeF2
ICl5
AlCl3
(a) SiCl4
Si has 4BP and 0 LP TETRAHEDRAL 109.5°
(b) H2S
S has 2BP and 2 LP V-SHAPED 105°
(c) XeF4
Xe has 4BP and 2 LP SQUARE-PLANAR 90°
(d) PCl3
P has 3BP and 1 LP TRIGONAL PYRAMID 107°
(e) CH3+
C has 3BP and 0 LP TRIGONAL PLANAR 120°
(f) XeF2
Xe has 2BP and 3 LP LINEAR 180°
(g) PCl6-
P has 6BP and 0 LP OCTAHEDRAL 90°
(h) ICl5
I has 5BP and 1 LP UMBRELLA 88°
(i) PCl4+
P has 4BP and 0 LP TETRAHEDRAL 109.5°
(j) AlCl3
Al has 3BP and 0 LP TRIGONAL PLANAR 120°
SHAPES OF ALKENE MOLECULES eg Ethene
121º
H
H
C
H
C
118º
H
Because of the structure
of the C=C bond, the six
atoms involved with, or
bonded to, a C=C bond
ALL LIE IN THE SAME
PLANE
Because of mutual repulsions, the three bonds are arranged as
far apart as possible.
This creates a TRIGONAL shape with the bonds about 120º apart.
However, the 4 electrons of the C=C bond (compared to the 2
electrons of the C-H bonds) cause stronger repulsions.
C-H bonds “pushed” slightly closer 118º
Similarly, for :
121º
H
CH3
C
Propene
C
H
118º
H
121º
H
CH3
C
But-2-ene
H3C
C
118º
H
NB Other atoms (i.e. H3) are NOT in same plane
THE SHAPES OF CARBON DIOXIDE and
SULPHUR DIOXIDE
O
C
O
O=C=O
180º
O
S
O
S
S
120º
S
Two equivalent double bonds
(2 e- bond pairs) to C and no
lone pairs
2 double bonds get as far
apart as possible
linear molecule
Two equivalent double bonds
(2 e- bond pairs) to S and one
lone pair
2 double bonds and lone pair
get as far apart as possible
V-shaped molecule
Shapes of molecules activity
36 of 33
© Boardworks Ltd 2009