Shapes of molecules & ions
VSEPR theory
VSEPR - the Valence Shell Electron Pair
Repulsion theory is used to obtain the
shape of simple molecules and ions
VSEPR theory
 VSEPR - the Valence Shell Electron Pair
Repulsion theory is used to obtain the shape
of simple molecules and ions
 It states that because of the repulsion
between pairs of electrons around the central
atom in the molecule or ion, the pairs of
electrons are arranged to be as far away from
each other as possible
Negative Charge Centres
For VSEPR, multiple bonds (double,
triple) count as if they are just one pair of
electrons
Negative Charge Centres
For VSEPR, multiple bonds (double,
triple) count as if they are just one pair of
electrons
So VSEPR uses repulsion between
negative charge centres rather than
between pairs of electrons
4 negative charge centres
4 bonding pairs - tetrahedral
Bond Angle =
109.5º
 E.g methane - CH4, CCl4, SiCl4
From: http://library.thinkquest.org/3659/structures/commonstructures.html
www.chemvc.com/~tim/VSEPR.html , http://www.sparknotes.com/testprep/books/sat2/chemistry/chapter4section8.rhtml
4 negative charge centres
3 bonding pairs (& 1 non-bonding pair)
trigonal pyramid
Bond Angle = 107º
E.g. ammonia - NH3, PCl3
From: http://library.thinkquest.org/3659/structures/commonstructures.html
www.chemvc.com/~tim/VSEPR.html , http://www.sparknotes.com/testprep/books/sat2/chemistry/chapter4section8.rhtml
The bond angle decreases because the
non-bonding pair of electrons exerts a
greater repulsion than the bonding
pair(s)
From: http://www.examstutor.com/chemistry/resources/studyroom/bonding/shapes_of_molecules/
4 negative charge centres
2 bonding pairs ( 2 non-bonding pairs)
 bent or v-shaped
Bond angle = 105º
E.g. H2O, H2S
From: http://library.thinkquest.org/3659/structures/commonstructures.html
www.chemvc.com/~tim/VSEPR.html , http://www.sparknotes.com/testprep/books/sat2/chemistry/chapter4section8.rhtml
2 negative charge centres
Shape is always linear
Bond angle is 180º
E.g.
From: www.physchem.co.za/Bonding/Shape.htm
www.tulane.edu/~bmitche/book/structrj.html
3 negative charge centres
 3 bonding electron
pairs, no non-bonding
pairs
 Trigonal planar
 Bond angle is 120°
 e.g. BF3, ethene, CO32-
 2 bonding pairs, 1
non-bonding pair
 Bent or v-shaped
 Bond angle is 120°
 E.g. SO2, ozone - O3
From: http://library.thinkquest.org/3659/structures/commonstructures.html ,
www.chm.bris.ac.uk/motm/so2/so2h.htm
5 and 6 charge centres
Elements in the third period can have
more than 8 electrons in their outer shell
because the 3 d orbitals are close in
energy to the 3p orbitals.
The VSEPR theory also applies
You do not have to know specific bond
angles
5 negative charge centres (HL)
Trigonal bipyramidal e.g. PCl5
From: http://library.thinkquest.org/3659/structures/commonstructures.html
5 negative charge centres (HL)
4 bonding pairs and 1 non-bonding pair
Distorted tetrahedral
From: www.chemvc.com/~tim/Predictions.html
6 negative charge centres (HL)
Octahedral
e.g. SF6
From: http://library.thinkquest.org/3659/structures/commonstructures.html
6 negative charge centres (HL)
 4 bonding pairs and 2 non-bonding pairs as
far apart as possible (above & below plane)
Square planar
e.g. XeF4
From: www.chemvc.com/~tim/Predictions.html
VESPR theory
All the previous molecular shapes are
based on VSEPR theory
VSEPR=Valence Shell Electron Pair
Repulsion theory
Pairs of electrons arrange themselves
around the central atom so that they are
as far apart from each other as possible