VSEPR theory posits that the geometry of a molecule is determined by the arrangement of electron pairs around a central atom, which minimizes repulsion between them. The theory categorizes molecular shapes based on the number of bond and lone pairs, leading to distinct geometries such as linear, trigonal planar, tetrahedral, and others. However, the model has limitations, including its inability to explain the shapes of certain polar molecules and transition metal complexes.

1. VSEPR TheoryVSEPR Theory
B.KRITHIKHAB.KRITHIKHA
XI-AXI-A

2.  Vsepr theory says that bonded atoms in aVsepr theory says that bonded atoms in a
molecule adopt that particular arrangement inmolecule adopt that particular arrangement in
space around the central atom which keepsspace around the central atom which keeps
them as far apart as possible.them as far apart as possible.
 The premise of VSEPR is that the valenceThe premise of VSEPR is that the valence
electron pairs surrounding an atom tend toelectron pairs surrounding an atom tend to
repel each other and will, therefore, adopt anrepel each other and will, therefore, adopt an
arrangement that minimizes this repulsion, thusarrangement that minimizes this repulsion, thus
determining the molecule's geometry.determining the molecule's geometry.
 The state will have minimum repulsions andThe state will have minimum repulsions and
thus corresponds to a state of minimumthus corresponds to a state of minimum
repulsion, energy and maximum stability.repulsion, energy and maximum stability.

3. PostulatesPostulates
 The shape of a molecule depends upon the number of valence shellThe shape of a molecule depends upon the number of valence shell
electron pairs (bonded or non-bonded) around the central atom.electron pairs (bonded or non-bonded) around the central atom.
 Pairs of electrons in the valence shell tend to repel each other as theirPairs of electrons in the valence shell tend to repel each other as their
electron clouds are negatively charged.electron clouds are negatively charged.
 The electron pairs tend to occupy those positions in space, whichThe electron pairs tend to occupy those positions in space, which
minimize repulsions and thus maximize distance from one another.minimize repulsions and thus maximize distance from one another.
 The valence shell is taken as a sphere with the electron pairsThe valence shell is taken as a sphere with the electron pairs
localizing on the spherical surface at maximum distance from onelocalizing on the spherical surface at maximum distance from one
another.another.
 A multiple bond is treated as a single electron pair and the two orA multiple bond is treated as a single electron pair and the two or
three electron pairs of a multiple bond are treated as a single superthree electron pairs of a multiple bond are treated as a single super
pair.pair.
 The VSEPR model is applicable to any such structure where two orThe VSEPR model is applicable to any such structure where two or
more resonance structures can represent a molecule.more resonance structures can represent a molecule.
 Decreasing order of repulsive interaction of electron pairs:Decreasing order of repulsive interaction of electron pairs:
Lone pair (lp) − Lone pair (lp) > Lone pair (lp) − Bond pair (bp) > BondLone pair (lp) − Lone pair (lp) > Lone pair (lp) − Bond pair (bp) > Bond
pair (bp) − Bond pair (bp).pair (bp) − Bond pair (bp).
 Repulsive forcesRepulsive forces decrease sharply with increasing angle betweendecrease sharply with increasing angle between
electron pairs. They areelectron pairs. They are strong at 90˚, weaker at 120˚ and weakest atstrong at 90˚, weaker at 120˚ and weakest at
180180˚.˚.

4. Reason for lone pair-loneReason for lone pair-lone
pair having the mostpair having the most
repulsionrepulsion
 The orbital of bonded electron pair is under theThe orbital of bonded electron pair is under the
influence of two nuclei and most of the electron cloud isinfluence of two nuclei and most of the electron cloud is
oriented between the nuclei of the two atoms. On theoriented between the nuclei of the two atoms. On the
other hand, the lone pair of electrons is under theother hand, the lone pair of electrons is under the
influence of only one nucleus and therefore its electroninfluence of only one nucleus and therefore its electron
cloud is spread out and tends to occupy more space.cloud is spread out and tends to occupy more space.
 Since the lone pair is flatter and occupies more spaceSince the lone pair is flatter and occupies more space
around the central atom, it can interact more and willaround the central atom, it can interact more and will
repel the electron pairs in the neighbouring orbitalsrepel the electron pairs in the neighbouring orbitals
strongly than do the electrons in the boned orbital.strongly than do the electrons in the boned orbital.
 Obviously, the repulsion between two lone pairs wouldObviously, the repulsion between two lone pairs would
be the largest. It will be lesser if one of them is bondbe the largest. It will be lesser if one of them is bond
pair and least if both are bond pairspair and least if both are bond pairs..

5. Geometries ofGeometries of
molecules based onmolecules based on
VSEPR theoryVSEPR theory
 Molecules are classified into theMolecules are classified into the
following two categories to predict thefollowing two categories to predict the
geometrical shapes of molecules usinggeometrical shapes of molecules using
VSEPR theory.VSEPR theory.
1.1. Molecules in which the central atom has noMolecules in which the central atom has no
lone pair (bond pairs only)lone pair (bond pairs only)
2.2. Molecules in which the central atom hasMolecules in which the central atom has
one or more lone pairs (bond pair and loneone or more lone pairs (bond pair and lone
pair)pair)

6. 1.1.Shape of moleculesShape of molecules
containing bond pairscontaining bond pairs
onlyonly
 In case of two electron pairsIn case of two electron pairs
around the central atom, thearound the central atom, the
only way to keep them as faronly way to keep them as far
apart as possible is to arrangeapart as possible is to arrange
them at an angle of 180˚ tothem at an angle of 180˚ to
each othereach other . This is. This is linear geometry.linear geometry.
 Eg:BeFEg:BeF22

7.  Similarly for three electron pairs aroundSimilarly for three electron pairs around
the central atom, the molecule adoptsthe central atom, the molecule adopts
trigonal planartrigonal planar geometry with 120˚geometry with 120˚
between each bond and four electronbetween each bond and four electron
pairs adoptpairs adopt tetrahedraltetrahedral geometry withgeometry with
109.5 ˚.109.5 ˚.
Trigonal planar TetrahedralTrigonal planar Tetrahedral

8.  When five electron are present,When five electron are present,
all bond angles and bond lengthsall bond angles and bond lengths
are not equal. Three electron pairsare not equal. Three electron pairs
are in the same plane at an angleare in the same plane at an angle
of 120˚ while other two areof 120˚ while other two are
perpendicular to the plane, bothperpendicular to the plane, both
making an angle of 90˚ with themaking an angle of 90˚ with the
plane.plane.
 This type of geometry isThis type of geometry is
trigonal bipyramidaltrigonal bipyramidal..
 Even the bond lengths are not equal. The three bondsEven the bond lengths are not equal. The three bonds
lying in the trigonal plane are called equatorial bonds.lying in the trigonal plane are called equatorial bonds.
The remaining two bonds are called axial bonds. It hasThe remaining two bonds are called axial bonds. It has
been observed that axial bonds are slightly longer thanbeen observed that axial bonds are slightly longer than
equatorial bonds.equatorial bonds.
 As the molecule is unsymmentrical, it is less stable andAs the molecule is unsymmentrical, it is less stable and
highly reactive.highly reactive.

9.  When six electron pairs are present, allWhen six electron pairs are present, all
bond angles are of 90˚.bond angles are of 90˚.
 When seven electron pairs are present, fiveWhen seven electron pairs are present, five
electron pairs are equatorial bonds in theelectron pairs are equatorial bonds in the
same plane each having 72˚ bond anglesame plane each having 72˚ bond angle
and two electro pairs are axial bondsand two electro pairs are axial bonds
making 90˚ with the bond.making 90˚ with the bond.

11. 2.Shape of molecules2.Shape of molecules
containing lone pairs andcontaining lone pairs and
bond pairsbond pairs
 If the valence shell has 1 lone pair,2If the valence shell has 1 lone pair,2
bond pairs(3 electron pairs), then thebond pairs(3 electron pairs), then the
geometry is slightly distorted fromgeometry is slightly distorted from
trigonal planar geometry and has V-trigonal planar geometry and has V-
shapes or angular shape.shapes or angular shape.
 Since the lone pair-lone pair repulsion isSince the lone pair-lone pair repulsion is
more than bond pair-bond pair repulsion,more than bond pair-bond pair repulsion,
the bonded pairs are pushed more closerthe bonded pairs are pushed more closer
and angle decreases from 120˚ to 119.5˚.and angle decreases from 120˚ to 119.5˚.

13.  Due to the presence of lone pairs, the geometry becomesDue to the presence of lone pairs, the geometry becomes
distorted and the bond angle reduces as the lone pairdistorted and the bond angle reduces as the lone pair
occupies more space and has more repulsion.occupies more space and has more repulsion.
 Sometimes, certain geometries can have two or moreSometimes, certain geometries can have two or more
ways of arrangements here, the most stable one will beways of arrangements here, the most stable one will be
taken. The most stable ones are usually when the lonetaken. The most stable ones are usually when the lone
pairs are at the equatorial positionspairs are at the equatorial positions..
 Example: when there are 3 bond pairs and 2 lone pairs,Example: when there are 3 bond pairs and 2 lone pairs,
there are 3 ways to positionthere are 3 ways to position
1.Two lone pairs ar equatorial positions1.Two lone pairs ar equatorial positions
2.One lp on axial one lp on equatorial2.One lp on axial one lp on equatorial
3.Both lp on axial position.3.Both lp on axial position.
 Here, the adopted geometry is (1) since it is the mostHere, the adopted geometry is (1) since it is the most
stable and has minimum repulsion.stable and has minimum repulsion.
 When there are 2 bp and 3 lp, the three lone pairs areWhen there are 2 bp and 3 lp, the three lone pairs are
present at the corners of an equilateral plane, the netpresent at the corners of an equilateral plane, the net
repulsion on the bonds due to lone pairs cance out and isrepulsion on the bonds due to lone pairs cance out and is
zero. So the molecule adopts linear geometryzero. So the molecule adopts linear geometry

14. Few examplesFew examples
 Water (HWater (H22O):O):
It is evident from theIt is evident from the
Lewis structure of waterLewis structure of water
molecule, there are twomolecule, there are two
bond pairs and two lonebond pairs and two lone
pairs in the valence shellpairs in the valence shell
of oxygen. Hence itsof oxygen. Hence its
structure is based onstructure is based on
tetrahedral geometry.tetrahedral geometry.
However its shape isHowever its shape is
angular with two loneangular with two lone
pairs on oxygen.pairs on oxygen.

15.  SulfurSulfur
tetrachloridetetrachloride
(SCl(SCl44):):
S(Z=16), Cl(Z=17)S(Z=16), Cl(Z=17)
Since there are fourSince there are four
bond pairs and one lonebond pairs and one lone
pair around sulfur in itspair around sulfur in its
valence shell, thevalence shell, the
structure of SCl4 isstructure of SCl4 is
based on trigonalbased on trigonal
bipyramidal geometry. Itbipyramidal geometry. It
has seesaw shape withhas seesaw shape with
a lone pair occupying thea lone pair occupying the
equatorial position.equatorial position.

16. LimitationLimitation
 It can’t explain the shape of molecules havingIt can’t explain the shape of molecules having
very polar bonds, eg. Li2O should have thevery polar bonds, eg. Li2O should have the
same structure as H2O but actually it is linear.same structure as H2O but actually it is linear.
 This fails to explain the shape of moleculesThis fails to explain the shape of molecules
having extensive delocalized pi electronhaving extensive delocalized pi electron
system.system.
 This theory is not able to predict the shape ofThis theory is not able to predict the shape of
certain transition metal complexes.certain transition metal complexes.
 It does not help in determining the exact bondIt does not help in determining the exact bond
angle.angle.