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The ideal electronic symmetry of a molecule consisting of a centralatom surrounded by a number of substituents (bonded atoms and
non-bonding electrons) is characteristic of the total number ofsubstituents, and is determined solely by geometric considerations -
- the substituents are arranged so as to maximize the distancesamongst them.
VSEPR
http://en.wikipedia.org/wiki/Sir_Ronald_Sydney_Nyholmhttp://en.wikipedia.org/wiki/Ronald_Gillespiehttp://en.wikipedia.org/wiki/VSEPR_theoryhttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Chemistryhttp://en.wikipedia.org/wiki/Model_(abstract)http://en.wikipedia.org/wiki/Sir_Ronald_Sydney_Nyholmhttp://en.wikipedia.org/wiki/Ronald_Gillespiehttp://en.wikipedia.org/wiki/VSEPR_theoryhttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Chemistryhttp://en.wikipedia.org/wiki/Model_(abstract)8/12/2019 VSEPR chemistry
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The electron pairs in the valence shel l around the central atom ofa molecule repel each other and tend to orient in space so as tominimize the repulsions and maximize the distance between them .
There are two types of valence shell electron pa irs viz. ,
i ) Bond pairs and
ii ) Lone pairs ond pairsare shared by two atoms and are attra cted by two
nuclei . Hence they occupy less space and cause less repulsion.
Lone pairs are not involved in bond formation and are in attrac tionwith only one nucleus. Hence they occupy more space. As a result,the lone pairs cause more repulsion.
The order of repulsion between different types of electron pairs isas follows:
Lone pair - Lone pair > Lone Pair - Bond pair > Bond pair -Bond pair
In VSEPR theory, the multiple bonds are treated as if they weresingle bonds. The elect ron pairs in multiple bonds are treatedcollectively as a single super pair.
THE BASIC ASSUMPTIONS OF THIS
THEORY ARE SUMMARIZED BELOW
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The repulsion caused by bonds increases with increase in
the number of bonded pairs between two atoms i.e., atriple bond causes more repulsion than a double bond
which in turn causes more repulsion than a single bond.
The shape of a molecule can be predicted from
the number and type of valence shell electron pairs
around the central atom.
When the valence shell of central atom contains only
bond pairs, the molecule assumes symmetrical geometry
due to even repulsions between them.
However the symmetry is distorted when there are alsolone pairs along with bond pairs due to uneven repulsion
forces.
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Primary & Secondary effects on bond
angle and shape:
The bond angle decreases due to the presence of lone pairs, which cause morerepulsion on the bond pairs and as a result the bond pairs tend to come closer.
The repulsion between electron pairs increases with increase inelectronegativity of central atom and hence the bond angle increases. The bond
pairs are closer and thus by shortening the distance between them, which inturn increases the repulsion. Hence the bonds tend to move away from eachother.
However the bond angle decreases when the electronegativities of ligand atomsare more than that of central atom. There is increase in the distance betweenbond pairs since they are now closer to ligand atoms. Due to this, they tend to
move closer resulting in the decrease in bond angle.
The bond angle decreases with increase in the size of central atom. Howeverthe bond angle increases with increase in the size of ligand atoms, whichsurround the central atom.
The bond angles are also changed when multiple bonds are present. It is due to
uneven repulsions.
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The shape of molecule and also the approximate bondangles can be predicted from the number and type ofelectron pairs in the valence shell of central atom as
tabulated below.
Steric Number
The steric numberof a molecule is the number of
atoms bonded to the central atom of a molecule plusthe number of lone pairs on the central atom. It is oftenused in VSEPR theory(valence shell electron-pairrepulsion theory) in order to determine the particularshape, ormolecular geometry, that will be formed.
RELATION BETWEEN NUMBER & TYPE OF
VALENCE ELECTRON PAIRS WITH THE SHAPE
OF MOLECULE
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STEPS INVOLVED IN PREDICTING THE SHAPES OF
MOLECULES USING VSEPR THEORYThe first step in determination of shape of a
molecule is to write the Lewis dot structureofthe molecule.
Then find out the number of bond pairs andlone pairs in the valence shell of centralatom.
While counting the number of bond pairs,
treat multiple bonds as if they were singlebonds. Thus electron pairs in multiple bondsare to be treated collectively as a singlesuper pair.
http://www.adichemistry.com/general/chemicalbond/covalentbond/covalent-bond.htmlhttp://www.adichemistry.com/general/chemicalbond/covalentbond/covalent-bond.html8/12/2019 VSEPR chemistry
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StericNo.
No. of lonepair electrons
on 'central'atom
No. of
bondinggroups (pair
electrons) on'central'
atom
Electron-pairGeometry(Structure)
MolecularGeometry
(Shape)
BondAngle
2 0 2 linear linear 180
3 0 3trigonalplanar
trigonalplanar
120
3 1 2trigonalplanar
bent less than 120
4 0 4 tetrahedral tetrahedral 109.5
4 1 3 tetrahedraltrigonal
pyramidalless than
109.5
4 2 2 tetrahedral bent less than109.5
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StericNo.
No. of lonepair
electronson 'central'
atom
No. ofbonding
groups (pairelectrons) on'central' atom
Electron-pair
Geometry
MolecularGeometry
BondAngle
5 0 5trigonal
bipyramidaltrigonal
bipyramidal90, 120 and 180
5 1 4StericNo.
No. of lonepair electrons
on 'central'
atom
No. of bondinggroups (pairelectrons) on
'central' atom
5 2 3trigonal
bipyramidalT-shaped 90 and 180
5 3 2
trigonal
bipyramidal linear 180
6 0 6 octahedral octrahedral 90 and 180
6 1 5 octahedralsquare
pyramidal90 and 180
6 2 4 octahedral squareplanar
90 and 180
http://intro.chem.okstate.edu/1314F97/Chapter9/5BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/5BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP2LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/2BP3LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/6BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/5BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/5BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/4BP2LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/4BP2LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/4BP2LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/4BP2LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/5BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/5BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/6BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/2BP3LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP2LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP2LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP2LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/5BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/5BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/5BP.html8/12/2019 VSEPR chemistry
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Even though the VSEPRmodel is useful to predict theshapes of molecules, it fails to predict the shapes ofisoelectronic species and transition metal
compounds. This model does not take relative sizesof substituents and stereochemically inactive lonepairs into account. As a result, VSEPR is notappropriate to apply to heavy d-block species that
experience the stereochemical inert pair effect.
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Shapes
Linear
Trigonal planar
Angular
Tetrahedral
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Trigonal pyramidal
Angular
Trigonal bipyramidal
See saw or
distorted tetrahedral
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T-Shape
Linear
Octahedral
Square pyramidal
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Square planar
Pentagonal bipyramidal
Pentagonal pyramidal
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