VSEPR chemistry

8/12/2019 VSEPR chemistry

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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)


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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
http://en.wikipedia.org/wiki/VSEPR_theoryhttp://en.wikipedia.org/wiki/Molecular_geometryhttp://en.wikipedia.org/wiki/Molecular_geometryhttp://en.wikipedia.org/wiki/Molecular_geometryhttp://en.wikipedia.org/wiki/Molecular_geometryhttp://en.wikipedia.org/wiki/VSEPR_theory


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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.html


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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
http://intro.chem.okstate.edu/1314F97/Chapter9/2BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/2BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/4BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/2BP2LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/2BP2LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/4BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/2BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/2BP.html


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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.html


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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.
http://chemwiki.ucdavis.edu/Wikitexts/UCD_Chem_124A:_Kauzlarich/ChemWiki_Module_Topics/VSEPRhttp://chemwiki.ucdavis.edu/Wikitexts/UCD_Chem_124A:_Kauzlarich/ChemWiki_Module_Topics/VSEPR


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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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VSEPR chemistry