VSEPR THEORY:

VSEPR THEORY:VSEPR THEORY:

How do we determine How do we determine the shapes of the shapes of

molecules and ions?molecules and ions?

VSEPR THEORYVSEPR THEORY

What does VSEPR stand What does VSEPR stand for?for?

VValence alence SShell hell EElectron lectron PPair air RRepulsionepulsion


Why is this important to Why is this important to know?know?

It explains how molecules It explains how molecules and ions behave.and ions behave.

VSEPR THEORY: VSEPR THEORY: Basic procedureBasic procedure

1) 1) Determine the central Determine the central atom atom (usually the atom (usually the atom with the lowest subscript with the lowest subscript and/or the atom capable of and/or the atom capable of forming the most bonds).forming the most bonds).


2) 2) Draw the electron dot Draw the electron dot structure and bar diagramstructure and bar diagram


3) 3) Determine the molecular Determine the molecular geometry using ALL geometry using ALL electron pairs AND atoms electron pairs AND atoms around the central atom.around the central atom.


4) 4) Modify the geometry to Modify the geometry to determine the molecular shape (if determine the molecular shape (if non-bonding electron pairs exist non-bonding electron pairs exist by ignoring them)by ignoring them)

VSEPR THEORY: VSEPR THEORY: Example: BeHExample: BeH22

1) Central Atom?1) Central Atom?

Be Be (only 1 atom)(only 1 atom)


2) Electron Dot?2) Electron Dot?

2) Bar Diagram?2) Bar Diagram?

H Be H

H—Be—H

Note that Be violates the octet rule—this is an exception!


3) Geometry? Hint: What is 3) Geometry? Hint: What is the furthest apart you can the furthest apart you can spread two atoms attached spread two atoms attached to a central atom?to a central atom?

H HBe


4) Shape? Ignore any 4) Shape? Ignore any unbonded pairs of electrons unbonded pairs of electrons —not necessary in this case.—not necessary in this case.

LINEARLINEAR

H HBe

VSEPR THEORY: VSEPR THEORY: Example: BFExample: BF33


B B (only 1 atom)(only 1 atom)


2) Electron Dot?2) Electron Dot?

2) Bar Diagram?2) Bar Diagram?F—B—F

F

Note that B violates the octet rule—this is an exception!

F B F F


3) Geometry? Hint: What is 3) Geometry? Hint: What is the furthest apart you can the furthest apart you can spread three atoms attached spread three atoms attached to a central atom?to a central atom?

BF

F

F

VSEPR THEORY: VSEPR THEORY: Example: BFExample: BF334) Shape? Ignore any 4) Shape? Ignore any

unbonded pairs of electrons unbonded pairs of electrons —not necessary in this case.—not necessary in this case.

trigonal planartrigonal planar

BF

F

F

VSEPR THEORY: VSEPR THEORY: Example: CHExample: CH44


C C (only 1 atom)(only 1 atom)

VSEPR THEORY: VSEPR THEORY: Example: CHExample: CH442) Electron Dot?2) Electron Dot?


H C H

H

H

H—C—H

H

H

VSEPR THEORY: VSEPR THEORY: Example: CHExample: CH44

3) Geometry? Hint: What is the 3) Geometry? Hint: What is the furthest apart you can spread four furthest apart you can spread four atoms attached to a central atom? atoms attached to a central atom? Think in 3D!Think in 3D!

CHH

H

H

VSEPR THEORY: VSEPR THEORY: Example: CHExample: CH44 4) Shape? Ignore any unbonded 4) Shape? Ignore any unbonded

pairs of electrons —not necessary pairs of electrons —not necessary in this case.in this case.

tetrahedraltetrahedral

CHH

H

H

VSEPR THEORY: VSEPR THEORY: Example: NHExample: NH33


N N (only 1 atom)(only 1 atom)

VSEPR THEORY: VSEPR THEORY: Example: NHExample: NH332) Electron Dot?2) Electron Dot?


H N H H

H—N—H H

VSEPR THEORY: VSEPR THEORY: Example: NHExample: NH33

3) Geometry? Hint: What is the 3) Geometry? Hint: What is the furthest apart you can spread three furthest apart you can spread three atoms plus one unbonded pair of atoms plus one unbonded pair of electrons attached to a central atom? electrons attached to a central atom? Think in 3D!Think in 3D!

HN

HH

~109.5o

VSEPR THEORY: VSEPR THEORY: Example: NHExample: NH33 4) Shape? Ignore any unbonded 4) Shape? Ignore any unbonded

pairs of electrons —it IS pairs of electrons —it IS necessary in this case.necessary in this case.

trigonaltrigonal

pyramidalpyramidalH

N

HH

~109.5o

VSEPR THEORY: VSEPR THEORY: Example: HExample: H22OO


O O (only 1 atom)(only 1 atom)

VSEPR THEORY: VSEPR THEORY: Example: HExample: H22OO2) Electron Dot?2) Electron Dot?


O H H

O—H H

VSEPR THEORY: VSEPR THEORY: Example: HExample: H22OO

3) Geometry? Hint: What is the 3) Geometry? Hint: What is the furthest apart you can spread two furthest apart you can spread two atoms plus two unbonded pairs of atoms plus two unbonded pairs of electrons attached to a central atom? electrons attached to a central atom? Think in 3D!Think in 3D!

HO

H

~109.5o

VSEPR THEORY: VSEPR THEORY: Example: HExample: H22OO 4) Shape? Ignore any unbonded 4) Shape? Ignore any unbonded

pairs of electrons —it IS pairs of electrons —it IS necessary in this case.necessary in this case.

bentbent

HO

H

~109.5o


In conclusion:In conclusion:

Since water (also called the Since water (also called the universal solvent) is bent it universal solvent) is bent it is able to dissolve ionic is able to dissolve ionic substances:substances:

O side tends to be – (the electron pairs hybridize into one group)

H sides tend to be +

HO

H

HO

H

This negative side tends to attract positive ions

These positive ends tend to attract negative ions

What is the VSEPR Theory?What is the VSEPR Theory?

The VSEPR Theory is used to predict the The VSEPR Theory is used to predict the shapes of molecules based on the repulsion of shapes of molecules based on the repulsion of the bonding and non-bonding electrons in the the bonding and non-bonding electrons in the molecule.molecule.

The shape is determined by the number of The shape is determined by the number of bonding and non-bonding electrons in the bonding and non-bonding electrons in the molecule.molecule.

In order to determine the shape, the Lewis In order to determine the shape, the Lewis diagram must be drawn first.diagram must be drawn first.

When determining the shape of a molecule When determining the shape of a molecule with multiple bonds, treat the multiple bonds as with multiple bonds, treat the multiple bonds as if they were single bonds (i.e. one bonding pair)if they were single bonds (i.e. one bonding pair)

Molecules with the central atom Molecules with the central atom surrounded by four bonding pairs (i.e. four surrounded by four bonding pairs (i.e. four

atoms)atoms) If the central atom is placed at the center of a If the central atom is placed at the center of a

sphere, than each of the four pairs of sphere, than each of the four pairs of electrons will occupy a position to be as far electrons will occupy a position to be as far apart as possible.apart as possible.

This will result in the electron pairs being at This will result in the electron pairs being at the corners of a regular tetrahedron, therefore the corners of a regular tetrahedron, therefore these molecules are said to have a these molecules are said to have a TETRAHEDRAL SHAPE.TETRAHEDRAL SHAPE.

The angle between each bond will be 109.5The angle between each bond will be 109.5°°

Example CClExample CCl44

109.5°

Cl

CCl Cl

Cl

Cl

C

Cl

Cl

Cl

Molecules with the central atom surrounded Molecules with the central atom surrounded by 3 bonding pairs and 1 non-bonding pairby 3 bonding pairs and 1 non-bonding pair

Four pairs of electrons will always arrange Four pairs of electrons will always arrange themselves tetrahedrally around the central atom.themselves tetrahedrally around the central atom.

The shape of the molecule is determined by the The shape of the molecule is determined by the arrangement of the atoms not the electrons.arrangement of the atoms not the electrons.

As a result such molecules will have a As a result such molecules will have a TRIANGULAR (TRIGONAL) PYRAMIDAL shape.TRIANGULAR (TRIGONAL) PYRAMIDAL shape.

Due to the repulsion, a non-bonding electron pair Due to the repulsion, a non-bonding electron pair requires more space than a bonding pair, the requires more space than a bonding pair, the angles in these molecules are angles in these molecules are 107107° not 109.5° as ° not 109.5° as in the tetrahedral molecules.in the tetrahedral molecules.

Example NHExample NH33

107°

NH H

H

Molecules with the central atom surrounded Molecules with the central atom surrounded by 2 bonding pairs and 2 non-bonding pairsby 2 bonding pairs and 2 non-bonding pairs

The four pairs of electrons will be arranged The four pairs of electrons will be arranged tetrahedrally but since only 2 pairs are tetrahedrally but since only 2 pairs are bonding electrons, the surrounding atoms are bonding electrons, the surrounding atoms are at 2 corners of the tetrahedron.at 2 corners of the tetrahedron.

As a result these molecules will have a BENT As a result these molecules will have a BENT OR V-SHAPE.OR V-SHAPE.

The repulsion between the non-bonding pairs The repulsion between the non-bonding pairs will result in a bond angle of 104.5will result in a bond angle of 104.5°.°.

For each pair of non-bonding electrons, the For each pair of non-bonding electrons, the bond angle decreases by 2.5°bond angle decreases by 2.5°

Example HExample H22OO

104.5°

HO

H

Molecules with the central atom Molecules with the central atom surrounded by 5 pairs of bonding pairssurrounded by 5 pairs of bonding pairs

As with the tetrahedral molecules, the electron As with the tetrahedral molecules, the electron pairs will arrange themselves as far apart as pairs will arrange themselves as far apart as possible.possible.

To achieve this, the atoms will arrange To achieve this, the atoms will arrange themselves in a TRIANGULAR (TRIGONAL) themselves in a TRIANGULAR (TRIGONAL) BIPYRAMIDAL SHAPE which consists of 2 BIPYRAMIDAL SHAPE which consists of 2 pyramids sharing the same base.pyramids sharing the same base.

In this type of molecule, the 3 atoms making the In this type of molecule, the 3 atoms making the base will lie in the same plane with the central base will lie in the same plane with the central atom in the middle of it. The other atoms will be atom in the middle of it. The other atoms will be positioned above and below this plane.positioned above and below this plane.

The bond angles within the base will 120The bond angles within the base will 120° and ° and the bond between the other atoms and the base the bond between the other atoms and the base will be 90°.will be 90°.

Example PFExample PF55

F

F

F

F

F

P

90

120

°

°

F PF

F

F

F

F PF

F

F

F

Molecules with the central atom surrounded Molecules with the central atom surrounded by 6 pairs of bonding pairsby 6 pairs of bonding pairs

In order for the 6 pairs of electrons to be as far In order for the 6 pairs of electrons to be as far apart as possible in this case, each bonding pair apart as possible in this case, each bonding pair will be at one corner of a REGULAR OCTAGONwill be at one corner of a REGULAR OCTAGON

The central atom is at the center of a square The central atom is at the center of a square plane made up of 4 atoms and the other 2 atoms plane made up of 4 atoms and the other 2 atoms will be placed above and below the plane.will be placed above and below the plane.

All bond angles will be 90All bond angles will be 90°°

Example SFExample SF66

S

90°

SF

F F

F

F

F

S

F

F F

F

F

F

Molecules with the central atom with an Molecules with the central atom with an incomplete octetincomplete octet

Molecules that only have 2 bonding pairs on the Molecules that only have 2 bonding pairs on the central atom will have a LINEAR SHAPE with a central atom will have a LINEAR SHAPE with a bond angle of 180bond angle of 180°°

Molecules that only have 3 bonding pairs on the Molecules that only have 3 bonding pairs on the central atom will have a TRIANGULAR central atom will have a TRIANGULAR (TRIGONAL) PLANAR SHAPE with bond (TRIGONAL) PLANAR SHAPE with bond angles of 120angles of 120°°

Example BeFExample BeF22 Example BFExample BF33

120°

180°

Be FF

BFF

F

Things to RememberThings to Remember

In order to predict the shape of a molecule you In order to predict the shape of a molecule you must draw the Lewis Dot Diagram for the must draw the Lewis Dot Diagram for the molecule, determine the number of bonding and molecule, determine the number of bonding and non-bonding electron pairs and compare this non-bonding electron pairs and compare this with the chart you have been given (the general with the chart you have been given (the general shapes must be memorized).shapes must be memorized).

Thank YouThank You

Arunava Das, PGT Chemistry, GDGPS Arunava Das, PGT Chemistry, GDGPS Siliguri, Darjeeling, West BengalSiliguri, Darjeeling, West Bengal

Ex – Research Scholar UGC CSIR DBT Ex – Research Scholar UGC CSIR DBT JRF II@ Molecular Biology Lab, I.I.Sc., JRF II@ Molecular Biology Lab, I.I.Sc., Bangalore and Soil Biology Lab, GKVK, Bangalore and Soil Biology Lab, GKVK,

BangaloreBangalore

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VSEPR THEORY: