Molecular Geometry and Hybridization: A Comprehensive Guide with Examples, Exams of Geometry

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Molecular Geometry

Valence Shell Electron Pair repulsion

The valence shell electron pair repulsion model (VSEPR model)assumes that electron pairs repel one another.

(VSEPR) model gives helps determine the correct placement ofatoms and nonbonding electrons in the molecule based on therepulsions of electrons in the molecule.

The most stable conformation for a molecule is the one which hasthe electron pairs as far away from each other as possible.

This produces a set of geometries which depend only on thenumber of valence shell electron pairs and not on the atomspresent.

Why study molecular geometry?

molecular geometry is vital in order to understand thepolarity of molecules

crucial to understanding reactions in organic, inorganic andbiochemistry.

Some sites on the molecule are more open to reaction thanother sites – helps in deducing reaction mechanism, withinformation about the correct orientation of the atoms in themolecule.

Example

CH

4

C: 1s

2

2s

2

2p

2

: 4 valence electrons

H: 1s

1

: 1 valence electron

(and there are 4 H’s)

Total of 8 valence e’sHence 4 pairs, tetrahedral pairsThere are 4 bonds (no lone pairs)Hence bonds are also tetrahedral:

molecular geometry is tetrahedral

C

H

H

H

H

or

C

H

H

H

H

No# e

Pairs

Arrangement of

Electron Pairs

No# of

σ

Bond

Pairs

No# of

LonePairs

Molecular geometry

Examples

2

X

2

0

Linear

CO

2

, N

3

3

X

3 2

0 1

Trigonal planar

Bent

SO

3

, CO

3

2-

SO

2

, O

3

, NO

2

4

X

4 3 2

0 1 2

Tetrahedral

Trigonal pyramid

Bent

CH

4

, NH

4

, PO

4

3-

H

3

O

, NH

3

H

2

O, NH

2

, ClO

2

5

X

5 4 3 2

0 1 2 3

Trigonal bipyramid

"See-saw"

T-shaped

Linear

PCl

5

, SF

5

SF

4

, PBr

4

ClF

3

, XeF

3

ICl

2

, XeF

2

6

X

6 5 4

0 1 2

Octahedral

Square pyramidal

Square planar

SF

6

, SiF

6

2-

, AsF

6

IF

5

, SF

5

, SbF

5

2-

ICl

4

, XeF

4

Molecular Geometry

Linear

Trigonal planar

Bent(1 l.p., 2b.p.)

Tetrahedral

Trigonal pyramidal

Bent (2 l.p., 2b.p.)

Molecular Geometry

Square pyramidal

Square planar

Octahedral

Examples

P O

Cl

Cl

Cl

F S F

F

F

Cl

F F

F

I F

F F

F F

Linear

Tetrahedral

'See-saw'

'T'-shaped

Square pyramid

C

O

O

Hybridization

covalent bonding requires a consideration of valence shell atomicorbitals. For second period elements eg. carbon, nitrogen andoxygen, these orbitals have been designated 2s, 2px, 2py & 2pz.

Ground state electronic configuration of methane Ground state electronic configuration of methane

The valence shell electron configuration of carbon is……..

If this was the configuration used in covalent bonding, carbon

would only be able to form two bonds.

sp

hybridization in methane

Carbon attempting to bond with four hydrogens, needs four orbitals.Therefore, the 2

s

orbital (core orbitals are almost never involved in

bonding) mixes with the three 2

p

orbitals to form four

sp

3 hybrids

(read as

s-p-three

The four equivalent hybrid atomic orbitals, each having 25% s and75% p character, have a specific orientation, and the four arenaturally oriented in a tetrahedral fashion. four

sp

3 hybridised orbitals are overlapped by hydrogen's 1

s

orbital,

yielding four

σ

(sigma) bonds. The four bonds are of the same length

and strength.

sp

hybridization in ethylene

Lewis structure of ethene

Shape – trigonal planar

2s-orbital combines with two of theavailable p-orbitals (sp

2

) – they point

towards the vertices of an equilateraltriangle, with axes 120

apart.

™

the single bond, formed from the sp

2

orbitals, and

π

bond

by

parallel

overlap of p-orbitals with bean-shaped probability areas above and below theplane of the six atoms.

sp hybridization in acetylene

C

2

H

2

(acetylene). Both carbon

atoms will be sp hybridized (ones orbital mixing with one porbital)and have one electron ineach of two unhybridized porbitals.

These p orbitals will undergoparallel overlap to form two pi-bonds at right angles to eachother.

Dipole moment

E.N.is huge, so that the majority

of electron density in thehydrogen-fluorine bond ends upon the much more E.N.fluorine.Resulting bond- Polar covalentwith a dipole moment.

Blue - partial positive charge onhydrogen atom; red- partialnegative charge on fluorine atom.

Carbon dioxide has two polarcovalent C-O bonds. However, thebond dipole moments exactlycancel each other since they arepointing in exactly oppositedirections.

What is the hybridisation of carbonatom in CO

2

Hint: consider shape Hint: consider shape

Dipole moments

Shape – bent

Hybridisation – sp

3

two very polar covalent bondsbetween oxygen and hydrogen.

the two bond dipole momentsadd up to give water a relativelylarge molecular dipole moment.

CF

4

has four polar covalent

bonds.

they are arranged in asymmetrical tetrahedralarray

all of the bond dipole momentvectors exactly cancel

Net dipole moment/ moleculardipole moment = 0