chapter 9 molecular geometry & bonding theories chapter 9 molecular geometry & bonding...

Chapter 9

Molecular Geometry & Molecular Geometry & Bonding TheoriesBonding Theories

Chapter 9

Molecular Geometry & Molecular Geometry & Bonding TheoriesBonding Theories

Overview

Molecular Shapes VSEPR Model

Predicting Shapes Effect of Nonbonding Electrons

Polarity of Molecules Covalent Bonding

Hybrid Orbitals sp, sp2, sp3 hybrids containing d orbitals

Multiple Bonds sigma () & pi () localized & delocalized

Molecular Orbitals electron configurations & bond order diamagnetism & paramagnetism

Molecular Shapes & VSEPR

Shapes defined by bond angles linear, 180° angles trigonal planar, 120° angles tetrahedral, 109.5° angles

VSEPR Valence Shell Electron Pair Repulsion theory electron pairs are arranged symmetrically with

maximum separation

Two electron pairs 180° apart linear geometry

••••

180°

Three electron pairs 120° apart trigonal planar geometry

••

•• ••

120°

Four electron pairs 109.5° apart tetrahedral geometry

••

•• ••••

109.5°

Five electron pairs angles of 90° and 120° trigonal bipyramidal (TBP) geometry

••

••••

••••

120°

90°

Six electron pairs angles of 90° octahedral geometry

••

••

••

••

••

••

90°

Geometries

Electron pair geometry arrangement of electron pairs around a central

atom Molecular Geometry

arrangement of atoms around a central atom When all electron pairs are bonding pairs

electron pair geometry = molecular geometry When there are unshared electron pairs

electron pair geometry molecular geometry

To determine electron pair geometry draw Lewis dot structure count shared & unshared electron pairs around central

atom• a multiple bond is counted as only one bonding pair when predicting geometry

determine electron pair geometry based on the number of electron pairs

• 2 pair = linear• 3 pair = trigonal planar• 4 pair = tetrahedral• 5 pair = trigonal bipyramidal• 6 pair = octahedral

Molecular Geometries with One or More Unshared Pairs

Two Pairs

electron pair geometry linear

bonding pairs 2

non-bonding pairs 0

molecular geometry linear

Two electron pairs

••••

180°

electron pair geometry

molecular geometry

Three Pairs

electron pair geometry trigonal planar

bonding pairs 3 2

non-bonding pairs 0 1

molecular geometry trig. pl. bent

Three electron pairs

••

•• ••

120°

••

trigonal planar

bent

electron pair geometry

molecular geometry

trigonal planar

Four Pairs

electron pair geometry tetrahedral

bonding pairs 4 3 2

non-bonding pairs 0 1 2

molecular geometry tet. trig. pyr. bent

Four electron pairs

••

•• ••••

109.5°

electron pair geometry

••

••

••

bent

trigonal pyramid

tetrahedral

molecular geometry

Five Pairs

electron pair geometry trigonal bipyramid

bonding pairs 5 4 3 2

non-bonding pairs 0 1 2 3

molecular geometry tbp seesaw T-shp. Lin.

Five electron pair

••

••••

••

••

120°

90°

electron pair geometrymolecular geometry

••

••

••

••••

••

TBP seesaw

T-shapedlinear

Six Pairs

electron pair geometry octahedral

bonding pairs 6 5 4 2

non-bonding pairs 0 1 2 4

molecular geometry oct sq.pyr. sq. pl. lin.

Six electron pairs

••

••

••

••

••••

90°

••

••

••

••

••

••

square pyramid

••

linear

square planar

octahedral

electron pair geometry

molecular geometry

Molecular Polarity

Molecules are always non-polar if all covalent bonds are non-polar N2, P4, Cl2

Molecules with polar bonds can be polar or non-polar H - Cl polar bond, polar molecular O=C=O two polar bonds but total molecule

is non-polar

H Cl+

-

O C O

+- -

equal but opposite forces cancel out non-polar molecule

O

H H+

+

-

are these dipole moments equal & opposite?

no

is this molecule polar? yes

C

Cl Cl- -

+

Cl Cl--

are these bond dipole moments equal & opposite?

yes

is this molecule polar? no

C

Cl Cl- -

H H+

+

are these bond dipole moments equal & opposite?

no

is this molecule polar? yes

Single and Multiple Bonds

(sigma) bonds always the first bond between two atoms single bonds are localized between two

atoms• orbitals from two atoms overlap, allowing electrons to be shared• electron density is on the internuclear axis

CC••

localized electrons

(pi) bonds the second & third bonds between two

atoms bond electrons can be delocalized over

several atoms to form resonance structures• electron density is above & below the internuclear axis

CC CC internuclear axis

electron density above & below-- bond

CC CC

electron density can move or delocalize

••

••

Hybridization allows for greater number of bonds types of hybridization

sp mixing of one s orbital & one p orbital•

2s 2p sp p sp2 mixing of one s orbital & two p orbitals

• 2s 2p sp2 p

sp3 mixing of one s orbital & three p orbitals•

2s 2p sp3

in sp hybridization the two sp hybrid orbitals form two bonds with linear geometry remaining two p orbitals form bonds

in sp2 hybridization the three hybrid orbitals form three bonds with trigonal planar

geometry the remaining one p orbital forms a bond

in sp3 hybridization the four hybrid orbitals form four bonds with tetrahedral geometry sp3 hybrid atoms can form no bonds as they have no unhybridized

p orbitals

Molecular Orbitals

mathematical combinations of atomic orbitals delocalized over whole molecule n atomic orbitals produce n molecular orbitals

• ½ are bonding orbitals and ½ are antibonding orbitals

bond order # bonding electrons - # antibonding electron

2

electron configuration of diatomic, homonuclear molecules

* * *

MO’s from s orbital combination

MO’s from p orbital combination

electron configuration of diatomic, homonuclear molecules with interaction of the 2s and 2p orbitals

* * *

relative positions switched

* * *

* * *

H2 N2

2 electrons 10 electrons

B.O. = 1 B.O. = 3

* * *

* * *

He2

4 electrons

B.O. = 0

O2

12 electrons

B.O. = 2