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