Chapter 9 Molecular Geometry

Lewis Theory-VSEPR Valence Bond Theory

Molecular Orbital Theory

Lewis Structures and the “Real” 3D-Shape of Molecules

Sulfanilamide

Lewis Theory of

Molecular Shape and Polarity

Structure Determines Properties!

Properties of molecular substances depend on the structure of the molecule.

The structure includes many factors, such as:

Skeletal arrangement of the atoms

Kind of bonding between the atoms

Shape of the molecule

Molecular Geometry

We can describe the shape of a molecule with terms that relate to geometric figures

These geometric figures have characteristic “corners” (indicating the positions of atoms)

The geometric figures also have characteristic angles that we call bond angles.

Lewis Theory Predicts“Electron Groups”

“Electron groups” - regions of electrons around an atom

Regions result from placing shared pairs of valence electrons between bonding nuclei

Regions result from placing unshared valence electrons on a single nuclei

Lewis Theory of Molecular Shapes

Electron groups repel each other.

Predicting the shapes of molecules

1) The arrangement of the electron groups will be determined by trying to minimize repulsions between them.

2) The arrangement of atoms (“molecular shape”) surrounding a central atom will be determined by where the bonding electron groups are.

3) “1” and “2” are not necessarily the same

VSEPR Theory

Valence shell electron pair repulsion (VSEPR) - Electron groups around the central atom will be most

stable when they are as far apart as possible.

A Lewis structure predicts the number of valence electron pairs around a central atom(s).

Each lone pair of electrons or odd electron constitutes one

electron group on a central atom.

Each bond constitutes one electron group, regardless of whether it is single, double, or triple

Electron Groups

There are three electron groups around N

one lone pair one single bond one double bond

O N O O N O

There are three electron groups around N

one odd electron one single bond one double bond

“steric number = 3”

Electron Group Geometry

There are five basic arrangements of electron groups around a central atom.

For molecules that exhibit resonance, it doesn’t matter

which resonance form you use – the electron group geometry will be the same.

linear triangular tetrahedral trigonal bipyramidal

octahedral

Linear Electron Geometry

When there are two electron groups around the central atom, they will occupy positions on opposite sides of the central atom.

This results in the electron groups taking a linear geometry.

The bond angle is 180°.

Cl Be Cl O C O

Trigonal Planar Electron Geometry

When there are three electron groups around the central atom, they will occupy positions in the shape of a triangle around the central atom.

This results in the electron groups in a trigonal planar geometry.

The bond angle is 120°

F Be

F

F

Tetrahedral Electron Geometry

When there are four electron groups around the central atom, they will occupy positions in the shape of a tetrahedron around the central atom.

This results in the electron groups taking a tetrahedral geometry.

The bond angle is 109.5°

F C

F

F

F

Trigonal Bipyramidal Electron Geometry

When there are five electron groups around the central atom, they will occupy positions in the shape of two tetrahedra that are base-to-base with the central atom in the center of the shared bases.

This results in the electron groups in a trigonal bipyramidal geometry.

The positions above and below the central atom are called the axial positions.

The positions in the same base plane as the central atom are called the

equatorial positions.

The bond angle between equatorial positions is 120°.

The bond angle between axial and equatorial positions is 90°.

Trigonal Bipyramidal Electron Geometry

Octahedral Electron Geometry

When there are six electron groups around the central atom, they will occupy positions in the shape of two square-base pyramids that are base-to-base with the central atom in the center of the shared bases

This results in the electron groups taking an octahedral geometry.

All positions around the central atom are equivalent.

The bond angle is 90°

Molecular Geometry

1) The actual geometry (“molecular geometry”) of a molecule may be different from the electron geometry.

2) When the electron groups are attached to atoms of different size, or when the bonding to one atom is different than the bonding to another, this will affect the molecular geometry around the central atom.

3) Lone pairs occupy space on the central atom, but are not “seen” as points on the molecular geometry.

Not Quite Perfect Geometry

Because the bonds and atom sizes are not identical in formaldehyde, the observed angles are slightly different from ideal.

C

O

H H

The nonbonding electrons are localized on the central atom, so area of negative charge

takes more space.

The bonding electrons are shared by two

atoms, so some of the negative charge is removed from the

central atom.

The Effect of Lone Pairs

The Effect of Lone Pairs

Lone pair groups “occupy more space” on the central atom than bonding electrons.

Relative sizes of repulsive force interactions:

Lone Pair – Lone Pair > Lone Pair – Bonding Pair >

Bonding Pair – Bonding Pair

This affects the bond angles, making the bonding pair – bonding pair angles smaller than expected.

Bond Angle Distortion from Lone Pairs

Molecular geometries derived from tetrahedral electron geometry.

VSEPR Theory

Bond Angle Distortion from Lone Pairs

Tetrahedral molecular

shapePyramidal molecular

shape

Bent molecular

shape

Bent or Angular Molecular Geometry: a Derivative of Trigonal Planar Electron Geometry

When there are three electron groups around the central atom, and one of them is a lone pair, the resulting shape

of the molecule is called a angular or bent shape.

The bond angle is less than 120°.

SO2

O S O

ClO2-Bent Molecular Geometry

O Cl O-

110 º

Trigonal Bipyramidal Electron Geometry

Molecular geometries derived from trigonal bipyramidal electron geometry.

See Saw Moleclular Geometry a Derivatives of Trigonal Bipyramidal Electron Geometry

When there are five electron groups around the central atom, and some are lone pairs, the lone pairs will occupy the equatorial positions because there is more room .

SF4

F S

F

F

F

When there are five electron groups around the central atom, and one is a lone pair, the result is called the seesaw shape.

See Saw Moleclular Geometry a Derivatives of Trigonal Bipyramidal Electron Geometry

SF4

F S

F

F

F

T-Shaped Molecular Geometry a Derivative of Trigonal Bipyramidal Electron Geometry

When there are five electron groups around the central atom, and two are lone pairs, the result is called the T-shaped.

BrF3

Linear Molecular Geometry a Derivatives of Trigonal Bipyramidal Electron Geometry

When there are five electron groups around the central atom, and three are lone pairs, the result is a linear shape .

XeF2

Molecular geometries derived from octahedral electron geometry.

Square Pyramidal Molecular Geometry a Derivatives of Octahedral Electron Geometry

When there are six electron groups around the central atom, and one is a lone pair, the result is called a square pyramid shape.

The bond angles between axial and equatorial positions is less than 90°

BrF5

When there are six electron groups around the central atom, and two are lone pairs, the result is called a square planar shape.

The bond angles between equatorial positions is 90°.

Square Planar Molecular Geometry a Derivatives of Octahedral Electron Geometry

XeF4

Predicting the Shapes Around Central Atoms

1. Draw the Lewis structure

2. Determine the number of electron groups around the central atom

3. Classify each electron group as bonding or lone pair, and count each type

4. Determine the shape and bond angles

Predict the geometry and bond angles of PCl31. Draw the Lewis structure

26 valence electrons

2. Determine the number of electron groups around central atom

four electron groups around P

3. Classify the electron groups

a) three bonding groups

b) one lone pair

Cl P Cl

Cl

Predict the geometry and bond angles of PCl3

4. Determine the shape and bond angles

a) four electron groups around P = tetrahedral electron geometry

b) three bonding + one lone pair = trigonal pyramidal molecular geometry

c) trigonal pyramidal = bond angles less than 109.5°

Predict the molecular geometry and bond angles in SiF5─

1. Draw the Lewis structure 40 valence electrons

2. Determine the number of electron groups around central atom

five electron groups around Si

3. Classify the electron groups

a) five bonding groups

b) 0 lone pairs

F Si F

F -

F F

Predict the molecular geometry and bond angles in SiF5─

4. Determine the shape and bond angles

a) five electron groups around Si = trigonal bipyramidal electron geometry

b) five bonding + 0 lone pairs = trigonal bipyramidal molecular geometry

c) trigonal bipyramidal = bond angles of than 120° (eq-eq) and 90º (ax-eq)

Predict the molecular geometry and bond angles in ClO2F

1. Draw the Lewis structure 26 valence electrons

2. Determine the number of electron groups around central atom

4 electron groups around Cl

3. Classify the electron groups

a) three bonding groups

b) one lone pair

Predict the molecular geometry and bond angles in ClO2F

4. Determine the shape and bond angles

a) four electron groups around Cl = tetrahedral electron geometry

b) 3 bonding + 1 lone pair = trigonal pyramidal molecular geometry

c) trigonal pyramidal = bond angles of <109.5°

Molecules with Multiple Central Atoms

Methanol

H N

H

C

H

H

C

O

O H

Glycine

Polarity of Molecules

Polarity of Molecules

For a molecule to be polar, it must

have polar bonds, and

have an unsymmetrical shape

Polarity affects the intermolecular forces of attraction

and therefore affects boiling points and solubilities

Nonbonding pairs affect molecular polarity.

When describing the polarity of a molecule, we must consider bond polarities as

VECTOR QUANTITIESquantities with magnitude and direction.

Common Cases of Adding Dipole Moments to Determine Whether a Molecule is Polar

The O─C bond is polar. The bonding electrons are pulled equally toward both O

ends of the molecule. The net result is a nonpolar molecule.

Molecular Polarity

The H─O bond is polar. Both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule.

Molecular Polarity

Predicting Polarity of Molecules

1. Draw the Lewis structure and determine the molecular geometry.

2. Determine whether the bonds in the molecule are polar.

3. Determine whether the polar bonds add together to give a net dipole moment.

Predict whether NH3 is a polar molecule

1. Draw the Lewis structure and determine the molecular geometry

a) eight valence electrons

b) three bonding + one lone pair = trigonal pyramidal molecular geometry

Predict whether NH3 is a polar molecule

2. Determine if the bonds are polar

a) electronegativity difference

b) if the bonds are not polar, we can stop here and declare the molecule will be nonpolar ENN = 3.0

ENH = 2.13.0 − 2.1 = 0.9 therefore the bonds are polar covalent

Predict whether NH3 is a polar molecule

3) Determine whether the polar bonds add together to give a net dipole moment

a) vector addition

b) generally, asymmetric shapes result in uncompensated polarities and a net dipole moment

The H─N bond is polar. All the sets of bonding electrons are pulled toward the N end of the molecule. The net result is a polar molecule.

Decide whether the following molecule is polar

ENO = 3.5N = 3.0Cl = 3.0S = 2.5 Trigonal

Bent

1. polar bonds, N-O2. asymmetrical shape

polar

Decide whether the following molecule is polar

TrigonalPlanar

1. polar bonds, all S-O2. symmetrical shape

nonpolar

ENO = 3.5S = 2.5

What about Tetrahedral Geometry ?

Some molecules are inherently polar because of the atoms which they contain and the arrangement of these atoms in space.

H2O NH3 CH2O HCl

δ− δ+ A crude representation of a polar molecule

Other molecules are considered nonpolar

CH4 BH3 C2H2 CO2

Nonpolarizedelectronclouds

Molecular Formula⬇

Structural Formula⬇

Dot Diagram⬇

Molecular Shape⬇

Molecular PolarityIntermolecular Forces

Melting Point, Boiling Point, Solubility