Tài liệu Chapter 10 Chemical Bonding II

Molecular Geometry • Molecules are 3-dimensional objects • We often describe the shape of a molecule with terms that relate to geometric figures • These geometric figures have character

Structure Determines Properties!

• properties of molecular substances depend on the structure of the molecule

• the structure includes many factors, including:

the skeletal arrangement of the atoms

the kind of bonding between the atoms

 ionic, polar covalent, or covalent

the shape of the molecule

• bonding theory should allow you to predict the shapes of molecules

Molecular Geometry

• Molecules are 3-dimensional objects

• We often describe the shape of a molecule

with terms that relate to geometric figures

• These geometric figures have characteristic

“corners” that indicate the positions of the

surrounding atoms around a central atom in

the center of the geometric figure

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

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Using Lewis Theory to Predict

Molecular Shapes

• Lewis theory predicts there are regions of

electrons in an atom based on placing shared

pairs of valence electrons between bonding

nuclei and unshared valence electrons located

on single nuclei

• this idea can then be extended to predict the

shapes of molecules by realizing these regions are all negatively charged and should repel

VSEPR Theory

• electron groups around the central atom will be most stable when they are as far apart as

possible – we call this valence shell electron

pair repulsion theory

since electrons are negatively charged, they should

be most stable when they are separated as much as possible

• the resulting geometric arrangement will allow

us to predict the shapes and bond angles in the molecule

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

6

VSEPR electron domain animation

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Molecular Geometries

• there are 5 basic arrangements of electron groups

around a central atom

 based on a maximum of 6 bonding electron groups

 though there may be more than 6 on very large atoms, it is very rare

• each of these 5 basic arrangements results in 5 different basic molecular shapes

 in order for the molecular shape and bond angles to be a

“perfect” geometric figure, all the electron groups must be bonds and all the bonds must be equivalent

• for molecules that exhibit resonance, it doesn’t matter which resonance form you use – the molecular

geometry will be the same

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Linear Geometry

• when there are 2 electron groups around the central atom, they will occupy positions opposite each other around the central atom

• this results in the molecule taking a linear geometry

• the bond angle is 180°

O

Linear Geometry

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Trigonal Geometry

• when there are 3 electron groups around the central atom, they will occupy positions in the shape of a

triangle around the central atom

• this results in the molecule taking a trigonal planar geometry

• the bond angle is 120°

F

Trigonal Geometry

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Not Quite Perfect Geometry

Because the bonds are

not identical, the

observed angles are

slightly different from

ideal.

15 Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Tetrahedral Geometry

• when there are 4 electron groups around the central atom, they will occupy positions in the shape of a

tetrahedron around the central atom

• this results in the molecule taking a tetrahedral

F

F

Tetrahedral Geometry

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Methane

Trigonal Bipyramidal Geometry

• when there are 5 electron groups around the central atom, they will occupy positions in the shape of a two tetrahedra that are base-to-base with the central atom in the center of the shared bases

• this results in the molecule taking 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°

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Trigonal Bipyramid

22

Octahedral Geometry

• when there are 6 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 molecule taking an octahedral

geometry

 it is called octahedral because the geometric figure has 8 sides

• all positions are equivalent

• the bond angle is 90°

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Octahedral Geometry

26

The Effect of Lone Pairs

• lone pair groups “occupy more space” on the central atom

because their electron density is exclusively on the

central atom rather than shared like bonding electron

groups

• relative sizes of repulsive force interactions is:

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

• this effects the bond angles, making them smaller

than expected

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Effect of Lone Pairs

The bonding electrons are shared by two atoms,

so some of the negative charge is removed from the central atom.

The nonbonding electrons are localized on the central atom, so area of negative charge takes more space.

Derivative Shapes

• the molecule’s shape will be one of basic

molecular geometries if all the electron groups are bonds and all the bonds are equivalent

• molecules with lone pairs or different kinds of surrounding atoms will have distorted bond

angles and different bond lengths, but the shape will be a derivative of one of the basic shapes

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Derivative of Trigonal Geometry

• when there are 3 electron groups around the central atom, and 1 of them is a lone pair, the resulting shape

of the molecule is called a trigonal planar - bent

O

 because it is a triangular-base pyramid with the central

atom at the apex

• when there are 4 electron groups around the central atom, and 2 are lone pairs, the result is called a

tetrahedral-bent shape

 it is planar

 it looks similar to the trigonal planar-bent shape, except the angles are smaller

• for both shapes, the bond angle is < 109.5°

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Trigonal Pyramidal Shape

Bond Angle Distortion from Lone Pairs

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Tetrahedral-Bent Shape

Chemistry, Julia Burdge, 2 nd e., McGraw Hill.

Bond Angle Distortion from Lone Pairs

Tro, Chemistry: A Molecular Approach 37

Tetrahedral-Bent Shape

1

O Cl

Derivatives of the Trigonal Bipyramidal Geometry

• when there are 5 electron groups around the central atom, and some are lone pairs, they will occupy the equatorial positions because there is more room

• when there are 5 electron groups around the central atom, and 1

is a lone pair, the result is called see-saw shape

 aka distorted tetrahedron

• when there are 5 electron groups around the central atom, and 2 are lone pairs, the result is called T-shaped

• when there are 5 electron groups around the central atom, and 3 are lone pairs, the result is called a linear shape

• the bond angles between equatorial positions is < 120°

• the bond angles between axial and equatorial positions is < 90°

 linear = 180° axial-to-axial

Tro, Chemistry: A Molecular Approach 39

Replacing Atoms with Lone Pairs

in the Trigonal Bipyramid System

Tro, Chemistry: A Molecular Approach 41

T-Shape

Tro, Chemistry: A Molecular Approach 43

Linear Shape

Derivatives of the Octahedral Geometry

• when there are 6 electron groups around the central

atom, and some are lone pairs, each even number lone pair will take a position opposite the previous lone pair

• when there are 6 electron groups around the central

atom, and 1 is a lone pair, the result is called a square pyramid shape

 the bond angles between axial and equatorial positions is < 90°

• when there are 6 electron groups around the central

atom, and 2 are lone pairs, the result is called a square planar shape

 the bond angles between equatorial positions is 90°

Tro, Chemistry: A Molecular Approach 45

Square Pyramidal Shape

Br

F

FFF

Square Planar Shape

Tro, Chemistry: A Molecular Approach 47

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

 remember, multiple bonds count as 1 group

4) Use Table 10.1 to Determine the Shape and

Bond Angles

VSEPR animation

Practice – Predict the Molecular Geometry and

Bond Angles in SiF5-1

Tro, Chemistry: A Molecular Approach 51

Practice – Predict the Molecular Geometry and

Bond Angles in SiF5─

Practice – Predict the Molecular Geometry and

Bond Angles in ClO2F

Tro, Chemistry: A Molecular Approach 53

Practice – Predict the Molecular Geometry and

Bond Angles in ClO2F

Representing 3-Dimensional Shapes on

a 2-Dimensional Surface

• one of the problems with drawing molecules is trying

to show their dimensionality

• by convention, the central atom is put in the plane of the paper

• put as many other atoms as possible in the same plane

and indicate with a straight line

• for atoms in front of the plane, use a solid wedge

• for atoms behind the plane, use a hashed wedge

Tro, Chemistry: A Molecular Approach 55

SF

FF

Tro, Chemistry: A Molecular Approach 57

Multiple Central Atoms

• many molecules have larger structures with many

interior atoms

• we can think of them as having multiple central atoms

• when this occurs, we describe the shape around each central atom in sequence

H

C H

||

|

O H

shape around left C is tetrahedral

shape around center C is trigonal planar

shape around right O is tetrahedral-bent

Describing the Geometry

of Methanol

Tro, Chemistry: A Molecular Approach 59

Describing the Geometry

of Glycine

Practice – Predict the Molecular Geometries in

H3BO3

Polarity of Molecules

• in order for a molecule to be polar it must

1) have polar bonds

 electronegativity difference - theory

 bond dipole moments - measured

2) have an unsymmetrical shape

 vector addition

• polarity affects the intermolecular forces of attraction

 therefore boiling points and solubilities

 like dissolves like

• nonbonding pairs affect molecular polarity, strong

pull in its direction

Tro, Chemistry: A Molecular Approach 63

Molecule Polarity

The H-Cl bond is polar The bonding electrons are pulled toward the Cl end of the molecule The net result is a polar molecule.

Vector Addition

Tro, Chemistry: A Molecular Approach 65

Molecule Polarity

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.

Tro, Chemistry: A Molecular Approach 67

Molecule Polarity

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

Molecule Polarity

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.

Tro, Chemistry: A Molecular Approach 69

Molecular Polarity Affects

Solubility in Water

• polar molecules are attracted to

other polar molecules

• since water is a polar molecule,

other polar molecules dissolve

well in water

and ionic compounds as well

• some molecules have both polar

and nonpolar parts

A Soap Molecule Sodium Stearate

Tro, Chemistry: A Molecular Approach 71

Practice - Decide Whether the Following Are Polar

Practice - Decide Whether the Following Are Polar

polar

nonpolar

1) polar bonds, N-O

2) asymmetrical shape 1) polar bonds, all S-O

N

O

3.0 3.0

3.5

O

O

OS

3.5

2.5

Tro, Chemistry: A Molecular Approach 73

Problems with Lewis Theory

• Lewis theory gives good first approximations of the bond angles in molecules, but usually cannot

be used to get the actual angle

• Lewis theory cannot write one correct structure for many molecules where resonance is

Valence Bond Theory

• Linus Pauling and others applied the principles

of quantum mechanics to molecules

• they reasoned that bonds between atoms would arise when the orbitals on those atoms

interacted to make a bond

• the kind of interaction depends on whether the orbitals align along the axis between the nuclei,

or outside the axis

Tro, Chemistry: A Molecular Approach 75

Orbital Interaction

• as two atoms approached, the partially filled or empty valence atomic orbitals on the atoms

would interact to form molecular orbitals

• the molecular orbitals would be more stable

than the separate atomic orbitals because they would contain paired electrons shared by both atoms

the interaction energy between atomic orbitals is negative when the interacting atomic orbitals

contain a total of 2 electrons

Orbital Diagram for the

Tro, Chemistry: A Molecular Approach 77

Valence Bond Theory - Hybridization

• one of the issues that arose was that the number of

partially filled or empty atomic orbital did not predict the number of bonds or orientation of bonds

C = 2s22p x1 2p y1 2p z0 would predict 2 or 3 bonds that are 90° apart, rather than 4 bonds that are 109.5° apart

• to adjust for these inconsistencies, it was

postulated that the valence atomic orbitals could

hybridize before bonding took place

one hybridization of C is to mix all the 2s and 2p

orbitals to get 4 orbitals that point at the corners of a tetrahedron

Unhybridized C Orbitals Predict the

Wrong Bonding & Geometry

Tro, Chemistry: A Molecular Approach 79

Valence Bond Theory

Main Concepts

quantum mechanical atomic orbitals or hybrid orbitals

orbitals overlap and there is a total of 2 electrons in the new molecular orbital

a) the electrons must be spin paired

the geometry of the overlapping orbitals

more bonds = more full orbitals = more stability

• better explain observed shapes of molecules

• same type of atom can have different

hybridization depending on the compound

C = sp, sp2, sp3

Tro, Chemistry: A Molecular Approach 81

Hybrid Orbitals

• H cannot hybridize!!

• the number of standard atomic orbitals combined = the number of hybrid orbitals formed

• the number and type of standard atomic orbitals

combined determines the shape of the hybrid orbitals

• the particular kind of hybridization that occurs is the

one that yields the lowest overall energy for the

molecule

 in other words, you have to know the structure of the

molecule beforehand in order to predict the hybridization

Orbital Diagrams with

Hybridization

• place electrons into hybrid and unhybridized

valence orbitals as if all the orbitals have equal energy

• when bonding,  bonds form between hybrid

orbitals and  bonds form between unhybridized orbitals that are parallel

Tro, Chemistry: A Molecular Approach 83

2sp3

sp3 Hybridization

• atom with 4 areas of electrons

tetrahedral geometry

109.5° angles between hybrid orbitals

• atom uses hybrid orbitals for all bonds and

Hybridization animation

sp3 Hybridization of C

Tro, Chemistry: A Molecular Approach 87

Tro, Chemistry: A Molecular Approach 89