structure determines properties!

Tro, Chemistry: A Molecular Approach 1

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 atomsthe kind of bonding between the atoms

ionic, polar covalent, or covalentthe shape of the molecule

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

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 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


Using Lewis Theory to PredictMolecular 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 theorysince 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


Electron Groups• the Lewis structure predicts the arrangement of valence

electrons around the central atom(s) • each lone pair of electrons constitutes one electron group

on a central atom• each bond constitutes one electron group on a central

atom regardless of whether it is single, double, or triple

O N O ••••••

••

•••• there are 3 electron groups on N1 lone pair1 single bond1 double bond


Molecular Geometries• there are 5 basic arrangements of electron groups

around a central atombased 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


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°

ClBeCl

O C O


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

F B F


Not Quite Perfect Geometry

Because the bonds are not identical, the observed angles are slightly different from ideal.


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 geometry

• the bond angle is 109.5°

F

F C F

F


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°


Trigonal Bipyramidal Geometry

P

ClCl

ClCl

Cl•••• ••••

••••

••••

••

••

••••

••

••

••


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°


Octahedral Geometry

S

F F

FF

F

F

•••• ••••

••••••••

••••••

••••

••••

••••

••


The Effect of Lone Pairs• lone pair groups “occupy more space” on the central

atombecause 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


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 shape

• the bond angle is < 120°

O S O

O S O

O S O

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


Derivatives of Tetrahedral Geometry

• when there are 4 electron groups around the central atom, and 1 is a lone pair, the result is called a pyramidal shapebecause 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°


Bond Angle Distortion from Lone Pairs


Tetrahedral-Bent Shape


Derivatives of theTrigonal 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


See-Saw Shape

F S F

F

F

••••

••••

••

••••••

••

••

••

••

••


T-Shape


Linear Shape


Derivatives of theOctahedral 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°


Square Pyramidal Shape

Br

FF

FF

F•••• ••••

••••

••••

••

••

••••

••

••

••••


Practice – Predict the Molecular Geometry and Bond Angles in SiF5

─

Si = 4e─

F5 = 5(7e─) = 35e─

(─) = 1e─

total = 40e─

5 Electron Groups on Si

5 Bonding Groups0 Lone Pairs

Shape = Trigonal Bipyramid

Si

FF

FF

F•••• ••••

••••

••••

••

••

••••

••

••

••

-1

Bond AnglesFeq-Si-Feq = 120°Feq-Si-Fax = 90°

Si Least Electronegative

Si Is Central Atom


Practice – Predict the Molecular Geometry and Bond Angles in ClO2F

Cl = 7e─

O2 = 2(6e─) = 12e─

F = 7e─

Total = 26e─

4 Electron Groups on Cl

3 Bonding Groups1 Lone Pair

Shape = Trigonal Pyramidal

Bond AnglesO-Cl-O < 109.5°O-Cl-F < 109.5°

Cl Least Electronegative

Cl Is Central AtomO Cl

O

F

•••••• ••••

••

••

••

••

••


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


SF6

S

F

FF

F F

FS

F F

FF

F

F

•••• ••••

••••••••

••••••

••••

••••

••••

••


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|

HOCCH|||OH

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

Describing the Geometry of Glycine

35

Practice – Predict the Molecular Geometries in H3BO3

B = 3e─

O3 = 3(6e─) = 18e─

H3 = 3(1e─) = 3e─

Total = 24e─

3 Electron Groups on B

B has3 Bonding Groups0 Lone Pairs

Shape on B = Trigonal Planar

B Least Electronegative

B Is Central Atom

oxyacid, so H attached to O

O B

O

OH H

H••••

••

••

••

•• 4 Electron Groups on O

O has2 Bonding Groups2 Lone Pairs

Shape on O = Tetrahedral Bent


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


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.


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.


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.


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 waterand ionic compounds as well

• some molecules have both polar and nonpolar parts


A Soap MoleculeSodium Stearate


Practice - Decide Whether the Following Are Polar

polarnonpolar

1) polar bonds, N-O2) asymmetrical shape 1) polar bonds, all S-O

2) symmetrical shape

O N Cl ••••••

••

••••

O S

O

O

•••••• ••

••••

••

••

TrigonalBent Trigonal

PlanarCl

NO

3.0

3.0

3.5O

O

OS

3.5

3.5 3.52.5


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 important

• Lewis theory often does not predict the correct magnetic behavior of moleculese.g., O2 is paramagnetic, though the Lewis structure

predicts it is diamagnetic

structure determines properties!

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