atomic bond theory.ppt

7/29/2019 ATOMIC BOND THEORY.ppt

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Chapter 8

Covalent

Bonding

Pre-AP ChemistryCharles Page High School

Stephen L. Cotton

Ball-and-stick model


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Section 8.1

Molecular Compounds

OBJECTIVES:

Distinguish between themelting points and

boiling points ofmolecularcompoundsand ioniccompounds.


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Section 8.1

Molecular Compounds

OBJECTIVES:

Describe the informationprovided by a molecular

formula.


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Bonds are

Forces that hold groups ofatoms together and make them

function as a unit. Two types:

1) Ionic bondstransferofelectrons (gained or lost; makes formula unit)

2) Covalent bondssharingofelectrons. The resulting

particle is called a molecule


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Covalent Bonds

The word covalent is acombination of the prefix co-

(from Latin com, meaning with

or together), and the verb

valere, meaning to be strong.

Two electrons shared togetherhave the strength to hold two

atoms together in a bond.


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Molecules

Many elements found in natureare in the form ofmolecules:

a neutral group of atoms joined

together by covalent bonds.

For example, air contains oxygen

molecules, consisting of two

oxygen atoms joined covalently

Called a diatomic molecule (O2

)


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How does H2 form?

The nuclei repel each other,since they both have a positivecharge (like charges repel).

++

(diatomic hydrogen molecule)

+ +
http://www.schools.pinellas.k12.fl.us/gallery/variety/Stop_sign.gif


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How does H2 form?

++

But, the nuclei are attracted tothe electrons

They share the electrons, and

this is called a covalent bond,

and involves only NONMETALS!


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Covalent bonds

Nonmetals hold on to their valenceelectrons. They cant give away electrons to bond.

But still want noble gas configuration. Get it by sharing valence electrons with

each other = covalent bonding

By sharing, both atoms get to countthe electrons toward a noble gasconfiguration.


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Covalent bonding

Fluorine has seven valenceelectrons (but would like to have 8)

F


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Covalent bonding

Fluorine has seven valenceelectrons

A second atom also has seven

F F


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Covalent bonding



By sharing electrons

F F


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Covalent bonding




F F


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Covalent bonding




F F


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Covalent bonding




F F


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Covalent bonding




F F


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Covalent bonding

Fluorine has seven valence electronsA second atom also has seven


both end with full orbitals

F F


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Covalent bonding




F F8 Valenceelectrons


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Covalent bonding




F F8 Valenceelectrons


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Molecular Compounds

Compounds that are bondedcovalently (like in water, or carbon

dioxide) are called molecular

compounds

Molecular compounds tend to have

relatively lowermelting and boilingpoints than ionic compounds this is

not as strong a bond as ionic


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Molecular Compounds

Thus, molecular compounds tend to

be gases or liquids at room

temperatureIonic compounds were solids

A molecular compound has a

molecular formula:Shows how many atoms of each

element a molecule contains


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Molecular Compounds

The formula for water is written asH2O

The subscript 2 behind hydrogen

means there are 2 atoms ofhydrogen; if there is only one atom,the subscript 1 is omitted

Molecular formulas do not tell anyinformation about the structure (thearrangement of the various atoms).


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- Page 215

These are some of the

different ways to represent

ammonia:

1. The molecular

formula showshow many atoms

of each element

are present

2. The structuralformula ALSO

shows the

arrangement of

these atoms!

3. The ball and stickmodel is

the BEST, because it showsa 3-dimensional arrangement.


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Section 8.2

The Nature of Covalent Bonding

OBJECTIVES:

Describe howelectronsare shared to formcovalent bonds, andidentify exceptions to theoctet rule.


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Section 8.2


OBJECTIVES:

Demonstrate howelectron dot structures

represent sharedelectrons.


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Section 8.2


OBJECTIVES:

Describe how atomsform double or triple

covalent bonds.


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Section 8.2

The Nature of Covalent Bonding OBJECTIVES:

Distinguish between acovalent bond and acoordinate covalent bond, and

describe how the strength of acovalent bond is related to itsbond dissociation energy.


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Section 8.2


OBJECTIVES:

Describe how oxygenatoms are bonded in

ozone.


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A Single Covalent Bond is...

A sharing of two valence electrons.Only nonmetals and hydrogen.

Different from an ionic bondbecause they actually form

molecules.

Two specific atoms are joined. In an ionic solid, you cant tell which

atom the electrons moved from or to


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Sodium Chloride Crystal Lattice

Ionic compounds

organize in a

characteristic

crystal lattice ofalternating

positive and

negative ions,repeated over and

over.


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How to show the formation

Its like a jigsaw puzzle. You put the pieces together to end up

with the right formula.

Carbon is a special example - can itreally share 4 electrons: 1s22s22p2?

Yes, due to electron promotion!

Another example: lets show how water isformed with covalent bonds, by using an

electron dot diagram


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Water

H

O

Each hydrogen has 1 valenceelectron

- Each hydrogen wants 1

more The oxygen has 6 valence

electrons

- The oxygen wants 2 more They share to make each

other complete


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Water

Put the pieces together The first hydrogen is happy

The oxygen still needs one more

H O


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Water

So, a second hydrogen attachesEvery atom has full energy levels

H OH

Note the two

unshared pairs

of electrons


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Examples:

1. Conceptual Problem8.1 on page 220

2. Do PCl3


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Multiple Bonds

Sometimes atoms share more thanone pairof valence electrons.

A double bond is when atoms share

two pairs of electrons (4 total)A triple bond is when atoms share

three pairs of electrons (6 total)

Table 8.1, p.222 - Knowthese 7elements as diatomic:

Br2 I2 N2 Cl2 H2 O2 F2

Whats the deal

with the oxygen

dot diagram?


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Dot diagram for Carbon dioxide

CO2 - Carbon is centralatom( more metallic )

Carbon has 4 valence

electronsWants 4 more

Oxygen has 6 valence

electrons

Wants 2 moreO

C


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Carbon dioxide

Attaching 1 oxygen leaves theoxygen 1 short, and the carbon 3

short

OC


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Carbon dioxide

Attaching the second oxygenleaves both of the oxygen 1 short,

and the carbon 2 short

OCO


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Carbon dioxide

The only solution is to share more

OCO


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Carbon dioxide


OCO


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Carbon dioxide


OCO


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Carbon dioxide


OCO


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Carbon dioxide


OCO


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Carbon dioxide


OCO


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Carbon dioxide

The only solution is to share moreRequires two double bonds

Each atom can count all the

electrons in the bond

OCO


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Carbon dioxide

The only solution is to share more Requires two double bonds

Each atom can count all the electrons in

the bond

OCO

8 valence

electrons


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Carbon dioxide



the bond

OCO

8 valence

electrons


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Carbon dioxide



the bond

OCO

8 valence

electrons


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How to draw them?

Use the handout guidelines:1) Add up all the valence electrons.

2) Count up the total number of

electrons to make all atoms happy.

3) Subtract; then Divide by 2

4) Tells you how many bonds to draw5) Fill in the rest of the valence

electrons to fill atoms up.


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Example

NH3, which is ammonia N central atom; has 5

valence electrons, wants 8

H - has 1 (x3) valenceelectrons, wants 2 (x3)

NH3 has 5+3 = 8

NH3 wants 8+6 = 14 (14-8)/2= 3 bonds

4 atoms with 3 bonds

N

H


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N HH

H

Examples

Draw in the bonds; start with singlesAll 8 electrons are accounted for

Everything is full done with this one.


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Example: HCN

HCN: C is central atom N - has 5 valence electrons, wants 8

C - has 4 valence electrons, wants 8

H - has 1 valence electron, wants 2 HCNhas 5+4+1 = 10

HCNwants 8+8+2 = 18

(18-10)/2= 4 bonds

3 atoms with 4 bonds this will require

multiple bonds - not to H however


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HCN

Put single bond between each atomNeed to add 2 more bonds

Must go between C and N (Hydrogen is full)

NH C


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HCN

Put in single bonds Needs 2 more bonds

Must go between C and N, not the H

Uses 8 electrons need 2 more toequal the 10 it has

NH C


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HCN

Put in single bonds Need 2 more bonds

Must go between C and N

Uses 8 electrons - 2 more to add Must go on the N to fill its octet

NH C


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Another way of indicating

bondsOften use a line to indicate a bond

Called a structural formula

Each line is 2 valence electrons

H HO = H HO


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Other Structural Examples

H C N

C O

H

H


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A Coordinate Covalent Bond...

When one atom donates bothelectrons in a covalent bond.

Carbon monoxide (CO) is a good

example:

OCBoth the carbon

and oxygen give

another singleelectron to share


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Coordinate Covalent Bond


Carbon monoxide (CO) is a good

example:

OC

Oxygengives both of

these

electrons,

since it hasno more

singles to

share.

This carbon

electron

moves tomake a pair

with the other

single.


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Coordinate Covalent Bond


Carbon monoxide (CO)

OCC O

The

coordinate

covalent bond

is shown withan arrow as:


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Coordinate covalent bond

Mostpolyatomic cations andanions contain covalent and

coordinate covalent bonds

Table 8.2, p.224

Sample Problem 8.2, p.225

The ammonium ion (NH41+)can

be shown as another example


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Bond Dissociation Energies...

The total energy required to breakthe bondbetween 2 covalently

bonded atoms

High dissociation energy usually

means the chemical is relatively

unreactive, because it takesa lot of energy to break it down.


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Resonance is...When more than one valid dot

diagram is possible.

Consider the two ways to draw ozone

(O3)

Which one is it? Does it go back and

forth?

It is a hybrid of both, like a mule; andshown by a double-headed arrow

found in double-bond structures!


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Resonance in Ozone

Neitherstructure is correct, it is

actually a hybrid of the two. To showit, draw all varieties possible, and join

them with a double-headed arrow.

Note the different location of the double bond


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ResonanceOccurs when more than one valid Lewis

structure can be written for a particular

molecule (due to position of double bond)

These are resonance structures of benzene.

The actual structure is an average (or hybrid)

of these structures.


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Resonance

in a

carbonate

ion (CO32-):

Resonance

in anacetate ion

(C2H3O21-):

Polyatomic ions note the different

positions of the double bond.


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The 3 Exceptions to Octet rule

For some molecules, it isimpossible to satisfythe octet rule

#1. usually when there is an odd

numberof valence electronsNO2 has 17 valence electrons,

because the N has 5, and each O

contributes 6. Note N page 228 It is impossible to satisfy octet rule,

yet the stable molecule does exist


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Exceptions to Octet rule Another exception: Boron

Page 228 shows boron trifluoride,and note that one of the fluoridesmight be able to make a coordinate

covalent bond to fulfill the boron

#2 -But fluorine has a highelectronegativity (it is greedy), so this

coordinate bond does not form #3 -Top page 229 examples exist

because they are in period 3 or beyond


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Section 8.3

Bonding TheoriesOBJECTIVES:

Describe the relationshipbetween atomic and

molecular orbitals.


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Section 8.3

Bonding TheoriesOBJECTIVES:

Describe how VSEPRtheory helps predict the

shapes of molecules.


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Molecular Orbitals are...

The model for covalent bondingassumes the orbitals are those ofthe individual atoms = atomic orbital

Orbitals that apply to the overallmolecule, due to atomic orbitaloverlap are the molecular orbitals

A bonding orbital is a molecularorbital that can be occupied bytwo electrons of a covalent bond


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Molecular Orbitals - definitions

Sigma bond- when two atomicorbitals combine to form the

molecular orbital that is

symmetrical along the axisconnecting the nuclei

Pi bond- the bonding electrons are

likely to be found above and belowthe bond axis (weaker than sigma)

Note pictures on the next slide


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- Pages 230 and 231

Sigma bond is symmetrical along theaxis between the two nuclei.

Pi bond is

above and

below thebond axis,

and is

weaker

than sigma


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VSEPR: stands for...

Valence Shell Electron PairRepulsion

Predicts the three dimensional shape ofmolecules.

The name tells you the theory:Valence shell = outside electrons.

Electron Pair repulsion = electron

pairs try to get as far away aspossible from each other.

Can determine the angles of bonds.


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VSEPR

Based on the number of pairs ofvalence electrons, both bondedand

unbonded.

Unbonded pair also called lone pair.CH4 - draw the structural formula

Has 4 + 4(1) = 8

wants 8 + 4(2) = 16

(16-8)/2 = 4 bonds


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VSEPR for methane (a gas): Single bonds fill

all atoms.

There are 4

pairs of

electrons

pushing away.

The furthest theycan get away is

109.5

C HH

H

H

This 2-dimensional drawing does

not show a true representation of

the chemical arrangement.


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4 atoms bonded

Basic shape istetrahedral.

A pyramid with a

triangular base. Same shape for

everything with

4 pairs. CH HH

H 109.5


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Other angles, pages 232 - 233

Ammonia (NH3) = 107oWater (H2O) = 105

o

Carbon dioxide (CO2) = 180o

Note the shapes of these thatare pictured on the next slide


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- Page 232

Methane has

an angle of

109.5o, calledtetrahedral

Ammonia has

an angle of107o, called

pyramidal

Note the unshared pair that is repulsion for other electrons.


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Section 8.4

Polar Bonds and MoleculesOBJECTIVES:

Describe howelectronegativity valuesdetermine thedistribution of charge in apolar molecule.


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Section 8.4


Describe what happensto polarmolecules whenthey are placed betweenoppositely charged metalplates.


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Section 8.4


Evaluate the strength ofintermolecular attractionscompared with thestrength of ionic andcovalent bonds.


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Section 8.4


Identify the reason whynetwork solids have high

melting points.

B d P l it


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Bond Polarity

Covalent bonding means sharedelectrons

but, do they share equally?

Electrons are pulled, as in a tug-of-war, between the atoms nuclei

In equal sharing (such as

diatomic molecules), the bondthat results is called a nonpolarcovalent bond

B d P l it


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Bond Polarity

When two differentatoms bondcovalently, there is an unequalsharing

the more electronegative atom willhave a stronger attraction, and will

acquire a slightly negative charge

called a polar covalent bond, or

simply polar bond.

El t ti it ?


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Electronegativity?

The ability of an

atom in a molecule

to attract sharedelectrons to itself.

Linus Pauling

1901 - 1994

T bl f El t ti iti


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Table of Electronegativities

B d P l it


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Bond Polarity

Refer to Table 6.2, p. 177(or handout)

Consider HCl

H = electronegativity of 2.1

Cl = electronegativity of 3.0

the bond is polar

the chlorine acquires a slightnegative charge, and the

hydrogen a slight positive charge

B d P l it


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Bond Polarity

Only partial charges, much lessthan a true 1+ or 1- as in ionic bond

Written as:

H Cl the positive and minus signs (with

the lower case delta: )denote partial charges.

d+ d-

d+andd-

B d P l it


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Bond Polarity

Can also be shown:

the arrow points to the moreelectronegative atom.

Table 8.3, p.238 shows how the

electronegativity can also indicate

the type of bond that tends to form

H Cl

P l l l


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Polar molecules

Sample Problem 8.3, p.239A polar bond tends to make the

entire moleculepolar

areas of difference

HCl has polar bonds, thus is a polar

molecule.A molecule that has two poles is

called dipole, like HCl

Polar molecules


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Polar molecules The effect of polar bonds on the

polarity of the entire molecule dependson the molecule shape

carbon dioxide has two polar bonds,and is linear = nonpolar molecule!

Polar molecules


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Polar molecules The effect of polar bonds on the

polarity of the entire molecule dependson the molecule shape

water has two polar bonds and a bent

shape; the highly electronegative oxygenpulls the e- away from H = very polar!

Polar molecules


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Polar molecules

When polar molecules areplaced between oppositelycharged plates, they tend to

become oriented with respectto the positive and negative

plates.Figure 8.24, page 239

Attractions between molecules


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Attractions between molecules They are what make solid and liquid

molecular compounds possible.

The weakest are called van derWaalsforces - there are two kinds:

#1. Dispersion forces

weakest of all, caused by motion of e-

increases as # e- increases

halogens start as gases; bromine isliquid; iodine is solid all in Group 7A

#2 Dipole interactions


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#2. Dipole interactions

Occurs when polar molecules areattracted to each other.

2. Dipole interaction happens in

waterFigure 8.25, page 240

positive region of one molecule

attracts the negative region of

another molecule.

#2 Dipole interactions


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#2. Dipole interactions

Occur when polar molecules areattracted to each other.

Slightly stronger than dispersion forces.

Opposites attract, but not completelyhooked like in ionic solids.

H F

d+ d-

H F

d+ d-

#2 Dipole Interactions


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#2. Dipole Interactions

d+ d-

d+ d-

#3 Hydrogen bonding


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#3. Hydrogen bonding

is the attractive force caused byhydrogen bonded to N, O, F, or Cl

N, O, F, and Cl are very

electronegative, so this is a verystrong dipole.

And, the hydrogen shares with the

lone pair in the molecule next to it. This is the strongest of the

intermolecular forces.

Order of Intermolecular attraction strengths


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Order of Intermolecular attraction strengths

1) Dispersion forces are theweakest

2)A little stronger are the dipole

interactions3) The strongest is the hydrogen

bonding

4) All of these are weaker thanionic bonds

#3 Hydrogen bonding defined:


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#3. Hydrogen bonding defined:

When a hydrogen atom is: a) covalentlybonded to a highly electronegative atom,AND b) is also weakly bonded to anunshared electron pairof a nearby

highly electronegative atom.The hydrogen is left very electron

deficient(it only had 1 to start with!)thus it shares with something nearby

Hydrogen is also the ONLY elementwith no shielding for its nucleus wheninvolved in a covalent bond!

Hydrogen Bonding


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Hydrogen Bonding(Shown in water)

H

H

O

d+ d-

d+

This hydrogen is bondedcovalently to: 1) the highly

negative oxygen, and 2) a

nearby unshared pair.

Hydrogen bonding allows H2O to be a


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Hydrogen bonding allows H2O to be a

liquid at room conditions.

H

H

O

Attractions and properties


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Why are some chemicals gases,some liquids, some solids?

Depends on the type of

bonding!

Table 8.4, page 244

Network solids solids in whichall the atoms are covalentlybonded to each other



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Figure 8.28, page 243Network solids melt at very high

temperatures, or not at all (decomposes)

Diamond does not really melt, butvaporizes to a gas at 3500 oC

and beyond

SiC, used in grinding, has a

melting point of about 2700 oC

Covalent Network Compounds


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Covalent Network CompoundsSome covalently bonded substances DO

NOT form discrete molecules.

Diamond, a network of

covalently bonded carbon

atoms

Graphite, a network of

covalently bonded carbon

atoms


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atomic bond theory.ppt

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