1 chapter 8 covalent bonding 2 i. octet rule l what is the octet rule? l the octet rule states that...

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Chapter 8

Covalent bonding

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I. Octet Rule What is the Octet Rule?

The octet rule states that atoms lose gain or share electrons in to acquire a full set of 8 valence electrons

Create a drawing of Mg and Cl, Al and Cl

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II. Covalent BondsA Chemical Bond occurs when electron

are shared.

Hydrogen and Hydrogen

A molecules are formed from the overlap of orbitals and sharing of electrons

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Covalent bondsa. Nonmetals hold onto their valence

electrons.

b. They can’t give away electrons to bond.

Still want noble gas configuration.

c. Get it by sharing valence electrons with each other.

d. By sharing both atoms get to count the electrons toward noble gas configuration.

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Its all in the distancea. Too far no bond

b. Too close electrons repel

c. Just right and a molecule is born

A molecules are formed from the overlap of orbitals and sharing of electrons

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Covalent clip

Covalent_Bonds.asf

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III. Molecule A. a covalently bonded compound.

– 1. Tend to occur between non metals that are close together on the periodic table.

– a. diatomic molecules – occur naturally in nature

a. this is a more stable arrangement.

H H F F Br Br Cl Cl N N

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I. Single Covalent BondA. A sharing of two valence electrons.

1.Only nonmetals and Hydrogen.

2.Different from an ionic bond because they actually form molecules.

3. Two specific atoms are joined.

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How does H2 form? The nuclei repel

++

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When Atoms Combine to make Molecules

Fig 8-1

Atoms contain both positive and negative charges. When they come Together they arrange themselves so that the attractive forges of oppositeCharges is greater than the repulsive forces of like charges

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How does H2 form?

++

The nuclei repel But they are attracted to electrons They share the electrons

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How to show how they formed It’s like a jigsaw puzzle. I have to tell you what the final formula

is. You put the pieces together to end up

with the right formula. For example- show how water is formed

with covalent bonds.

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Water

H

O

Each hydrogen has 1 valence electron

Each hydrogen wants 1 more

The oxygen has 6 valence electrons

The oxygen wants 2 more

They share to make each other happy

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Water Put the pieces together The first hydrogen is happy The oxygen still wants one more

H O

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Water The second hydrogen attaches Every atom has full energy levels

H OH

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use acetate sheets to work on page 244 1-5 to create lewis structures

Use a square of acetate to show each atom (use vis a vis pens only)

Show the overlap of orbitals Draw the outcome in your notebook-use

structural formula (line to represent pair) Circle the shared pairs

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Lewis structure 1. PH3

2. H2S

3. HCl

4. CCl4

5. SiH4

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Lewis structure Use molecular model kit to build 1. PH3

2. H2S

3. HCl

4. CCl4

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Covalent Bond Formation

Covalent bond forms by overlap of orbitals. Two types of bonds

Sigma bond: all single bonds are sigma bonds

Pi bond: in multiple bonds: the first one is sigma, all other bonds are pi.

There areSingle bondsMultiple bonds (double and triple only)

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Two types of Bonds Sigma bonds from

overlap of orbitals along the axis connecting the nuclei between the atoms

Pi bond ( bond): perpendicular overlap of p-orbitals above and below the axis connecting the atoms

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Sigma bond: s-s Orbital Overlap

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Pg 247- # 12 a-e

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III. Multiple BondsA. Sometimes atoms share more than

one pair of valence electrons.

B. A double bond is when atoms share two pair (4) of electrons.

C. A triple bond is when atoms share three pair (6) of electrons.

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Carbon dioxide CO2 - Carbon is central

atom ( I have to tell you)

Carbon has 4 valence electrons

Wants 4 more Oxygen has 6 valence

electrons Wants 2 more

O

C

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Carbon dioxide Attaching 1 oxygen leaves the oxygen 1

short and the carbon 3 short

OC

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Carbon dioxide Attaching the second oxygen leaves

both 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 The only solution is to share more

OCO

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Carbon dioxide The only solution is to share more

OCO

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Carbon dioxide The only solution is to share more

OCO

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Carbon dioxide The only solution is to share more

OCO

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Carbon dioxide The only solution is to share more

OCO

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Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in the

bond

OCO

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Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in the

bond

OCO8 valence electrons

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Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in the

bond

OCO8 valence electrons

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Carbon dioxide The only solution is to share more Requires two double bonds Each atom gets to count all the atoms in the

bond

OCO

8 valence electrons

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How to draw them Add up all the valence electrons. Count up the total number of electrons

to make all atoms complete octet rule. Subtract. Divide by 2 Tells you how many bonds - draw them. Fill in the rest of the valence electrons

to fill atoms up.

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Examples NH3

N - has 5 valence electrons wants 8

H - has 1 valence electrons wants 2

NH3 has 5+3(1) = 8

NH3 wants 8+3(2) = 14

(14-8)/2= 3 bonds 4 atoms with 3 bonds

N

H

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N HHH

Examples Draw in the bonds All 8 electrons are accounted for Everything is full

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Examples HCN C is central atom N - has 5 valence electrons wants 8 C - has 4 valence electrons wants 8 H - has 1 valence electrons wants 2 HCN has 5+4+1 = 10

HCN wants 8+8+2 = 18

(18-10)/2= 4 bonds 3 atoms with 4 bonds -will require multiple

bonds - not to H

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HCN Put in single bonds Need 2 more bonds Must go between C and N

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

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 N to fill octet

NH C

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Another way of indicating bonds

Often 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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Structural Examples

H C N

C OH

H

C has 8 electrons because each line is 2 electrons

Ditto for N

Ditto for C here Ditto for O

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Build the molecules using the molecular model kit.

HCN H2O NH3

CH4

C2H2

PH3

H2S http://www.youtube.com/watch?v=NYFE5usl

aNo

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http://www.youtube.com/watch?v=NYFE5uslaNo

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IV. Bond strengthA. bond strength is the energy needed to

break a covalent bond1. Bond length and atom size help

determine bond strengtha. The shorter the bond length The greater the bond strength

i. single bond – longest length ii. Double bond – medium lengthiii triple bond – shortest length

WHICH HAS THE GREATEST BOND STRENGTH?

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B. Energy is needed to create and break a covalent bond.

1.Energy is released when a covalent bond forms

2. Bond dissociation energy- is needed to break a bond

a. always a positive number

i. It takes 159KJ/mol to break F2

Would it take more or less to break N2 - why?

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V. Chemical reaction energyA. Endothermic reaction – energy is

needed1. More energy is needed to break the bond

than is needed to create new bond.

AB + energy A + B

NH4SCN + Ba(OH) + energy ( freezes to wood because heat is pulled from the reaction

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B. Exothermic reaction – energy is released

1. more energy is released when bonds form than is needed to break bonds

A + B AB + energy

CaCl + baking soda and water energy is released as heat

Hw: pg 247 1-12

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I. Naming compoundsA. Two types

1. Ionic - metal and non metal or polyatomics.

2. Covalent- we will just learn the rules for 2 non-metals.

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Covalent compounds Two words, with prefixes. Prefixes tell you how many. mono, di, tri, tetra, penta, hexa, Hepta,

octa, nona, deca First element whole name with the

appropriate prefix, except mono. Second element, -ide ending with

appropriate prefix. Practice

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Writing Formulas Two sets of rules, ionic and covalent To decide which to use, decide what the

first word is. If is a metal or polyatomic use ionic. If it is a non-metal use covalent.

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PREFIXESMono - one di - twoTri- threeTetra- four penta- five hexa- six Hepta- sevenOcta - eight nona - nine deca - ten

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CO2

CO CCl4

N2O4

XeF6

N4O4

P2O10

Naming Covalent Compounds

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Covalent compounds The name tells you how to write the

formula

Sulfur dioxide diflourine monoxide nitrogen trichloride diphosphorus pentoxide

Work on naming ditto

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I. Acids Substances that produce H+ ions when

dissolved in water. All acids begin with H. Two types of acids: Oxyacids Non Oxyacids- Binary Acids

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A. Binary Acids 1. Binary Acids( Hydrogen and one

other element

– A. hydro + element name + IC + acid

– Examples

– 1. HCl-

– 2. HF-

3. HBr-

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B. Oxyacids 1. oxyacids- (hydrogen + oxyanions)

– a. name anion + (ic or ous) + acid• i. use of ic or ous depends on the number of oxygen

atoms in the oxyanion

Examples:HNO3- nitric acid HNO2 –Nitrous acid

H3 PO4-phosphoric acid

H2PO3 phosphorous acid

HC2H3O2 –acetic acid

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H2SO4

sulfuric acid

H2SO3

sulfurous acid

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Formulas for acids hydrofluoric acid- HF

carbonic acid- H2CO3

hydrosulfuric acid

phosphorous acid

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Hydrates Some salts trap water crystals when

they form crystals. These are hydrates. Both the name and the formula needs to

indicate how many water molecules are trapped.

In the name we add the word hydrate with a prefix that tells us how many water molecules.

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Hydrates In the formula you put a dot and then

write the number of molecules. Calcium chloride dihydrate =

CaCl22 Chromium (III) nitrate hexahydrate =

Cr(NO3)3 6H2O

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I. ResonanceA. When more than one dot diagram with

the same connections are possible.

1. SO2

Which one is it? Does it go back and forth. It is a mixture of both, like a mule.

7020 Oct 97 Bonding and structure (2) 70

Sulfur Dioxide, SO2

••O OS

••

••

••

••••••

bring inleft pair

OR bring inright pair

These equivalent structuresare called:

RESONANCE STRUCTURES. The proper Lewis structure

is a HYBRID of the two.

Each atom has OCTET . . . . . BUT there is a +1 and -1 formal charge

••O OS

••

••

••

••••

••O OS••

••

••

••

••

+— —+

Rules 1-3 O—S —O

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Draw the resonance structure

O3

SO3

SO2

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Coordinate Covalent Bond When one atom donates both electrons

in a covalent bond. Carbon monoxide CO

OC

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Coordinate Covalent Bond When one atom donates both electrons

in a covalent bond. Carbon monoxide CO

OC

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Coordinate Covalent Bond When one atom donates both electrons

in a covalent bond. Carbon monoxide CO

OC

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How do we know Have to draw the diagram and see what

happens. Often happens with polyatomic ions and

acids.

Work on drawing lewis structures from packet

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