Chapter 8 “Covalent Bonding”

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Chapter 8

“Covalent Bonding”

Pre-AP Chemistry

Charles Page High School

Stephen L. Cotton

Ball-and-stick model

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Bellringer #3 (Feb 10th, 2011) Based on Section 8.1:

– How is an IONIC COMPOUND different from a MOLECULAR COMPOUND?

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Agenda (Feb 10th, 2011) Bellringer #3 Cornell Notes 8.1 Check Turn in ALL Sub work Write 8.1 Objectives 8.1 Review Hmwk: 8.2 Cornell Notes Next Test: Unit 7 and 8 combined (next

week Weds or Thurs)

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Section 8.1Molecular Compounds

OBJECTIVES:

–Distinguish between the melting points and boiling points of molecular compounds and ionic compounds.

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Section 8.1Molecular Compounds

OBJECTIVES:

–Describe the information provided by a molecular formula.

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Bonds are…Bonds are… Forces that hold groups of atoms Forces that hold groups of atoms

together and make them function together and make them function as a unit. Two types:as a unit. Two types:

1)1) Ionic bondsIonic bonds – – transfertransfer of of electrons electrons (gained or lost; makes (gained or lost; makes formula unitformula unit))

2)2) Covalent bondsCovalent bonds – – sharingsharing of of electrons. The resulting electrons. The resulting particle is called a particle is called a ““moleculemolecule””

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Covalent BondsThe word covalent is a

combination of the prefix co- (from Latin com, meaning “with” or “together”), and the verb valere, meaning “to be strong”.

Two electrons shared together have the strength to hold two atoms together in a bond.

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MoleculesMolecules Many elements found in nature

are in the form of molecules: a neutral group of atoms joined

together by covalent bonds. For example, air contains oxygen For example, air contains oxygen

molecules, consisting of two molecules, consisting of two oxygen atoms joined covalentlyoxygen atoms joined covalently

Called a “Called a “diatomicdiatomic molecule molecule” (O” (O22))

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How does H2 form?

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

++

(diatomic hydrogen molecule)

+ +

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How does H2 form?

++

But, the nuclei are attracted to the electrons

They share the electrons, and this is called a “covalent bond”, and involves only NONMETALS!

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Covalent bondsNonmetals hold on to their valence

electrons. They can’t 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 count

the electrons toward a noble gas configuration.

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Covalent bonding Fluorine has seven valence

electrons (but would like to have 8)

F

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electrons A second atom also has seven

F F

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electrons A second atom also has seven By sharing electrons…

F F

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F F

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F F

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F F

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F F

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Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons…

…both end with full orbitals

F F

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Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons… …both end with full orbitals

F F8 Valence electrons

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Covalent bonding Fluorine has seven valence electrons A second atom also has seven By sharing electrons… …both end with full orbitals

F F8 Valence electrons

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Molecular Compounds Compounds that are bonded

covalently (like in water, or carbon dioxide) are called molecular compounds

Molecular compounds tend to have relatively lower melting and boiling points 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 temperature

–Ionic 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 as

H2O–The subscript “2” behind hydrogen

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

Molecular formulas do not tell any information about the structure (the arrangement of the various atoms).

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Peer to Peer (1 minute) Ionic or Molecular Compound?

– NaCl

– CO2

– MgF2

– H2O

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- Page 215

These are some of the different ways to represent ammonia:

1. The molecular formula shows how many atoms of each element are present

2. The structural formula ALSO shows the arrangement of these atoms!

3. The ball and stick model is the BEST, because it shows a 3-dimensional arrangement.

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Exit Slip 8.1 Name various ways an IONIC

COMPOUND is different from a MOLECULAR COMPOUND. (write a detailed paragraph using your notes from today)

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Bellringer #4 (Feb 11th, 2011) Give an example of an IONIC

compound and a MOLECULAR compound.

How many electrons would the following atoms need to achieve noble gas configuration:

– C?

– P?

– H?

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Agenda (Feb 11th, 2011) Bellringer #4 Check 8.2 Cornell Notes Review 8.2: Single, Double, and

Triple Bonded Molecular Compounds

Hmwk: 8.2 Section Assessment (pg.229) #13-16 & #18-21

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Section 8.2The Nature of Covalent Bonding

OBJECTIVES:

–Describe how electrons are shared to form covalent bonds, and identify exceptions to the octet rule.

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OBJECTIVES:

–Demonstrate how electron dot structures represent shared electrons.

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OBJECTIVES:

–Describe how atoms form double or triple covalent bonds.

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OBJECTIVES:

–Describe how oxygen atoms 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 bond

because they actually form molecules.

Two specific atoms are joined. In an ionic solid, you can’t tell which

atom the electrons moved from or to

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Sodium Chloride Crystal LatticeSodium Chloride Crystal Lattice

•Ionic compounds Ionic compounds organize in a organize in a characteristic characteristic crystal latticecrystal lattice of of alternating alternating positive and positive and negative ions, negative ions, repeated over and repeated over and over.over.

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How to show the formation… It’s like a jigsaw puzzle. You put the pieces together to end up

with the right formula. Carbon is a special example - can it

really share 4 electrons: 1s22s22p2?

– Yes, due to electron promotion! Another example: lets show how water is

formed with covalent bonds, by using an electron dot diagram

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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 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 attaches Every atom has full energy levels

H OH

Note the two “unshared” pairs of electrons

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Examples:

1. Conceptual Problem 8.1 on page 220

2. Do PCl3

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Multiple Bonds Sometimes atoms share more than

one pair of 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 - Know these 7

elements as diatomic:

Br2 I2 N2 Cl2 H2 O2 F2 What’s the deal with the oxygen dot diagram?

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Dot diagram for Carbon dioxide CO2 - Carbon is central

atom ( more metallic ) 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 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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OCO

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OCO

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OCO

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OCO

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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

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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

OCO8 valence electrons

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the bond

OCO8 valence electrons

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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 draw

5) 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) valence

electrons, 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 HHH

Examples Draw in the bonds; start with singles All 8 electrons are accounted for Everything is full – done with this one.

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Bellringer #5 (Feb 14th, 2011)Write the structure for the

MOLECULAR compound HCN

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Agenda (Feb 14th, 2011) Bellringer #5 HW Check Continue 8.2 Review:

Resonance,Naming of Covalent Compounds, Polyatomic Ions

Hmwk:

– Covalent Compound Wkst

– 8.4 Cornell Notes

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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 HCN has 5+4+1 = 10

HCN wants 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 atom Need 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 to

equal 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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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; and

shown by a double-headed arrow found in double-bond structures!

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Resonance in OzoneResonance in Ozone

Neither structure is correct, it is actually a hybrid of the two. To show it, draw all varieties possible, and join them with a double-headed arrow.

Note the different location of the double bond

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ResonanceResonanceOccurs 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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Molecular compounds are easier! Ionic compounds use charges to

determine how many of each.

–You have to figure out charges.

–May need to criss-cross numbers. Molecular compounds: the name

tells you the number of atoms.

–Uses prefixes to tell you the exact number of each element present!

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Prefixes (Table 9.4, p.269) 1 = mono- 2 = di- 3 = tri- 4 = tetra- 5 = penta- 6 = hexa- 7 = hepta- 8 = octa-

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Prefixes

9 = nona- 10 = deca- To write the name, write two words:

One exception is we don’t write mono if there is only one of the first element.

Prefix name Prefix name -ide

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Practice by naming these(Exit Slip 8.2a #1):

N2O

NO2

Cl2O7

CBr4

CO2

BaCl2 (This one will not use prefixes, since it is an ionic compound!)

= dinitrogen monoxide(also called nitrous oxide or laughing gas)

= nitrogen dioxide

= dichlorine heptoxide

= carbon tetrabromide= carbon dioxide

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Write formulas for these (Exit Slip 8.2a #2):

diphosphorus pentoxide tetraiodine nonoxide sulfur hexafluoride nitrogen trioxide carbon tetrahydride phosphorus trifluoride aluminum chloride (Ionic compound)

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Polyatomic ions are… Groups of atoms that stay together and

have an overall charge, and one name. Usually end in –ate or -ite

Acetate: C2H3O21-

Nitrate: NO31-

Nitrite: NO21-

Permanganate: MnO41-

Hydroxide: OH1- and Cyanide: CN1-?

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Sulfate: SO42-

Sulfite: SO32-

Carbonate: CO32-

Chromate: CrO42-

Dichromate: Cr2O72-

Phosphate: PO43-

Phosphite: PO33-

Ammonium: NH41+

Know Table 9.3 on page 257

If the polyatomic ion begins with H, then combine the word hydrogen with the other polyatomic ion present:

H1+ + CO32- → HCO3

1-

hydrogen + carbonate → hydrogen carbonate ion

(One of the few positive polyatomic ions)

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Naming Compounds with Polyatomic Ions

FIRST NAME: Name of the FIRST atom in the compound

LAST NAME: Name of POLYATOMIC ION

Ex:

– MgSO4 = Magnesium Sulfate

– Ca(C2H3O2)2= Calcium Acetate

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Exit Slip 8.2a #3 Use pg.257 to assist you in naming the

following IONIC COMPOUND with POLYATOMIC IONS

– NaClO4

– LiCN

– Mg3(PO4)2

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Bellringer #6 (Feb 15th, 2011) Write the name for the following

compounds (you may need pg.257):

– SiO2

– C2H6

– Na2(SO4)

– Li2CO3

What is ELECTRONEGATIVITY?

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Agenda (Feb 15th, 2011) Bellringer #6 HW Check Write 8.4 Objectives Hmwk: 8.4 Wkst Review

– Writing formulas with POLYATOMIC IONS

– Section 8.4 (polar, non-polar, and ionic compounds; bond strength)

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Bond Polarity Covalent bonding means shared

electrons–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 bond that results is called a nonpolar covalent bond

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Bond Polarity When two different atoms bond

covalently, there is an unequal sharing

–the more electronegative atom will have a stronger attraction, and will acquire a slightly negative charge

–called a polar covalent bond, or

simply polar bond.

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Table of Electronegativities

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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 slight negative charge, and the hydrogen a slight positive charge

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Bond Polarity Only partial charges, much less

than a true 1+ or 1- as in ionic bond Written as:

HCl the positive and minus signs (with

the lower case delta: ) denote partial charges.

and

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Bond Polarity Can also be shown:

–the arrow points to the more electronegative atom.

Table 8.3, p.238 shows how the electronegativity can also indicate the type of bond that tends to form

H Cl

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

entire molecule “polar”

–areas of “difference” HCl has polar bonds, thus is a polar

molecule.

–A molecule that has two poles is called dipole, like HCl

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Attractions between molecules They are what make solid and liquid

molecular compounds possible. The weakest are called van der Waal’s

forces - there are two kinds:#1. Dispersion forces

weakest of all, caused by motion of e-

increases as # e- increaseshalogens start as gases; bromine is liquid; iodine is solid – all in Group 7A

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#2. Dipole interactions Occurs when polar molecules are

attracted to each other. 2. Dipole interaction happens in

water

–Figure 8.25, page 240

–positive region of one molecule attracts the negative region of another molecule.

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#2. Dipole interactions Occur when polar molecules are

attracted to each other. Slightly stronger than dispersion forces. Opposites attract, but not completely

hooked like in ionic solids.

H F

H F

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#3. Hydrogen bonding …is the attractive force caused by

hydrogen bonded to N, O, F, or Cl N, O, F, and Cl are very

electronegative, so this is a very strong dipole.

And, the hydrogen shares with the lone pair in the molecule next to it.

This is the strongest of the intermolecular forces.

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Remember!!!Three types of

INTERMOLECULAR FORCES:

–Van der Walls (dispersion forces)

–Dipole

–Hydrogen (strongest of the three)

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Order of Intermolecular attraction strengths

1) Dispersion forces are the weakest

2) A little stronger are the dipole interactions

3) The strongest is the hydrogen bonding

4) All of these are weaker than ionic bonds

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Attractions and properties Why are some chemicals gases,

some liquids, some solids?

–Depends on the type of bonding!

–Table 8.4, page 244 Network solids – solids in which

all the atoms are covalently bonded to each other

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Attractions and properties Figure 8.28, page 243 Network solids melt at very high

temperatures, or not at all (decomposes)

–Diamond does not really melt, but vaporizes to a gas at 3500 oC and beyond

–SiC, used in grinding, has a melting point of about 2700 oC

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Covalent Network CompoundsCovalent 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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Chapter 8 “Covalent Bonding”

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