8.7 Covalent Bonding

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8.7 Covalent Bonding8.7 Covalent Bonding

• Molecules arise from localized attractive Molecules arise from localized attractive forces between atoms, which we call covalent forces between atoms, which we call covalent bondsbonds

• Atoms in the molecule are connected strongly, but Atoms in the molecule are connected strongly, but molecules are not strongly attracted to each other.molecules are not strongly attracted to each other.

• Molecular compounds are usually gases or liquids Molecular compounds are usually gases or liquids unless the molecules are very largeunless the molecules are very large

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Single Covalent BondsSingle Covalent Bonds

• Sharing of 1 pair of electronsSharing of 1 pair of electrons

• Each atom has one half-filled valence Each atom has one half-filled valence orbital that overlap one anotherorbital that overlap one another

• HH.. + + ..H H H:H H:H

• Single bond represented asSingle bond represented asH:H or H–HH:H or H–HCalled a Lewis formula or electron-dot Called a Lewis formula or electron-dot formulaformula

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8.10 Lewis Symbols8.10 Lewis Symbols

• Lewis Symbols:Lewis Symbols:The number of The number of valence electrons valence electrons available for available for bonding are bonding are indicated by indicated by unpaired dots.unpaired dots.

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Lewis SymbolsLewis Symbols

• We generally place the We generally place the electrons on four sides electrons on four sides of a square around the of a square around the element symbol.element symbol.

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Single bonds between like atomsSingle bonds between like atoms

• HalogensHalogens

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Single bonds between unlike atoms

Single bonds between unlike atoms

• HFHF Figure 7.10Figure 7.10

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Some atoms can form bonds with more than one atom

Some atoms can form bonds with more than one atom

• CClCCl44

Figure 7.10Figure 7.10

How many How many valence electrons valence electrons are supplied by are supplied by each atom?each atom?

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Lewis Model of Covalent Bonding (Localized electron model)

Lewis Model of Covalent Bonding (Localized electron model)

1. A covalent bond is formed by the sharing of at least two valence electrons between two atoms.

2. At normal temperatures and pressures, for non-metals, the maximum number of valence electrons per atom is eight (with the exception of two for H and He).• Each shared electron is counted twice (one per

atom).

3.3. A A stable moleculestable molecule is formed when sharing of is formed when sharing of valence electrons lowers the chemical energy.valence electrons lowers the chemical energy.

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Procedure for Drawing Lewis Structures

Procedure for Drawing Lewis Structures

1.1. Arrange the atoms (symbols) around the central Arrange the atoms (symbols) around the central atom. atom.

2.2. Count up total valence electronsCount up total valence electrons3.3. Draw a line to represent a single bond (2 shared Draw a line to represent a single bond (2 shared

electrons) between the central atom and each electrons) between the central atom and each surrounding atom. This is a 2-electron bond surrounding atom. This is a 2-electron bond between the cores.between the cores.

4.4. Place remaining electrons on the outside atoms, Place remaining electrons on the outside atoms, then the central atomthen the central atom

5.5. Shift electrons, as necessary, to make multiple Shift electrons, as necessary, to make multiple bonds and satisfy the octet rulebonds and satisfy the octet rule

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Whiteboard Work #1:Whiteboard Work #1:

• Draw the Lewis Structures for the Draw the Lewis Structures for the following molecules:following molecules:• BFBF33

• NHNH33

• NONO22

• COCO22

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8.11 Exceptions to the Octet “Rule”

8.11 Exceptions to the Octet “Rule”

• Incomplete OctetsIncomplete Octets• Usually only seen for Be, B, Al• Leads to stable, yet reactive molecules!

• Coordinate Covalent BondingCoordinate Covalent Bonding• Molecules with too few electron pairs can Molecules with too few electron pairs can

bond with molecules with unshared electron bond with molecules with unshared electron pairs to form a new shared-electron-pair pairs to form a new shared-electron-pair bondbond

• BFBF33 + NH + NH33 F F33BNHBNH33

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Exceptions to the Octet Rule


• Why is BHWhy is BH44-- more stable than BH more stable than BH33??

• Why is BFWhy is BF44-- more stable than BF more stable than BF33??

• Why does aluminum chloride exist in the Why does aluminum chloride exist in the gaseous state as Clgaseous state as Cl22AlClAlCl22AlClAlCl22 (that is, (that is,

AlAl22ClCl66) instead of AlCl) instead of AlCl33??

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8.11 Exceptions to the Octet Rule

8.11 Exceptions to the Octet Rule

• Odd-Electron MoleculesOdd-Electron Molecules

• Also stable, yet reactive!Also stable, yet reactive!

• Examine the Lewis formula for NOExamine the Lewis formula for NO22

• Why does NOWhy does NO22 combine with itself to combine with itself to

form Nform N22OO44??

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CO2CO2

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Multiple BondsMultiple Bonds

• Can share more than one pair of electrons Can share more than one pair of electrons to form double or triple bondsto form double or triple bonds

2 electron pairs2 electron pairs 3 electron pairs3 electron pairs

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Whiteboard Work #2:Whiteboard Work #2:

• Draw the Lewis structure for SOSO22

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8.12 Resonance Structures8.12 Resonance Structures

• Lewis formulas don’t always accurately Lewis formulas don’t always accurately represent bonds. Sometimes it takes two represent bonds. Sometimes it takes two formulas to adequately represent the formulas to adequately represent the bonds.bonds.

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Resonance StructuresResonance Structures

• In the sulfur dioxide example, we could move the electron pair from either oxygen to form the double bond. The two structures we drew were equivalent. These are examples of equivalent resonance structures.

• The Lewis-dot representation suggests that the molecule exists as either one structure or the other, and that it sort of “flips” back and forth. This is NOT the case.

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• Rather the electron pair of the double bond is sort of “smeared out” over both bond positions. This “smearing out” is referred to as delocalization, and can be represented using a dashed line.

• Each S–O bond is neither a single or a double bond. The three electron pairs distribute themselves among the 2 bonding positions and is basically a 3/2 = 1.5 bond.

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ResonanceResonance

• How many different valid Lewis formulas How many different valid Lewis formulas can you write for the following molecules or can you write for the following molecules or ions? How do they differ?ions? How do they differ?

SOSO22 SOSO33

HH22SOSO44 COCO3322--

NONO33-- HNOHNO33

NCSNCS--

• The different resonance forms represent The different resonance forms represent delocalized bonding.delocalized bonding.

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Write Lewis formulas for the following molecules or ions:


• SOSO22

• SOSO33

• HH22SOSO44

• COCO3322--

• NONO33--

• HNOHNO33

• NCSNCS--

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Formal ChargeFormal Charge

• Can be used to decide between alternate Can be used to decide between alternate Lewis structuresLewis structures

• There is considerable controversy as to There is considerable controversy as to whether the concept of formal charge whether the concept of formal charge dictating electron distribution is in fact dictating electron distribution is in fact correct.correct.

• You can read more about it in your text.You can read more about it in your text.

• Is helpful for evaluating Lewis structuresIs helpful for evaluating Lewis structures

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Whiteboard Work #3Whiteboard Work #3

• Draw the Lewis structure for I31–

• Draw the Lewis structure for SbF5

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Exception to the Octet RuleException to the Octet Rule

• Some bonding situations result in the central Some bonding situations result in the central atom having > 4 electron pairs ( > 8 atom having > 4 electron pairs ( > 8 electrons) around it:electrons) around it:• S can form SHS can form SH22 with S having 8 valence electrons with S having 8 valence electrons

• S can form SClS can form SCl44 with S having 10 valence with S having 10 valence

electronselectrons

• S can form SClS can form SCl66 with S having 12 valence with S having 12 valence

electronselectrons

• How is this possible??? (Think QM…)How is this possible??? (Think QM…)

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Expanded OctetsExpanded Octets

• Non-Metal atoms in Period 3 or higher can have more than 8 electrons around them (i.e. in their valence shell or top shelf)

• Why is this? Recall period 3 atoms (and period 4, 5, 6… atoms) have d-orbitals that can hold those extra electrons!

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When do we find expanded octets? What When do we find expanded octets? What is the origin of the octet rule?is the origin of the octet rule?



• Expanded Valence ShellsExpanded Valence Shells

• What do you do if there are too many What do you do if there are too many electrons to be accommodated by octets?electrons to be accommodated by octets?

• Write Lewis formulas for the following:Write Lewis formulas for the following:SFSF44 SFSF66 IFIF44

++ XeFXeF44 XeF XeF22

PFPF55 BrFBrF33 BrFBrF55

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Modern View of Lewis Bonding Model

Modern View of Lewis Bonding Model

1.1. A A stable moleculestable molecule is formed when sharing of electrons is formed when sharing of electrons lowers the chemical energy.lowers the chemical energy.

2.2. For For non-metalsnon-metals, the number of valence electrons equals , the number of valence electrons equals the column number. The maximum number of valence the column number. The maximum number of valence electrons per core is 8 (2 for H and He).electrons per core is 8 (2 for H and He).

3.3. UsuallyUsually a covalent bond is formed by sharing 2 valence a covalent bond is formed by sharing 2 valence electrons between 2 cores.electrons between 2 cores.

4.4. At low temperaturesAt low temperatures, a non-reactive substance , a non-reactive substance usuallyusually has the maximum number of electrons per core (octet has the maximum number of electrons per core (octet rule). There are some exceptions.rule). There are some exceptions.

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• Website:Website:

http://www.kentchemistry.com/links/http://www.kentchemistry.com/links/bonding/lewisdotstruct.htmbonding/lewisdotstruct.htm

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• NHNH33

• NHNH44++

• CClCCl22FF22

• SOClSOCl22

• SOSO22

• COCO22

• COCO3322--

• SOSO3322--

• HH22SOSO44

• HCNHCN

• CNCN--

• NCSNCS--

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Lewis StructuresLewis Structures

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Ch. 8 Group QuizCh. 8 Group Quiz

• Write Lewis formulas for the Write Lewis formulas for the following molecules or ions:following molecules or ions:

1.1. PClPCl33

2.2. COCO

3.3. HH22OO22 (HOOH)(HOOH)

4.4. SOSO4422--

Put your names and class Put your names and class date in the heading!!!date in the heading!!!

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Origins of the “Octet Rule”Origins of the “Octet Rule”

• Octet rule: we know that Octet rule: we know that ss22pp66 is a noble gas is a noble gas configuration. We assume that an atom is configuration. We assume that an atom is stable when surrounded by 8 electrons (4 stable when surrounded by 8 electrons (4 electron pairs). (there are many electron pairs). (there are many exceptions)exceptions)

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Octet RuleOctet Rule

• What ion or compound is formed from What ion or compound is formed from the following to approximate a noble gas the following to approximate a noble gas electronic configuration? What is the electronic configuration? What is the configuration? configuration?

• NaNa 1s1s222s2s222p2p663s3s11

• NaNa++ 1s1s222s2s222p2p66

• HH 1s 1s11

• HH++ or H or H-- 1s1s0 0 or 1sor 1s22

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• ClCl 1s1s222s2s222p2p663s3s223p3p55

• ClCl-- 1s1s222s2s222p2p663s3s223p3p66

• OO 1s1s222s2s222p2p44

• OO22-- 1s1s222s2s222p2p66

• H + OH + O 1s1s11 + 1s + 1s222s2s222p2p44

• HH22OO 1s1s222s2s222p2p66 for O, 1s for O, 1s22 for H for H

• Na + ONa + O 1s1s222s2s222p2p663s3s11 + 1s + 1s222s2s222p2p44

• NaNa22OO 1s1s222s2s222p2p6 6 for Na and O for Na and O


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• C + H C + H 1s1s222s2s222p2p22 + 1s + 1s11 • CHCH44 1s1s222s2s222p2p6 6 for C, 1sfor C, 1s22 for H for H

• C + ClC + Cl 1s1s222s2s222p2p22 + 1s + 1s222s2s222p2p663s3s223p3p55 • CClCCl4 4 1s1s222s2s222p2p66 for C, for C,

1s1s222s2s222p2p663s3s223p3p6 6 for Clfor Cl• C + OC + O 1s1s222s2s222p2p22 + 1s + 1s222s2s222p2p44 • COCO22 1s1s222s2s222p2p6 6 for C and Ofor C and O


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1.1. NONO

1.1. NONO22

2.2. NN22OO44

2.2. BHBH33

3.3. BHBH44--

3.3. AlClAlCl33

4.4. SFSF4 4

4.4. SFSF66

5.5. IFIF44++

5.5. XeFXeF44

6.6. XeFXeF22

6.6. PFPF5 5

• BrFBrF55

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8.7 Covalent Bonding