Chemical BondingChemical Bonding

Chapters 8 and 9Chapters 8 and 9

Chemical BondsChemical Bonds

What is a bond?What is a bond?A force that holds atoms togetherA force that holds atoms togetherWe will look at it in terms of energyWe will look at it in terms of energyBond Energy is the NRG required to Bond Energy is the NRG required to

break a bondbreak a bondWhy are compounds formed?Why are compounds formed?Bonds give the system the lowest NRGBonds give the system the lowest NRG

Bond Energy is the energy Bond Energy is the energy required to break a bond.required to break a bond.

REMEMBER:REMEMBER:• It It alwaysalways takes energy to break a takes energy to break a

chemical bond. chemical bond. AND,AND,• To form a bond, requires a To form a bond, requires a loweringlowering of of

energy.energy.

Types of BondsTypes of BondsIonic BondsIonic Bonds – electrostatic forces that – electrostatic forces that

exist between ions of opposite chargeexist between ions of opposite chargeCovalent BondsCovalent Bonds – sharing of electrons – sharing of electrons

between two atomsbetween two atomsMetallic BondsMetallic Bonds – found typically in – found typically in

transition metals; each atom is bonded transition metals; each atom is bonded to several neighboring atoms; bonding to several neighboring atoms; bonding electrons are relatively free to move electrons are relatively free to move throughout the structure of the metal.throughout the structure of the metal.

Ionic BondingIonic Bonding• An atom with a low ionization energy An atom with a low ionization energy

reacts with an atom with high electron reacts with an atom with high electron affinity.affinity.

• Typically a metal with a nonmetalTypically a metal with a nonmetal• The electron transfers atoms.The electron transfers atoms.• The ions each achieve a Noble Gas The ions each achieve a Noble Gas

electron configuration = low energy electron configuration = low energy state.state.

• Opposite charges hold ions together.Opposite charges hold ions together.

Crystal LatticeCrystal Lattice• A repeating A repeating ++ and and crystal lattice results. crystal lattice results.

NaCl, sodium chloride

Ionic CompoundsIonic Compounds• Makes a solid crystal.Makes a solid crystal.• Ions align themselves to maximize Ions align themselves to maximize

attractions between opposite charges,attractions between opposite charges, and to minimize repulsion between like and to minimize repulsion between like

ions.ions.• Chemical formula is actually the Chemical formula is actually the

empirical formula, called the “formula empirical formula, called the “formula unit”unit”

ClNaClNaClNaClNaClNaClNaClNaClNaClNaClNaNaClNaClNaClNaClNaCl = “NaCl”NaClNaClNaClNaClNaCl = “NaCl”ClNaClNaClNaClNaClNaClNaClNaClNaClNaClNa

Features of Ionic CompoundsFeatures of Ionic Compounds

• Brittle, hard, crystalline solids at room Brittle, hard, crystalline solids at room temperaturetemperature

• High melting pointsHigh melting points• Metals bonded to non-metalsMetals bonded to non-metals• Elements from opposites sides of the Elements from opposites sides of the

Periodic TablePeriodic Table• Dissolve in water to form ionsDissolve in water to form ions• Conduct an electric current in waterConduct an electric current in water

Coulomb’s LawCoulomb’s LawExpresses the NRG of interaction Expresses the NRG of interaction

between a pair of ions.between a pair of ions.

E= 2.31 x 10E= 2.31 x 10-19-19 J · nm (Q J · nm (Q11QQ22) / r) / rE = energy of interaction between a pair E = energy of interaction between a pair

of ions (in Joules)of ions (in Joules)r = distance (in nm) between ion centersr = distance (in nm) between ion centers QQ1 1 andand QQ22 = charges of the ions = charges of the ionsOpposite charges means (–E)Opposite charges means (–E)Endo or Exo? What does that mean Endo or Exo? What does that mean

about NRG in the system?about NRG in the system?

Size of IonsSize of Ions

• Ion size increases down a group.Ion size increases down a group.• Cations are smaller than the atoms they Cations are smaller than the atoms they

came from.came from.• Anions are larger.Anions are larger.• across a row they get smaller, and then across a row they get smaller, and then

suddenly larger.suddenly larger.

Periodic TrendsPeriodic Trends• Across the period effective nuclear Across the period effective nuclear

charge increases so they get smaller.charge increases so they get smaller.• Energy level changes between anions Energy level changes between anions

and cations.and cations.

Li+1

Be+2

B+3

C+4

N-3O-2 F-1

Size of Isoelectronic ionsSize of Isoelectronic ions• Positive ions have more protons so they are Positive ions have more protons so they are

smaller.smaller.• A stronger + charged nucleus pulls the A stronger + charged nucleus pulls the

electrons inward.electrons inward.

Al+3

Mg+2

Na+1 Ne F-1 O-2 N-3

Lattice EnergyLattice EnergyThe energy release that occurs when The energy release that occurs when

separated gaseous ions are packed separated gaseous ions are packed together to form an ionic solidtogether to form an ionic solid

XX+x+x(g) + Y(g) + Y-y-y(g) (g) X XyyYYxx (s) + energy (s) + energy

Lattice NRG Lattice NRG (E(Eelel) = k(Q) = k(Q11QQ22)/r)/rk = constantk = constant QQ1 1 andand QQ22= charges of ions= charges of ionsr = distance between ion centersr = distance between ion centers

Example:Example:Which has the most exothermic lattice Which has the most exothermic lattice

energy, NaCl or KCl?energy, NaCl or KCl?NaCl … why?NaCl … why?Since both have the same charges (+1 Since both have the same charges (+1

and -1), the distance between the and -1), the distance between the charges needs to be considered. Since charges needs to be considered. Since Na+ is smaller than K+, the distance Na+ is smaller than K+, the distance between the centers of Na and Cl is less between the centers of Na and Cl is less and therefore has a greater lattice NRG.and therefore has a greater lattice NRG.

Example 2 and 3Example 2 and 3Arrange the following ionic compounds Arrange the following ionic compounds

in order of increasing lattice energy: in order of increasing lattice energy: NaF, CsI, and CaONaF, CsI, and CaO

CsI < NaF < CaOCsI < NaF < CaOWhich substance would you expect to Which substance would you expect to

have the greatest lattice energy, AgCl, have the greatest lattice energy, AgCl, CuO or CrN?CuO or CrN?

CrNCrN

Covalent BondsCovalent Bonds• Bond is a force which causes a group of Bond is a force which causes a group of

atoms to behave as a single unit.atoms to behave as a single unit.• Electrons are shared by atoms.Electrons are shared by atoms.• Electron orbitals must overlap.Electron orbitals must overlap.

The Covalent BondThe Covalent Bond

• The electrons in each atom are attracted The electrons in each atom are attracted to the nucleus of the other.to the nucleus of the other.

• The electrons repel each other,The electrons repel each other,• The nuclei repel each other.The nuclei repel each other.• They reach a distance with the lowest They reach a distance with the lowest

possible energy.possible energy.• The distance between is the bond length.The distance between is the bond length.

0

En

erg

y

Internuclear Distance

0

En

erg

y

Internuclear Distance

0

En

erg

y

Internuclear Distance

0

En

erg

y

Internuclear Distance

0

En

erg

y

Internuclear Distance

Bond Length(They reach a distance apart

with the lowest possible energy.)

0

En

erg

y

Internuclear Distance

Bond Energy

Features of Covalent Compounds(aka: “Molecular Compounds”)

1. Electrons are shared, (Single, Double or Triple Bonds are possible.)

2. Non-metals bonded to Non-metals.

3. Includes all diatomic molecules.

4. Relatively low melting/boiling points.

5. No repeating formula, particle is a single unit called a “molecule”.

How We Represent Covalent Compounds

1. Molecular Formulas

2. Structural Formulas H

H - C - H

H

CH4

Show the bondsbetween the atoms.

Covalent vs. Ionic BondingCovalent vs. Ionic Bonding

• Covalent is sharing, ionic is stealing.Covalent is sharing, ionic is stealing.• Totally different from each other.Totally different from each other.• Reality Check:Reality Check: There are many There are many

compounds which exhibit both traits!compounds which exhibit both traits!• These are called These are called polar covalent bondspolar covalent bonds..• The electrons are shared, but shared The electrons are shared, but shared

unequally.unequally.

Polar Covalent BondsPolar Covalent Bonds• The electrons are shared, but they are The electrons are shared, but they are

not shared evenly.not shared evenly.• One atom has the pair more often than One atom has the pair more often than

the other.the other.• A “polar” molecule results: One end is A “polar” molecule results: One end is

slightly positive, while the other is slightly slightly positive, while the other is slightly negative.negative.

• A partial charge is called a “dipole”.A partial charge is called a “dipole”.

H - F

+ -

Example:

H - F+ -

H - F

+-H - F+

-

H - F

+-

H - F +-

H - F+-

H - F

+-

H - F

+-

H - F+ -

H - F

+-H - F+

-

H - F

+-

H - F +-

H - F+-

H - F

+-

H - F

+-

+-

H - F+ -

H - F+ -

H - F+ - H - F

+ -

H - F+ -

H - F+ -

H - F+ -

H - F+ -

- +

How Do We Know Which Atom Has How Do We Know Which Atom Has the Electron Pair More Often?the Electron Pair More Often?

• Answer: Answer: Electronegativity ValuesElectronegativity Values• Electronegativity - The ability of an atom Electronegativity - The ability of an atom

to attract shared electrons to itself.to attract shared electrons to itself.• The more electronegative an atom, the The more electronegative an atom, the

more often it has the shared pair.more often it has the shared pair.• Greater electronegativity = Greater electronegativity = pole pole

ElectronegativityElectronegativity• E.N. values are assigned for almost E.N. values are assigned for almost

every element (Figure 8.6, p. 285)every element (Figure 8.6, p. 285)• Gives us relative electronegativities of Gives us relative electronegativities of

all elements.all elements.• Tends to increase left to right and Tends to increase left to right and

decreases as you go down a group.decreases as you go down a group.• Noble gases aren’t discussed.Noble gases aren’t discussed.• Difference in electronegativity between Difference in electronegativity between

atoms tells us how polar.atoms tells us how polar.

Helpful Number Line:Helpful Number Line:• Determine the Electronegativity Value Determine the Electronegativity Value

difference between two atomsdifference between two atoms

0 0.4 2.0 3.4

Non-polarcovalent

Polar covalent ionic

Electronegativity difference

Bond Type

Zero

Intermediate

Large

Covalent

Polar Covalent

Ionic

Co

valent C

haracter

decreases

Ion

ic Ch

aracter increases

Polar Covalent Bond-Polar Covalent Bond-How it is drawnHow it is drawn

H - F+ -

Reminder on Reminder on Writing Formulas and NomenclatureWriting Formulas and Nomenclature

Ionic Compounds:Ionic Compounds:Name the cation then name the anionName the cation then name the anionWhen writing formulas, add subscripts to When writing formulas, add subscripts to

make sure that the charges balancemake sure that the charges balanceCovalent Compounds:Covalent Compounds:When naming, use prefixes for the When naming, use prefixes for the

subscripts, the 2subscripts, the 2ndnd atom will end in –ide atom will end in –ide Write formulas, assign subscripts based Write formulas, assign subscripts based

on the prefixeson the prefixes

Lewis Dot Structures are Models to Lewis Dot Structures are Models to represent both ionic and covalent represent both ionic and covalent

What is a Model?What is a Model?• Explains how nature operates.Explains how nature operates.• Derived from observations.Derived from observations.• It simplifies and categorizes the It simplifies and categorizes the

information.information.• A model must be sensible, but it has A model must be sensible, but it has

limitations.limitations.

Properties of a ModelProperties of a Model• A human invention, not a blown up A human invention, not a blown up

picture of nature.picture of nature.• Models can be wrong, because they are Models can be wrong, because they are

based on speculations and based on speculations and oversimplification.oversimplification.

• You must understand the assumptions in You must understand the assumptions in the model, and look for weaknesses.the model, and look for weaknesses.

• We learn more when the model is wrong We learn more when the model is wrong than when it is right.than when it is right.

Lewis StructuresLewis Structures• Show how the valence electrons are Show how the valence electrons are

arranged.arranged.• One dot for each valence electron.One dot for each valence electron.• A stable compound has all its atoms A stable compound has all its atoms

with a noble gas configuration.with a noble gas configuration.• Hydrogen follows the duet rule.Hydrogen follows the duet rule.• The rest follow the octet rule.The rest follow the octet rule.• Bonding pair is the one between the Bonding pair is the one between the

symbols.symbols.

X32

14

75

8

6

The X represents the symbol for the element.

The dots are placed around the symbol in the order shown above.

RulesRules1.1. Sum the # of ALL the valence electrons.Sum the # of ALL the valence electrons.2.2. Determine the central atom. The Determine the central atom. The least least

electronegative element is centralelectronegative element is central (H never (H never central, C nearly always central)central, C nearly always central)

3.3. Write symbols for the atoms to show which Write symbols for the atoms to show which atoms are attached and connect them with a atoms are attached and connect them with a single bond single bond (a dash)(a dash)

4.4. Complete the octets of the atoms attached to Complete the octets of the atoms attached to the central atom (except for H, follows a duet the central atom (except for H, follows a duet rule).rule).

5.5. Place Place any leftover electrons on the central any leftover electrons on the central atomatom. Not enough electrons - consider a . Not enough electrons - consider a double or triple bond.double or triple bond.

A useful equation:A useful equation:

( happy - have ) ( happy - have ) 2 = # bonds 2 = # bonds (what they want - what they have) (what they want - what they have) 2 = # bonds 2 = # bonds

H2O

(12 - 8) 2 = 2 bonds

H O H

Practice StructuresPractice StructuresPClPCl33 IClIClCHCH44

CHCH22ClCl22HCNHCNNONO++

COCO22

HH22

NHNH33

CC22HH44

BrOBrO33-1-1

OO22

ClOClO22-1-1

OHOH-1-1

POPO44-3-3

Partial Ionic CharacterPartial Ionic Character• There are probably no totally ionic There are probably no totally ionic

bonds between individual atoms.bonds between individual atoms.

% Io

nic

Ch

arac

ter

Electronegativity difference

25%

50%

75%

How do we deal with it?How do we deal with it?• If bonds can’t be ionic, what are ionic If bonds can’t be ionic, what are ionic

compounds?compounds?• An An ionic compoundionic compound will be defined as will be defined as

any substance that any substance that conducts electricity conducts electricity when meltedwhen melted..

• Also use the generic term Also use the generic term saltsalt..• As it turns out, most compounds fall As it turns out, most compounds fall

somewhere between ionic and covalent.somewhere between ionic and covalent.

The bond is a human invention.The bond is a human invention.It is a method of explaining the energy It is a method of explaining the energy

change associated with forming change associated with forming molecules.molecules.

Bonds don’t exist in nature, but are Bonds don’t exist in nature, but are useful. useful.

We have a model of a bond.We have a model of a bond.

Exceptions to the octetExceptions to the octet• Less than an OctetLess than an Octet

BHBH33 Be and B often do not achieve octet, Be and B often do not achieve octet,

but form highly reactive compoundsbut form highly reactive compounds• More than an OctetMore than an Octet

SFSF66 and I and I33--

Third row and larger elements can Third row and larger elements can exceed the octet.exceed the octet.

How? Use 3d orbitalsHow? Use 3d orbitalsOdd Number of ElectronsOdd Number of Electrons

NO, NONO, NO22, and ClO, and ClO22

Exceptions to the octetExceptions to the octet• When we must exceed the octet, When we must exceed the octet,

extra electrons go on central atom.extra electrons go on central atom.

ClFClF33

XeOXeO33

IClICl44--

BeClBeCl22

Resonance StructuresResonance Structures

When more than one valid Lewis When more than one valid Lewis structure can be written for a particular structure can be written for a particular molecule.molecule.

Actual structure is an average of the Actual structure is an average of the depicted resonance structuresdepicted resonance structures

Drawn by writing the variant structures Drawn by writing the variant structures connected by a double-headed arrowconnected by a double-headed arrow

What to do when more than one Lewis What to do when more than one Lewis structure works? structure works? Use Formal ChargeUse Formal Charge

• Assign formal chargesAssign formal charges on atoms to on atoms to help decide which is best.help decide which is best.

• Trying to use the oxidation numbers to Trying to use the oxidation numbers to put charges on atoms in molecules put charges on atoms in molecules doesn’t work.doesn’t work.

• Molecules try to achieve as low a formal Molecules try to achieve as low a formal charge as possible.charge as possible.

• Negative formal charges should be on Negative formal charges should be on the most electronegative elements.the most electronegative elements.

Formal ChargeFormal ChargeNumber of valence electrons on the free Number of valence electrons on the free

atom atom minusminus number of valence electrons number of valence electrons assigned to the atom in the moleculeassigned to the atom in the moleculeLone pair e- belong to atom in questionLone pair e- belong to atom in questionShared e- are divided equally between the Shared e- are divided equally between the

sharing atomssharing atomsThe sum of the formal charges of all The sum of the formal charges of all

atoms in a given molecule or ion must atoms in a given molecule or ion must equal the overall charge on that speciesequal the overall charge on that speciesIf the charge on an ion is -2, the sum of the If the charge on an ion is -2, the sum of the

formal charges must be -2.formal charges must be -2.

Resonance ExamplesResonance Examples

NONO33-1-1

SOSO33

HCOHCO22-1-1

Bond Lengths are AveragesBond Lengths are Averages• Have made a table of the averages of Have made a table of the averages of

different types of bonds pg. 305different types of bonds pg. 305• single bond one pair of electrons is single bond one pair of electrons is

shared.shared.• double bond two pair of electrons are double bond two pair of electrons are

shared.shared.• triple bond three pair of electrons are triple bond three pair of electrons are

shared.shared.• More bonds, shorter bond lengthMore bonds, shorter bond length..

HowHow do they share electrons? do they share electrons?Localized Electron ModelLocalized Electron Model

• Simple model, easily applied.Simple model, easily applied.• A molecule is composed of atoms that are A molecule is composed of atoms that are

bound together by sharing pairs of bound together by sharing pairs of electrons using the atomic orbitals of the electrons using the atomic orbitals of the bound atoms.bound atoms.

Three Parts:Three Parts:

1)1) Valence electrons using Lewis structuresValence electrons using Lewis structures

2)2) Prediction of geometry using VSEPRPrediction of geometry using VSEPR

3)3) Description of the types of orbitals Description of the types of orbitals

VSEPR: the 3-D shapeVSEPR: the 3-D shape• Lewis structures tell us how the atoms Lewis structures tell us how the atoms

are connected to each other.are connected to each other.• They don’t tell us anything about shape.They don’t tell us anything about shape.• The shape of a molecule can greatly The shape of a molecule can greatly

affect its properties.affect its properties.• VValence alence SShell hell EElectron lectron PPair air RRepulsion epulsion

Theory allows us to predict geometryTheory allows us to predict geometry

VSEPRVSEPR• Molecules take a shape that Molecules take a shape that puts puts

electron pairs as far away from each electron pairs as far away from each other as possibleother as possible..

• Have to draw the Lewis structure to Have to draw the Lewis structure to determine electron pairs.determine electron pairs.

• count # bonding pairscount # bonding pairs• count # nonbonding (lone) pairscount # nonbonding (lone) pairs• Lone pair take up more space.Lone pair take up more space.• Multiple bonds count as one pair.Multiple bonds count as one pair.

VSEPRVSEPR• The number of pairs determinesThe number of pairs determines

bond anglesbond anglesunderlying structureunderlying structure

• The number of atoms determines The number of atoms determines actual shapeactual shape

VSEPRVSEPRElectron

pairsBond

AnglesUnderlyingShape

2 180° Linear

3 120° Trigonal Planar

4 109.5° Tetrahedral

590° &120°

Trigonal Bipyramidal

6 90° Octagonal

Actual shapeActual shape

ElectronPairs

BondingPairs

Non-Bonding

Pairs Shape

2 2 0 linear

3 3 0 trigonal planar

3 2 1 bent4 4 0 tetrahedral4 3 1 trigonal pyramidal4 2 2 bent

Actual ShapeActual Shape

ElectronPairs

BondingPairs

Non-Bonding

Pairs Shape

5 5 0 trigonal bipyrimidal

5 4 1 See-saw

5 3 2 T-shaped5 2 3 linear

Actual Shape

ElectronPairs

BondingPairs

Non-Bonding

Pairs Shape

6 6 0 Octahedral

6 5 1 Square Pyramidal

6 4 2 Square Planar6 3 3 T-shaped6 2 1 linear

How well does it work?How well does it work?• Does an outstanding job for such a simple Does an outstanding job for such a simple

model.model.• Predictions are almost always accurate.Predictions are almost always accurate.• Like all simple models, it has exceptions.Like all simple models, it has exceptions.• We’ll spend some time in this class We’ll spend some time in this class

drawing structures and making models to drawing structures and making models to better understand the different VSEPR better understand the different VSEPR modelsmodels

HybridizationHybridizationThe mixing of two or more atomic The mixing of two or more atomic

orbitals of similar energies on the same orbitals of similar energies on the same atom to produce new orbitals of equal atom to produce new orbitals of equal energies.energies.

Creates Hybrid OrbitalsCreates Hybrid OrbitalsMethane goes from 2sMethane goes from 2s22 2p 2p22 to 2sp to 2sp33

Draw the sub orbitals according to Draw the sub orbitals according to Hund’s ruleHund’s rule

Types of Hybrid OrbitalsTypes of Hybrid Orbitals

Two orbital sets = sp Two orbital sets = sp Three orbital sets = spThree orbital sets = sp22

Four orbital sets = spFour orbital sets = sp33

Five orbital sets = spFive orbital sets = sp33ddSix orbital sets = spSix orbital sets = sp33dd22

Bond TypesBond TypesSigma Bonds (Sigma Bonds ())Bond in which the electron pair is shared in Bond in which the electron pair is shared in

an area centered on a line running between an area centered on a line running between the atoms the atoms

Lobes of bonding orbital point toward each Lobes of bonding orbital point toward each otherother

Pi Bonds (Pi Bonds ())Electron pair above and below the Electron pair above and below the bond bondCreated by overlapping of non-hybridized p Created by overlapping of non-hybridized p

orbitalsorbitals

Single bonds consist of one Single bonds consist of one bond bondDouble bonds consist of one Double bonds consist of one bond bond

and one and one bond bondTriple bonds consist of one Triple bonds consist of one bond bond

and 2 and 2 bonds bonds