Bonding and Molecular Structure: Bonding and Molecular Structure: Fundamental ConceptsFundamental Concepts

Valence e- and BondingValence e- and BondingCovalentCovalent

IonicIonic

Resonance & Exceptions to Octet Resonance & Exceptions to Octet RuleRule

Bond Energy & LengthBond Energy & Length

Structure, Shape & Polarity of Structure, Shape & Polarity of CompoundsCompounds

AP Notes Chapter 8AP Notes Chapter 8

What is a Bond?What is a Bond? A force that holds atoms together.A force that holds atoms together. Why?Why? We will look at it in terms of energy.We will look at it in terms of energy. Bond energy the energy required to Bond energy the energy required to

break a bond.break a bond. Why are compounds formed?Why are compounds formed? Because it gives the system the lowest Because it gives the system the lowest

energy.energy.

Covalent compounds?Covalent compounds?

The electrons in each atom are The electrons in each atom are attracted to the nucleus of the other.attracted 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. The reach a distance with the lowest The reach a distance with the lowest

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

length.length.

Bond Formation

Thus Hydrogen is Diatomic!

Covalent CharacterCovalent Character

e-e-

E

He + He

He2

Inter-nuclear Distance

Why Isn’t Helium Diatomic?Why Isn’t Helium Diatomic?

. .

..

F + F FF + F F22

2p2p ____ ____ ___ ___ ____ ____ ____ ____ ___ ___ ____ ____ 2p2p

2s ____2s ____ ____ 2s ____ 2s

FF FF

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

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

The electron moves.The electron moves. Opposite charges hold the atoms Opposite charges hold the atoms

together.together.

Li + ClLi + Cl1s1s222s2s11 [Ne] 3s [Ne] 3s223p3p55

2s2s ___ ___ 3p3p _____ _____ ___ _____ _____ ___

1s1s _____ _____ 3s3s _____ _____ [Ne][Ne]

Li + ClLi + Cl

2s2s ___ ___ 3P3P _____ _____ _____ _____ _____ _____

1s1s _____ _____ 3s3s _____ _____

[Ne][Ne]

LiClLiCl

2s2s ___ ___ 3P3P _____ _____ _____ _____ _____ _____

1s1s _____ _____ 3s3s _____ _____

[Ne][Ne]

ElectronegativityElectronegativity

Describes the relative Describes the relative ability of an atom ability of an atom

within a molecule to within a molecule to attract a shared pair attract a shared pair

of electrons to itself. of electrons to itself.

ElectronegativityElectronegativity

Pauling electronegativity Pauling electronegativity values, which are unit-values, which are unit-

less, are the norm.less, are the norm.

Figure 9.9 – Kotz & Treichel

ElectronegativityElectronegativityRange from 0.7 to 4.0Range from 0.7 to 4.0

Bond: A - BBond: A - B

EN = | ENEN = | ENAA - EN - ENBB | |

Bond CharacterBond Character

““Ionic Bond” - Principally Ionic Ionic Bond” - Principally Ionic Character Character

““Covalent Bond” - PrincipallyCovalent Bond” - Principally Covalent Covalent Character Character

Determining Principal Determining Principal Character of BondCharacter of Bond

covalent ionicEN

~0 ~41.7

F - F EN = 0

Non-polar

N - O EN = |3.0 - 3.5| = 0.5

Slightly polar

N O

Ca - O EN = |1.0 - 3.5| = 2.5

Ionic Bond with somecovalent character

CaO

ElectronegativityElectronegativity The ability of an electron to attract The ability of an electron to attract

shared electrons to itself.shared electrons to itself. Pauling methodPauling method Imaginary molecule HXImaginary molecule HX Expected H-X energy = Expected H-X energy =

H-H energy + X-X energyH-H energy + X-X energy22

= (H-X) = (H-X) actualactual - (H-X) - (H-X)expectedexpected

ElectronegativityElectronegativity is known for almost every elementis known for almost every element Gives us relative electronegativities Gives us relative electronegativities

of all elements.of all elements. Tends to increase left to right.Tends to increase left to right. 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 Difference in electronegativity

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

Electronegativity difference

Bond Type

Zero

Intermediate

Large

Covalent

Polar Covalent

Ionic

Co

valent C

haracter

decreases

Ion

ic Ch

aracter increases

Dipole MomentsDipole Moments A molecule with a center of negative A molecule with a center of negative

charge and a center of positive charge and a center of positive charge is charge is dipolar dipolar (two poles),(two poles),

or has a or has a dipole moment.dipole moment. Center of charge doesn’t have to be Center of charge doesn’t have to be

on an atom.on an atom. Will line up in the presence of an Will line up in the presence of an

electric field.electric field.

How It is drawnHow It is drawn

H - F+ -

Which Molecules Have Which Molecules Have Them?Them?

Any two atom molecule with a polar Any two atom molecule with a polar bond.bond.

With three or more atoms there are With three or more atoms there are two considerations.two considerations.

There must be a polar bond.There must be a polar bond. Geometry can’t cancel it out.Geometry can’t cancel it out.

Ionic Radii -- Cations Ionic Radii -- Cations

Ionic Radii -- AnionsIonic Radii -- Anions

Molecular Polarity

Vector Sum of Bond Polarities

BrBr Mg

MgBr2

Mg - Br EN = |1.2 - 2.8| = 1.6

Covalent BOND w/much ionic character, BUT

NON-POLAR molecule

LewisStructures

The most important requirement for the

formation of a stable compound is that the atoms achieve noble gas e- configuration

Valence Shell Valence Shell ElectronElectron

Pair Repulsion ModelPair Repulsion Model(VSEPR)(VSEPR)

The structure around a The structure around a given atom is determined given atom is determined principally by minimizing principally by minimizing electron-pair repulsionselectron-pair repulsions

VSEPRVSEPRElectron

pairsBond

AnglesUnderlyingShape

2 180° Linear

3 120° Trigonal Planar

4 109.5° Tetrahedral

590° &120°

Trigonal Bipyramidal

6 90° Octagonal

LEWIS LEWIS STRUCTURESSTRUCTURES

1 : draw skeleton of species: draw skeleton of species2 : count e: count e-- in species in species3 : subtract 2 e: subtract 2 e-- for each bond for each bond

in skeletonin skeleton4 : distribute remaining e: distribute remaining e--

Distinguish Between ELECTRONIC

Geometry&

MOLECULARGeometry

CH4

Electronic geometry: tetrahedral

Molecular geometry: tetrahedral

Bond angle = 109.50

H3O+

Electronic geometry: tetrahedral

Molecular geometry: trigonal pyramidal

Bond angle ~ 1070

H2O

Electronic geometry: tetrahedral

Molecular geometry: bent

Bond angle ~ 104.50

NH2-

Electronic geometry: tetrahedral

Molecular geometry: bent

Bond angle ~ 104.50

“Octet Rule” holds for connecting atoms, but may not for the central

atom.

BaI2

Electronic geometry: linear

Molecular geometry: linear

Bond angle =1800

BF3

Electronic geometry: trigonal planar

Molecular geometry: trigonal planar

Bond angle =1200

PF5

Electronic geometry: trigonal bipyramidal

Molecular geometry: trigonal bipyramidal

Bond angle = 1200 & 900

SF4Electronic geometry: trigonal bipyramidal

Molecular geometry: see-saw

Bond angle = 1200 & 900

ICl3

Electronic geometry: trigonal bipyramidal

Molecular geometry: T-shape

Bond angle <= 900

I3-

Electronic geometry: trigonal bipyramid

Molecular geometry: linear

Bond angle = 1800

PCl6-

Electronic geometry: octahedral

Molecular geometry: octahedral

Bond angle = 900

BrF5

Electronic geometry: octahedral

Molecular geometry: square pyramidal

Bond angle ~ 900

ICl4-

Electronic geometry: octahedral

Molecular geometry: square planar

Bond angle = 900

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

What happens when there are not enough electrons

to “satisfy” the central atom?

EXAMPLESEXAMPLES

EtheneEthene

Acetic AcidAcetic Acid

OxygenOxygen

NitrogenNitrogen

RESONANCE&

FORMAL CHARGE

ResonanceResonance Sometimes there is more than one Sometimes there is more than one

valid structure for an molecule or ion.valid structure for an molecule or ion. NONO33

-- Use double arrows to indicate it is Use double arrows to indicate it is

the “average” of the structures.the “average” of the structures. It doesn’t switch between them.It doesn’t switch between them. NONO22

-- Localized electron model is based on Localized electron model is based on

pairs of electrons, doesn’t deal with pairs of electrons, doesn’t deal with odd numbers.odd numbers.

EXAMPLESEXAMPLES

Nitrate ionNitrate ion

OzoneOzone

FORMAL CHARGEFORMAL CHARGEthe charge the charge assignedassigned to an atom to an atom in a molecule or polyatomic ion in a molecule or polyatomic ion

FC atom = Family# - [LPE + ½(BE)]

Sum FC’s atoms = ion charge

Closer sum FC’s is to zero more stable

Formal ChargeFormal Charge For molecules and polyatomic For molecules and polyatomic

ions that exceed the octet there ions that exceed the octet there are several different structures.are several different structures.

Use charges on atoms to help Use charges on atoms to help decide which.decide which.

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

Formal ChargeFormal Charge The difference between the number The difference between the number

of valence electrons on the free atom of valence electrons on the free atom and that assigned in the molecule.and that assigned in the molecule.

We count half the electrons in each We count half the electrons in each bond as “belonging” to the atom.bond as “belonging” to the atom.

SOSO44-2-2

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

Negative formal charges should be Negative formal charges should be on electronegative elements.on electronegative elements.

Assignment of eAssignment of e--

1. Lone pairs belong1. Lone pairs belong entirely to atom in entirely to atom in question question

2. Shared e2. Shared e-- are are divideddivided equally between the equally between the two sharing atoms two sharing atoms

The sum of the formal charges of all atoms in a species must equal the overall charge on the species.

A useful equationA useful equation(happy-have) / 2 = bonds(happy-have) / 2 = bonds

POClPOCl33 P is central atomP is central atom

SOSO44-2-2

S is central atomS is central atom

SOSO33-2-2

S is central atomS is central atom

POPO44-2-2

P is central atomP is central atom

SClSCl22 S is central atom S is central atom

Exceptions to the octetExceptions to the octet BHBH33 Be and B often do not achieve octetBe and B often do not achieve octet Have less than and octet, for Have less than and octet, for

electron deficient molecules.electron deficient molecules. SFSF66 Third row and larger elements can Third row and larger elements can

exceed the octetexceed the octet Use 3d orbitals?Use 3d orbitals? II33

--

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

If nonequivalent Lewis structures exist, the one(s) that best describe the bonding in the species has...

FAVORED LEWIS FAVORED LEWIS STRUCTURESSTRUCTURES

1. formal charges closest to 1. formal charges closest to zero zero

2. negative formal charge2. negative formal charge

is on the most is on the most electronegative atomelectronegative atom

EXAMPLESEXAMPLES

Carbon dioxideCarbon dioxide

Thiocyanate ionThiocyanate ion

Sulfate ionSulfate ion

BONDENERGY &

LENGTH

Bond EnergiesBond Energies

E = (Bonds Broken) – (Bonds E = (Bonds Broken) – (Bonds Made)Made)

Bonds form between atoms because bonded atoms exhibit a lower energy.

Thus, energy is required to break bonds and energy is released when bonds are formed.

Bond Order = # bonds to a Bond Order = # bonds to a specific set specific set of elements of elements

C-C the BO=1C-C the BO=1

C=C the BO=2C=C the BO=2

C C the BO=3C C the BO=3

Fractions are Fractions are possiblepossible

COVALENT BONDSCOVALENT BONDS

Bond DissociationBond Dissociation

EnergyEnergy

Table 9.9 (text) Table 9.9 (text)

Bond Energy (kJ/mol)H-F 565H-Cl 432H-Br 366H-I 299

Bond Energy (kJ/mol)Cl-Cl 242Br-Br 193I-I 151

Bond Energy (kJ/mol)

835 CC

610 CC

346 CC

Bond Energy (kJ/mol)

945 NN

418 NN

163 NN

Use bond energies to predict Hc for acetylene (C2H2).

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

0

En

erg

y

Internuclear Distance

Bond Energy

Bond: Energy Length (kJ/mol) (pm)

121 835 CC

134 610 CC

154 346 CC

Bond: Energy Length (kJ/mol) (pm)

122 745 OC

143 358 OC

Binary Ionic Binary Ionic CompoundsCompounds

metalmetal(s)(s) + non-metal + non-metal (g)(g) ---> salt ---> salt(s)(s)

Lattice EnergyLattice EnergyEnergy change occurring Energy change occurring

when separated gaseous when separated gaseous ions are packed together ions are packed together to form an ionic solidto form an ionic solid

MM++(g) + NM(g) + NM--(g) --> M-NM(g) --> M-NM

What is the lattice energy of NaCl(s)?

Na+(g) + Cl-(g) ---> NaCl(s)

Lattice EnergiesLattice EnergiesLiCl 834 : BeClLiCl 834 : BeCl22 3004 3004

NaCl 769 : MgClNaCl 769 : MgCl22 2326 2326

KCl 701 : CaClKCl 701 : CaCl22 2223 2223

LiLi22O 2799 : BeO 4293O 2799 : BeO 4293

NaNa22O 2481 : MgO 3795O 2481 : MgO 3795

KK22O 2238 : CaO 3414O 2238 : CaO 3414

LE = Lattice Energy

r

QQkLE 21

Where: k = proportionality constant dependent on structure of solid and on electron configuration of the ions

Where: Q1 & Q2 = charges on the ions

Where: r = the shortest distancebetween the centers of the cation and anion