Prof. J. T. Spencer, CHE 106 1 Chapter Eight Copyright © James T. Spencer 1995 - 1998 All Rights Reserved

Prof. J. T. Spencer, CHE 106

1 Chapter EightChapter Eight

Copyright © James T. Spencer 1995 - 1998Copyright © James T. Spencer 1995 - 1998All Rights ReservedAll Rights Reserved


2 Chapter EightChapter Eight• Ions versus Molecules.Ions versus Molecules.• Ionic BondingIonic Bonding - Ions which are held together - Ions which are held together

primarily by electrostatic forces (between primarily by electrostatic forces (between oppositely charged ions).oppositely charged ions).

• Covalent BondingCovalent Bonding - sharing of electrons - sharing of electrons between atoms in molecules.between atoms in molecules.

• Metallic BondingMetallic Bonding - sharing of electrons between - sharing of electrons between a very large array of atoms in which the electron a very large array of atoms in which the electron are relatively free to move throughout the large are relatively free to move throughout the large array.array.


3 Lewis StructuresLewis Structures• Valence electronsValence electrons - electrons which - electrons which

participate in bonding (outer shell participate in bonding (outer shell electrons, usually from an unfilled electrons, usually from an unfilled shell).shell).

• Electron Bookkeeping - Electron Bookkeeping - Lewis Lewis SymbolsSymbols (or more commonly (or more commonly Lewis Lewis StructuresStructures) keep track of valence ) keep track of valence electrons involved in bonding electrons involved in bonding (ignoring core electrons). (ignoring core electrons).


4

•Lewis symbols/structuresLewis symbols/structures consist of; consist of;•Elemental SymbolElemental Symbol•a “dot” for each valence electron a “dot” for each valence electron •form compounds by sharing or form compounds by sharing or exchanging electrons to achieve exchanging electrons to achieve nearest Noble Gas (group 18) nearest Noble Gas (group 18) configuration. Since all noble configuration. Since all noble gases have 8 valence electrons, gases have 8 valence electrons, many atoms react to have eight many atoms react to have eight electrons (electrons (octet ruleoctet rule).).

Lewis StructuresLewis Structures


5 Lewis StructuresLewis Structures

LiLi BeBe BB CC NN OO FF NeNe..........

.. .... .... .. .. .. .. .. .... .. .. .... ..

.. ..

.. .... .... ..:: ::::

ElementElement Elec. Config.Elec. Config. Valence ElectronsValence ElectronsLiLi 1s1s222s2s11 11BeBe 1s1s222s2s22 22BB 1s1s222s2s222p2p11 33CC 1s1s222s2s222p2p22 44NN 1s1s222s2s222p2p33 55OO 1s1s222s2s222p2p44 66FF 1s1s222s2s222p2p55 77NeNe 1s1s222s2s222p2p66 88


6

• Octet RuleOctet Rule - elements tend to surround - elements tend to surround themselves with eight valence electrons themselves with eight valence electrons through electron sharing or exchange (to through electron sharing or exchange (to fill shell). Many exceptions, however.fill shell). Many exceptions, however.

Lewis StructuresLewis Structures

NaNa..

.. .... .... ..::ClCl.. .. ..

::ClClNaNa+ + + +-1 -1

.. .. .. .. .. .. .. ...... ..:: FF .. .. ..::FF .. ..:: FF .. ..::FF::

....Electron Exchange - Electron Exchange - Ionic BondingIonic Bonding

Electron Sharing - Electron Sharing - Covalent BondingCovalent Bonding


7 Ionic BondingIonic BondingNaNa(g)(g) + Cl + Cl(g)(g) NaNa+1+1

(g)(g) + Cl + Cl-1-1(g)(g)

E° = IE° = I11(Na) - EA(Cl) = +146.8 kJ mol(Na) - EA(Cl) = +146.8 kJ mol-1-1

NaNa ClClr = radiusr = radius

NaNa+1+1(g)(g) + Cl + Cl-1-1

(g)(g)

NaNa00(g)(g) + Cl + Cl00

(g)(g)

r = radiusr = radius

EEnneerrggyy

harpooningharpooning


8

• Formation of gaseous ions is Formation of gaseous ions is EndothermicEndothermic. Why . Why does NaCl form if ion formation is endothermic?does NaCl form if ion formation is endothermic?

• Answer: Answer: Lattice EnergyLattice Energy - Defined - energy - Defined - energy required to completely separate a solid ionic required to completely separate a solid ionic compound into its gaseous ions. The reverse compound into its gaseous ions. The reverse (“condensation” to solid is very exothermic)(“condensation” to solid is very exothermic)

NaNa+1+1(g)(g) + Cl + Cl-1-1(g) (g) NaClNaCl(s)(s)

E° = Lattice Energy = -788 kJ molE° = Lattice Energy = -788 kJ mol-1-1

NaNa(g)(g) + Cl + Cl(g)(g) NaNa+1+1(g)(g) + Cl + Cl-1-1(g)(g)

E° = IE° = I11(Na) - EA(Cl) = +146.8 kJ mol(Na) - EA(Cl) = +146.8 kJ mol-1-1

Ionic BondingIonic Bonding


9 Lattice EnergyLattice Energy• Endothermic written Endothermic written

from solid to gas and from solid to gas and exothermic from gas to exothermic from gas to solid.solid.

• Higher charged ions Higher charged ions have much greater lattice have much greater lattice energiesenergies

• The closer the ions can The closer the ions can come together, the larger come together, the larger the lattice energy.the lattice energy.

Cmpd Cmpd Lattice EnergyLattice Energy (kJ mol(kJ mol-1-1))

LiFLiF 10241024LiILiI 744744NaFNaF 911911NaClNaCl 788788NaINaI 693693KFKF 815815KBrKBr 682682KIKI 641641MgFMgF22 29102910SrClSrCl22 21392139MgOMgO 39383938

E = k QE = k Q11QQ22

dd

k = const.k = const.QQ11 = charge ion 1 = charge ion 1QQ22 = charge ion 2 = charge ion 2d = distanced = distance


10 Ionic CompoundsIonic Compounds

+-

-

-

-

-

-

-

-

++

+

++

++

-

-

+

+ +-

+

-+-

-

- -

- -

-

+

+ +

+

Unit Cell Unit Cell

Cell FaceCell Face


11 Ionic CompoundsIonic CompoundsTiO2 Surface


12 Ionic CompoundsIonic CompoundsLiF LatticeLiF Lattice


13 Ionic CompoundsIonic Compounds• Why do ionic compounds form?Why do ionic compounds form?• Which ionic compounds are stable?Which ionic compounds are stable?• How do we determine the How do we determine the

stoichiometry of the stable formsstoichiometry of the stable forms–i.e., MgCl or MgCli.e., MgCl or MgCl22 or MgCl or MgCl33 ? ?

ThermodynamicsThermodynamicsUse Hess’ Law Use Hess’ Law

and Born-Haber Cyclesand Born-Haber Cycles


14 Born-Haber CycleBorn-Haber CycleHess’s LawHess’s Law - if a reaction is carried out in a series of steps, - if a reaction is carried out in a series of steps, H H for the reaction will be equal to the sum of the for the reaction will be equal to the sum of the H’s for the H’s for the individual steps.individual steps.


15 Born-Haber CycleBorn-Haber Cycle

Na(s) + Na(s) + 11//22 Cl Cl22(g)(g)

Na(g) + Na(g) + Cl(g)Cl(g)

NaNa+1+1(g) + (g) + ClCl-1-1(g)(g)

NaCl(s)NaCl(s)

Hess’s LawHess’s Law - if a reaction is carried out in a series of steps, - if a reaction is carried out in a series of steps, H H for the reaction will be equal to the sum of the for the reaction will be equal to the sum of the H’s for the H’s for the individual steps.individual steps.

HHRxRx

EEnneerrggyy


16 Born-Haber CycleBorn-Haber Cycle

Na(s) + Na(s) + 11//22 Cl Cl22(g)(g)

Na(g) + Na(g) + Cl(g)Cl(g)

NaNa+1+1(g) + (g) + ClCl-1-1(g)(g)

II11 EAEA

HsubHsub BDEBDE

- Lattice - Lattice Energy (U)Energy (U)

NaCl(s)NaCl(s)

Hess’s LawHess’s Law - if a reaction is carried out in a series of steps, - if a reaction is carried out in a series of steps, H H for the reaction will be equal to the sum of the for the reaction will be equal to the sum of the H’s for the H’s for the individual steps.individual steps.

U cannot be U cannot be determined determined

directlydirectly

HHRxRx

HHRxnRxn = = HHsubsub + I + I11 + BDE + EA + U + BDE + EA + U

EEnneerrggyy


17

HHRxRx = = Hsub + IHsub + I11 + BDE + EA + U + BDE + EA + U

ExothermicExothermic

EndothermicEndothermic

Born-Haber CycleBorn-Haber Cycle

NaClNaCl


18Why NaCl and not NaClWhy NaCl and not NaCl22


Na(s) + Na(s) + ClCl22(g)(g)

Na(g) + Na(g) + 2 2 Cl(g)Cl(g)

NaNa+1+1(g) + 2(g) + 2 ClCl-1-1(g)(g)

II11

NaClNaCl22(s)(s)EEnneerrggyy

HHRxRx

NaNa+2+2(g) + 2(g) + 2 ClCl-1-1(g)(g)


19Why NaCl and not NaClWhy NaCl and not NaCl22


Na(s) + Na(s) + ClCl22(g)(g)

Na(g) + Na(g) + 2 2 Cl(g)Cl(g)

NaNa+1+1(g) + 2(g) + 2 ClCl-1-1(g)(g)

II11 EAEA

HsubHsub BDEBDE

- Lattice - Lattice Energy (U)Energy (U)

NaClNaCl22(s)(s)EEnneerrggyy

HHRxRx

HHRxRx = = Hsub + IHsub + I11 + I + I22 + BDE + EA + U + BDE + EA + U

NaNa+2+2(g) + 2(g) + 2 ClCl-1-1(g)(g)

II22Very Very

EndothermicEndothermic


20

HHRxRx = = Hsub + IHsub + I11 + BDE + EA + U + BDE + EA + U

HHRxRx = = Hsub + IHsub + I11 + I + I22 + BDE + EA + U + BDE + EA + U

ExothermicExothermicEndothermicEndothermic

Born-Haber CycleBorn-Haber CycleVery EndothermicVery EndothermicNaClNaCl22

NaClNaCl


21

HHff° for MgO° for MgOBorn-Haber CycleBorn-Haber Cycle

Mg(s) + Mg(s) + 1 1//22 O O22(g)(g)

Mg(g) + Mg(g) + O(g) O(g)

MgMg+1+1(g) + O(g) + O-1-1(g)(g)

MgO(s)MgO(s)

EEnneerrggyy HHRxRx

MgMg+2+2(g) + O(g) + O-2-2(g)(g)larger U offsets increased

endothermicity from I2 and EA2


22

HHff° for MgO° for MgOBorn-Haber CycleBorn-Haber Cycle

Mg(s) + Mg(s) + 1 1//22 O O22(g)(g)

Mg(g) + Mg(g) + O(g) O(g)

MgMg+1+1(g) + O(g) + O-1-1(g)(g)

II11 EAEA

HsubHsub BDEBDE

- Lattice - Lattice Energy Energy

(U)(U)

MgO(s)MgO(s)

EEnneerrggyy HHRxRx

HHRxRx = = Hvap + IHvap + I11 + I + I22 + BDE + EA + BDE + EA11 + EA + EA22 + U + U

MgMg+2+2(g) + O(g) + O-2-2(g)(g)

II22 EAEA

larger U offsets increased

endothermicity from I2 and EA2


23 When a positive ion is formed, electrons When a positive ion is formed, electrons

are always lost first from the subshell are always lost first from the subshell with the largest n value.with the largest n value.

To determine electronic configurations of To determine electronic configurations of Cations, remove the highest energy Cations, remove the highest energy electrons (usually the last one added electrons (usually the last one added EXCEPT for transition metal ions - which EXCEPT for transition metal ions - which have NO n(max)s electrons).have NO n(max)s electrons).

Electronic Config. of Electronic Config. of IonsIons

2525MnMn

2525MnMn+1+1

4s 3d 3d 3d 3d 3d 4p 4p 4p


24 Ionic RadiiIonic Radii• Sizes of ions determines stability and packing Sizes of ions determines stability and packing

arrangements (structures) of ionic compounds.arrangements (structures) of ionic compounds.

• Size depends upon ZSize depends upon Zeffeff and the total number of and the total number of electrons of the ion.electrons of the ion.– Cations are smaller than the neutral atomsCations are smaller than the neutral atoms– Anions are larger than the neutral atomsAnions are larger than the neutral atoms– size of Ions of the same charge increases down and size of Ions of the same charge increases down and

L.L.• Isoelectronic Series - ions with the same number of Isoelectronic Series - ions with the same number of

electrons (although different charges):electrons (although different charges):

NN-3-3 = O = O-2-2 = F = F-1-1 = Ne = Na = Ne = Na+1+1 = Mg = Mg+2+2 = Al = Al+3+3 = 10 elec. = 10 elec.


25

• Isoelectronic seriesIsoelectronic series - ions with the same number - ions with the same number of electronsof electrons

rraaddiiuuss

atomic numberatomic number

Ionic RadiiIonic Radii

LiLi++

BeBe+2+2

NaNa++

KK++ RbRb++

MgMg+2+2

CaCa+2+2 SrSr+2+2

AlAl+3+3

ScSc+3+3 YY+3+3

TiTi+4+4 ZrZr+4+4

FF--

ClCl--BrBr--

OO-2-2

SS-2-2SeSe-2-2

smaller w/ incr + chargesmaller w/ incr + chargesmaller with smaller max nsmaller with smaller max n


26 Covalent BondingCovalent Bonding• Atoms may achieve noble gas configurations by Atoms may achieve noble gas configurations by

sharing electronssharing electrons.. .. .. .. .. .. .. ...... ..:: FF .. .. ..::FF .. ..:: FF .. ..::FF::

..CC..

....4 H +4 H +.. ..CC.. ....HH HHHH

HH

......

..

Each atom achieves an outer shell with 8 electrons (at least)Each atom achieves an outer shell with 8 electrons (at least)Lewis structures show this electron sharingLewis structures show this electron sharing


27 Covalent BondingCovalent Bonding• Wave function (wave Wave function (wave

properties)properties)• Can “constructively” and Can “constructively” and

“destructively add waves “destructively add waves (just like ripples on a (just like ripples on a pond).pond).

1s orbital1s orbital22

0 radius0 radius

1s orbitals1s orbitals220 radius0 radius 0 radius0 radius

H atomH atom 11

H atomH atom 22

Move together to Move together to overlap wavesoverlap waves


28 Covalent BondingCovalent Bonding

0 0 0 radius0 radius

22

H atomH atom 11

H atomH atom 22

0 0 0 radius0 radius

22

H atomH atom 11

H atomH atom 22

Destructive Destructive Addition of Addition of Waves Waves (out of (out of phase)phase)

Constructive Constructive Addition of Addition of WavesWaves(in phase)(in phase)



++

++

s orbital - in-phase addition (bonding)s orbital - in-phase addition (bonding)

s orbital - out-of-phase addition (antibonding)s orbital - out-of-phase addition (antibonding)



++

++

p orbital - in-phase addition (bonding)p orbital - in-phase addition (bonding)

p orbital - out-of-phase addition (antibonding)p orbital - out-of-phase addition (antibonding)


31 Multiple BondsMultiple Bonds• Bond Order (bo) denotes how many electron pairs Bond Order (bo) denotes how many electron pairs

are shared between two atoms;are shared between two atoms;– single bondsingle bond (bo = 1) has one shared electron (bo = 1) has one shared electron

pair. pair. longestlongest– double bonddouble bond (bo = 2) has two shared electron (bo = 2) has two shared electron

pair.pair.– triple bondtriple bond (bo = 3) has three shared electron (bo = 3) has three shared electron

pair. pair. shortestshortest

SingleSingle DoubleDouble TripleTriple.. .. .. ..:: FF ::FF::.. .. .. ..

.. .. .. ..OO OO::::.. .. .. .. ::NN :::::: NN::

F = 1sF = 1s222s2s222p2p55 O = 1sO = 1s222s2s222p2p44 N = 1sN = 1s222s2s222p2p33


32Polarity and Polarity and

ElectronegativityElectronegativity• Electrons shared equally between like atoms is a Electrons shared equally between like atoms is a

non-polar covalentnon-polar covalent bond (i.e., H bond (i.e., H22, F, F22, O, O22, etc...)., etc...).

• Electrons may be unequally shared due to Electrons may be unequally shared due to differences in Zdifferences in Zeffeff between unlike nuclei (i.e., HF). between unlike nuclei (i.e., HF). Referred to as Referred to as polar covalentpolar covalent bonding. bonding.

• Electron density shifts toward the greater ZElectron density shifts toward the greater Zeffeff nuclei.nuclei.

• ElectronegativityElectronegativity - used to estimate the polarity of - used to estimate the polarity of a bond - the ability of an atom to attract electrons a bond - the ability of an atom to attract electrons to itself.to itself.


33 ElectronegativityElectronegativity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

1H 2He

3Li 4Be 5B 6C 7N 8O 9F 10Ne

11 Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar

19K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br 36Kr

37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53I 54Xe

55Cs 56Ba 57La 72Hf 73Ta 74W 75Re 76Os 77Ir 78Pt 79Au 80Hg 81Tl 82Pb 83Bi 84Po 85At 86Rn

87Fr 88Ra 89Ac 104Unq 105Unp 106Unh 107Ns 108Hs 109Mt

58Ce 59Pr 60Nd 61Pm 62Sm 63Eu 64Gd 65Tb 66Dy 67Ho 68Er 69Tm 70Yb 71Lu

90Th 91Pa 92U 93Np 94Pu 95Am 96Cm 97Bk 98Cf 99Es 100Fm 101Md 102No 103Lr

metalsmetalsmetalloidsmetalloidsnon-metalsnon-metals

Lower Lower ENEN

Higher Higher ENEN

F EN (highest) = 4.0F EN (highest) = 4.0Cs EN (lowest) = 0.7Cs EN (lowest) = 0.7


34 Bond PolarityBond Polarity• The larger the difference in EN between the two The larger the difference in EN between the two

bonded atoms, the more unequal the electron bonded atoms, the more unequal the electron sharing.sharing.

• Bonds with unequal electron sharing are a Bonds with unequal electron sharing are a polar polar bondsbonds with the higher EN atom “holding” the with the higher EN atom “holding” the greater electron density.greater electron density.

FF FF NN FF BB FF LiLi FF

ElementElement ElectronegativityElectronegativityF F 4.04.0N N 3.03.0BB 2.02.0LiLi 1.01.0

EN = 0EN = 0 EN = 1.0EN = 1.0 EN = 2.0EN = 2.0 EN = 3.0EN = 3.0

More PolarMore Polar

IONICIONIC


35 Bond PolarityBond Polarity

HH FF EN = 4.0 - 2.1 = 1.9EN = 4.0 - 2.1 = 1.9Polar Covalent BondPolar Covalent Bond

ElementElement ENENF F 4.04.0OO 3.53.5N N 3.03.0CC 2.52.5BB 2.02.0LiLi 1.01.0PP 2.12.1ClCl 3.03.0BrBr 2.82.8

Classify the Bonds (give Classify the Bonds (give EN):EN):

S S SSBB ClClPP FFPP ClClPP OOLiLi FFNN FFOO OO

non-polarnon-polar 00polar covalentpolar covalent 1.01.0polar covalentpolar covalent 1.91.9polar covalentpolar covalent 0.90.9polar covalentpolar covalent 1.41.4ionicionic 3.03.0polar covalentpolar covalent 1.01.0non-polarnon-polar 0.00.0


36Drawing Lewis Drawing Lewis

StructuresStructures• Lewis Structure describe many properties of Lewis Structure describe many properties of

molecules. To draw Lewis structures;molecules. To draw Lewis structures;– Sum the valence electrons for all atomsSum the valence electrons for all atoms– Write symbols for the atoms and connect atoms in Write symbols for the atoms and connect atoms in

correct arrangement (with at least a single bond)correct arrangement (with at least a single bond)– Complete the octets of the atoms bonded to the Complete the octets of the atoms bonded to the

central atom (with some exceptions)central atom (with some exceptions)– Place any remaining electrons on the central atomPlace any remaining electrons on the central atom– If short electrons to gain central atom’s octet, try If short electrons to gain central atom’s octet, try

using multiple bonds.using multiple bonds.


37 Lewis StructuresLewis Structures

LiLi BeBe BB CC NN OO FF NeNe..........

.. .... .... .. .. .. .. .. .... .. .. .... ..

.. ..

.. .... .... ..:: ::::

ElementElement Elec. Config.Elec. Config. Valence ElectronsValence ElectronsLiLi 1s1s222s2s11 11BeBe 1s1s222s2s22 22BB 1s1s222s2s222p2p11 33CC 1s1s222s2s222p2p22 44NN 1s1s222s2s222p2p33 55OO 1s1s222s2s222p2p44 66FF 1s1s222s2s222p2p55 77NeNe 1s1s222s2s222p2p66 88


38 Formal ChargeFormal Charge• Bookkeeping of valence electronsBookkeeping of valence electrons

– all the unshared electrons are assigned to the all the unshared electrons are assigned to the atom on which they are foundatom on which they are found

– Half of the bonding electrons are assigned to each Half of the bonding electrons are assigned to each atom in a bondatom in a bond

– Formal charge - the number of valence electrons Formal charge - the number of valence electrons on the isolated atom minus the number of on the isolated atom minus the number of electrons assigned to the atom in the Lewis electrons assigned to the atom in the Lewis structure.structure.

formal ch. = [atomic valence e-] - [assigned e-]formal ch. = [atomic valence e-] - [assigned e-]• The best Lewis structure (most stable) will be the one The best Lewis structure (most stable) will be the one

with the smallest formal charges and the one in with the smallest formal charges and the one in which formal negative charges reside on the most which formal negative charges reside on the most electronegative atoms.electronegative atoms.


39 Formal ChargeFormal Charge

HH H B HH B H HH

-1-1

B in neutral atom has 3 valence electronsB in neutral atom has 3 valence electronsin [BHin [BH44]]--, B assigned 4 electrons, B assigned 4 electrons

formal charge on B = -1formal charge on B = -1

AtomAtom Neutral AssignedNeutral Assigned Formal Ch.Formal Ch.NN 55 55 00O (R)O (R) 66 66 00O (L)O (L) 66 77 -1-1

O N OO N O

-1-1


40 Lewis StructuresLewis Structures• Best way to learn Lewis structures is PRACTICE!Best way to learn Lewis structures is PRACTICE!• Example (in class):Example (in class):

– PClPCl33

– PHPH33

– HH22OO– HClOHClO22

– SeClSeCl22

• [BH[BH44]]--

• [SO[SO33]]-2-2

–COCO33-2-2

–HCNHCN

““Normal”Normal”

ChargedCharged

Multiple BondingMultiple Bonding


41Lewis Structure: Lewis Structure:

PClPCl33(1) (1) Sum Valence ElectronsSum Valence Electrons::

P P = 1 x 5= 1 x 5 = 5= 5ClCl = 3 x 7 = 3 x 7 = = 2121TotalTotal = 26 electrons = 13 pairs= 26 electrons = 13 pairs

(2) (2) Write symbols and include atom connectionsWrite symbols and include atom connections::

Cl P ClCl P Cl ClCl

(3) (3) Complete Octets and place all remaining electronsComplete Octets and place all remaining electrons::

Cl P ClCl P Cl ClCl

(4) (4) CheckCheck (including formal charges). (including formal charges).

= electron pair= electron pair



PHPH33(1) (1) Sum Valence ElectronsSum Valence Electrons::

P P = 1 x 5= 1 x 5 = 5= 5HH = 3 x 1 = 3 x 1 = = 33TotalTotal = 8 electrons = 4 pairs= 8 electrons = 4 pairs


H P HH P H HH


H P HH P H HH





HH22OO(1) (1) Sum Valence ElectronsSum Valence Electrons::

H H = 2 x 1= 2 x 1 = 2= 2OO = 1 x 6 = 1 x 6 = = 66TotalTotal = 8 electrons = 4 pairs= 8 electrons = 4 pairs


H O HH O H


H O HH O H





HClOHClO22(1) (1) Sum Valence ElectronsSum Valence Electrons::Cl Cl = 1 x 7= 1 x 7 = 7= 7OO = 2 x 6= 2 x 6 = 12= 12HH = 1 x 1 = 1 x 1 = = 11TotalTotal = 20 electrons = 10 pairs= 20 electrons = 10 pairs


H Cl OH Cl O OO


H Cl OH Cl O OO





SeClSeCl22(1) (1) Sum Valence ElectronsSum Valence Electrons::

Se = 1 x 6Se = 1 x 6 = 6= 6ClCl = 2 x 7 = 2 x 7 = = 1414TotalTotal = 20 electrons = 10 pairs= 20 electrons = 10 pairs


Cl Se ClCl Se Cl


Cl Se ClCl Se Cl





[BH[BH44]]--

(1) (1) Sum Valence ElectronsSum Valence Electrons::B B = 1 x 3= 1 x 3 = 3= 3HH = 4 x 1= 4 x 1 = 4= 4chargecharge = = 11TotalTotal = 8 electrons = 4 pairs= 8 electrons = 4 pairs

(2) (2) Write symbols and include atom connectionsWrite symbols and include atom connections:: HH H B HH B H HH

(3) (3) Complete Octets and place all remaining electronsComplete Octets and place all remaining electrons:: HH H B HH B H HH



-1-1

-1-1



NONO22--

(1) (1) Sum Valence ElectronsSum Valence Electrons::N N = 1 x 5= 1 x 5 = 5= 5OO = 2 x 6= 2 x 6 = 12= 12charge (-1) charge (-1) = = 11TotalTotal = 18 electrons = 9 pairs= 18 electrons = 9 pairs


O N OO N O


O N OO N O



-1-1

-1-1



COCO33-2-2

(1) (1) Sum Valence ElectronsSum Valence Electrons::C C = 1 x 4= 1 x 4 = 4= 4OO = 3 x 6= 3 x 6 = 18= 18charge (-1) charge (-1) = = 22TotalTotal = 24 electrons = 12 pairs= 24 electrons = 12 pairs


O C OO C O OO


O C OO C O OO



-2-2

-2-2



SOSO33-2-2

(1) (1) Sum Valence ElectronsSum Valence Electrons::C C = 1 x 6= 1 x 6 = 6= 6OO = 3 x 6= 3 x 6 = 18= 18charge (-1) charge (-1) = = 22TotalTotal = 26 electrons = 13 pairs= 26 electrons = 13 pairs


O S OO S O OO


O S OO S O OO



-1-1

-1-1



HCNHCN(1) (1) Sum Valence ElectronsSum Valence Electrons::

C C = 1 x 4= 1 x 4 = 4= 4NN = 1 x 5= 1 x 5 = 5= 5HH = 1 x 1 = 1 x 1 = = 11TotalTotal = 10 electrons = 5 pairs= 10 electrons = 5 pairs


H C NH C N


H C NH C N




51Resonance Resonance StructuresStructures

• Certain molecules cannot adequately be Certain molecules cannot adequately be described using a single Lewis structuredescribed using a single Lewis structure– Ozone is perfect exampleOzone is perfect example– takes two Lewis structures to describe ozonetakes two Lewis structures to describe ozone

O OO O

O O O OO O O O



• If the below description of ozone is correct, how is If the below description of ozone is correct, how is this possible?this possible?– Double bond is shorter than single bond so Double bond is shorter than single bond so

molecule would be lopsidedmolecule would be lopsided

O OO O

O O O OO O O O



• When several possible Lewis structures can be When several possible Lewis structures can be written which vary only in electron arrangement written which vary only in electron arrangement (not atom connections), each possible structure is (not atom connections), each possible structure is a a resonance structureresonance structure (connected by double (connected by double headed arrows).headed arrows).

• The molecule “exists” as a weighted blend of the The molecule “exists” as a weighted blend of the possible Lewis structurespossible Lewis structures

BB

FF

FFFFBB

FF

FFFFBB

FF

FFFF



• Resonance is of crucial importance in organic Resonance is of crucial importance in organic chemistrychemistry– primary group showing resonance is the primary group showing resonance is the

aromaticsaromatics– aromatics are based on benzenearomatics are based on benzene


55Octet Rule Octet Rule ExceptionsExceptions

• Exceptions to the Octet Rule:Exceptions to the Octet Rule:

– Molecules with odd number of electrons (i.e., Molecules with odd number of electrons (i.e., ClOClO22, NO, NO22, NO, etc...)., NO, etc...).

– Molecules in which the atom has less than an Molecules in which the atom has less than an octet (i.e., BFoctet (i.e., BF33, H, H22, etc...)., etc...).

– Molecules in which an atom has more than an Molecules in which an atom has more than an octet (i.e., PCloctet (i.e., PCl55, [ICl, [ICl44]]-1-1, XeOF, XeOF44, etc...)., etc...).

–only possible for period 3 and beyond only possible for period 3 and beyond elements (requires n = 3 or greater)elements (requires n = 3 or greater)


56Octet Rule Octet Rule ExceptionsExceptions

• Exceptions to the Octet Rule:Exceptions to the Octet Rule:

– Molecules with odd number of electrons (i.e., ClOMolecules with odd number of electrons (i.e., ClO22, , NONO22, NO, etc...)., NO, etc...).

• Complete pairing of electrons is impossibleComplete pairing of electrons is impossible

– Molecules in which the atom has less than an octet Molecules in which the atom has less than an octet (i.e., BF(i.e., BF33, H, H22, etc...)., etc...).

• Most encountered in molecules of Boron and Most encountered in molecules of Boron and BerylliumBeryllium

• allows the B or Be to bond with another atom allows the B or Be to bond with another atom using a coordinate covalent bondusing a coordinate covalent bond


57 Octet ExpansionsOctet ExpansionsPClPCl55 IClICl44

-1-1 XeOFXeOF44

P = 1 x 5 P = 1 x 5 = 5= 5 I = 1 x 7I = 1 x 7 = 7= 7 Xe = 1 x 8Xe = 1 x 8 = 8= 8Cl = 5 x 7 Cl = 5 x 7 = 35= 35 4 x 7 4 x 7 = 28= 28 O = 1 x 6O = 1 x 6 = 6= 640 e- = 40 e- = 20 e- pairs20 e- pairs charge charge = 1= 1 F = 4 x 7F = 4 x 7 = 28= 28

18 e- pairs18 e- pairs 21 e- pairs21 e- pairs

PP

ClClClCl

ClClClCl

ClClII ClClClClClCl

ClCl XeXeOO

FF FFFFFF

Unshared electron pairs on central atomUnshared electron pairs on central atom

-1-1


58 Lewis StructuresLewis Structures• Practice:Practice:

– CHClCHCl33 13 prs13 prs– NHNH44

++ 4 prs4 prs– XeOXeO44 16 prs16 prs– HNOHNO22 9 prs9 prs– CC22NN22 9 prs9 prs– SClSCl22 10 prs10 prs– ClFClF33 14 prs14 prs





ClClClCl

ClClCCHH HH

HH

HHNNHH

+1+1





ClClClCl

ClClCCHH HH

HH

HHNNHH

+1+1

OOOO

OOXeXeOO OO

OONNHH





ClClClCl

ClClCCHH HH

HH

HHNNHH

+1+1

OOOO

OOXeXeOO OO

OONNHH

ClClClClSS

FFFF

FFClCl

NNNN CCCC


62 Bond StrengthsBond Strengths• Stability of molecules is related to the strengths of Stability of molecules is related to the strengths of

the bonds within the moleculesthe bonds within the molecules• Bond strengths can be determined Bond strengths can be determined

thermochemicallythermochemically– Bond Dissociation EnergyBond Dissociation Energy (BDE, (BDE, H)H)

FF--F (g)F (g) 2 F (g)2 F (g) H = 155 kJ molH = 155 kJ mol-1-1

O=O (g)O=O (g) 2 O (g)2 O (g) H = 495 kJ molH = 495 kJ mol-1-1

NNN (g)N (g) 2 N (g)2 N (g) H = 941 kJ molH = 941 kJ mol-1-1

..

.. ..

.. ....


63 Bond StrengthsBond Strengths• BDE for diatomic molecules may be measured BDE for diatomic molecules may be measured

directly since only one bond exists within the directly since only one bond exists within the molecule.molecule.

• For polyatomic molecules (with many bonds), its For polyatomic molecules (with many bonds), its difficult to determine the BDE directly so an difficult to determine the BDE directly so an average BDE is typically used.average BDE is typically used.– Atomization Reactions:Atomization Reactions:

H Si H (g)H Si H (g) Si (g) + 4 H (g) Si (g) + 4 H (g) H = 1292 kJ molH = 1292 kJ mol-1-1

ave BDE (CH)= 1292 kJ molave BDE (CH)= 1292 kJ mol-1-1/ 4= 323 kJ mol/ 4= 323 kJ mol-1-1HH

HH .... .... ..

•BDEs are endothermic (positive)BDEs are endothermic (positive)


64Bond Dissociation Bond Dissociation

EnergiesEnergies• Larger BDE’s generally lead to lower reactivity.Larger BDE’s generally lead to lower reactivity.• BDE’s follow bond order (b.o. and BDE 3 > 2 > 1).BDE’s follow bond order (b.o. and BDE 3 > 2 > 1).

BondBond Ave. BDE (kJ molAve. BDE (kJ mol-1-1)) BOBOC-C C-C 348348 1.01.0C=CC=C 614614 2.02.0CCCC 839839 3.03.0N-NN-N 391391 1.01.0N=NN=N 418418 2.02.0NNNN 941941 3.03.0C-NC-N 293293 1.01.0C=NC=N 615615 2.02.0CCNN 891891 3.03.0


65 Bond EnergiesBond Energies• Bond energies may be used to estimate Bond energies may be used to estimate H for H for

reactions where bonds are broken or formed.reactions where bonds are broken or formed.– Determine which bonds are broken Determine which bonds are broken

(endothermic) (endothermic) BREAK IN (+)BREAK IN (+)– Sum the BDE for these bondsSum the BDE for these bonds– Determine which bonds are formed Determine which bonds are formed

(exothermic) (exothermic) MAKE OUT (-)MAKE OUT (-)– Sum the BDE for these bondsSum the BDE for these bonds– This gives you the Estimate for This gives you the Estimate for H H

H = H = (BDE bonds broken) - (BDE bonds broken) - (BDE bonds formed) (BDE bonds formed)


66H estimates from H estimates from

BDE’sBDE’sestimate estimate H for the reaction: H for the reaction:

H - C - O - H + H - BrH - C - O - H + H - Br H - C - Br + H - O - HH - C - Br + H - O - HHH

HH

HH

HH

H = + BDE C-O + BDE H-Br - BDE C-Br - BDE O-HH = + BDE C-O + BDE H-Br - BDE C-Br - BDE O-HH = + 358 + 366 - 276 - 463H = + 358 + 366 - 276 - 463H = 724 - 739H = 724 - 739H = -15 kJ molH = -15 kJ mol-1-1




C = C + H - O - O - HC = C + H - O - O - H H - O - C - C - O - HH - O - C - C - O - HHH

HH

HH

HH

HH

HH

HH

HH




C = C + H - O - O - HC = C + H - O - O - H H - O - C - C - O - HH - O - C - C - O - HHH

HH

HH

HH

HH

HH

HH

HH

H = BDE C=C + BDE O-O - (2(BDE C-O) - BDE C-CH = BDE C=C + BDE O-O - (2(BDE C-O) - BDE C-CH = +614 + 146 - (2(358) - 348H = +614 + 146 - (2(358) - 348H = 760 - 1064 H = 760 - 1064 H = -304 H = -304


69 Bond LengthsBond Lengths• As the number of bonds between two atoms As the number of bonds between two atoms

increases (greater bond order), the bond becomes increases (greater bond order), the bond becomes shorter and stronger.shorter and stronger.

BondBond Ave. BDE (kJ molAve. BDE (kJ mol-1-1)) BOBO Bond LengthBond LengthC-C C-C 348348 1.01.0 1.541.54C=CC=C 614614 2.02.0 1.341.34CCCC 839839 3.03.0 1.201.20N-NN-N 391391 1.01.0 1.471.47N=NN=N 418418 2.02.0 1.241.24NNNN 941941 3.03.0 1.101.10C-NC-N 293293 1.01.0 1.431.43C=NC=N 615615 2.02.0 1.381.38CCNN 891891 3.03.0 1.161.16


70 End Chapter EightEnd Chapter Eight• Ionic BondingIonic Bonding• Octet Rule and Lewis Symbols and Lewis Octet Rule and Lewis Symbols and Lewis

StructuresStructures• Hf° for ionic compounds - esp. Lattice energies Hf° for ionic compounds - esp. Lattice energies

and Born-Haber cycles.and Born-Haber cycles.• Ionic RadiiIonic Radii• Covalent BondingCovalent Bonding• Bond Polarity and electronegativityBond Polarity and electronegativity• Lewis Structures (incl. resonance structures, Lewis Structures (incl. resonance structures,

formal charge, etc...)formal charge, etc...)• Covalent Bond Lengths and StrengthsCovalent Bond Lengths and Strengths

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Prof. J. T. Spencer, CHE 106 1 Chapter Eight Copyright © James T. Spencer 1995 - 1998 All Rights Reserved