prof. j. t. spencer, che 106 1 chapter eight copyright © james t. spencer 1995 - 1998 all rights...
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Prof. J. T. Spencer, CHE Lewis Structures Valence electrons - electrons which participate in bonding (outer shell electrons, usually from an unfilled shell).Valence electrons - electrons which participate in bonding (outer shell electrons, usually from an unfilled shell). Electron Bookkeeping - Lewis Symbols (or more commonly Lewis Structures) keep track of valence electrons involved in bonding (ignoring core electrons).Electron Bookkeeping - Lewis Symbols (or more commonly Lewis Structures) keep track of valence electrons involved in bonding (ignoring core electrons).TRANSCRIPT
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
Prof. J. T. Spencer, CHE 106
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.
Prof. J. T. Spencer, CHE 106
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).
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
10 Ionic CompoundsIonic Compounds
+-
-
-
-
-
-
-
-
++
+
++
++
-
-
+
+ +-
+
-+-
-
- -
- -
-
+
+ +
+
Unit Cell Unit Cell
Cell FaceCell Face
Prof. J. T. Spencer, CHE 106
11 Ionic CompoundsIonic CompoundsTiO2 Surface
Prof. J. T. Spencer, CHE 106
12 Ionic CompoundsIonic CompoundsLiF LatticeLiF Lattice
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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.
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
17
HHRxRx = = Hsub + IHsub + I11 + BDE + EA + U + BDE + EA + U
ExothermicExothermic
EndothermicEndothermic
Born-Haber CycleBorn-Haber Cycle
NaClNaCl
Prof. J. T. Spencer, CHE 106
18Why NaCl and not NaClWhy NaCl and not NaCl22
Born-Haber CycleBorn-Haber Cycle
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)
Prof. J. T. Spencer, CHE 106
19Why NaCl and not NaClWhy NaCl and not NaCl22
Born-Haber CycleBorn-Haber Cycle
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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.
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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)
Prof. J. T. Spencer, CHE 106
29 Covalent BondingCovalent Bonding
++
++
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)
Prof. J. T. Spencer, CHE 106
30 Covalent BondingCovalent Bonding
++
++
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)
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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.
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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.
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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.
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
42Lewis Structure: Lewis Structure:
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
(2) (2) Write symbols and include atom connectionsWrite symbols and include atom connections::
H P HH P H HH
(3) (3) Complete Octets and place all remaining electronsComplete Octets and place all remaining electrons::
H P HH P H HH
(4) (4) CheckCheck (including formal charges). (including formal charges).
= electron pair= electron pair
Prof. J. T. Spencer, CHE 106
43Lewis Structure: Lewis Structure:
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
(2) (2) Write symbols and include atom connectionsWrite symbols and include atom connections::
H O HH O H
(3) (3) Complete Octets and place all remaining electronsComplete Octets and place all remaining electrons::
H O HH O H
(4) (4) CheckCheck (including formal charges). (including formal charges).
= electron pair= electron pair
Prof. J. T. Spencer, CHE 106
44Lewis Structure: Lewis Structure:
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
(2) (2) Write symbols and include atom connectionsWrite symbols and include atom connections::
H Cl OH Cl O OO
(3) (3) Complete Octets and place all remaining electronsComplete Octets and place all remaining electrons::
H Cl OH Cl O OO
(4) (4) CheckCheck (including formal charges). (including formal charges).
= electron pair= electron pair
Prof. J. T. Spencer, CHE 106
45Lewis Structure: Lewis Structure:
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
(2) (2) Write symbols and include atom connectionsWrite symbols and include atom connections::
Cl Se ClCl Se Cl
(3) (3) Complete Octets and place all remaining electronsComplete Octets and place all remaining electrons::
Cl Se ClCl Se Cl
(4) (4) CheckCheck (including formal charges). (including formal charges).
= electron pair= electron pair
Prof. J. T. Spencer, CHE 106
46Lewis Structure: Lewis Structure:
[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
(4) (4) CheckCheck (including formal charges). (including formal charges).
= electron pair= electron pair
-1-1
-1-1
Prof. J. T. Spencer, CHE 106
47Lewis Structure: Lewis Structure:
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
(2) (2) Write symbols and include atom connectionsWrite symbols and include atom connections::
O N OO N O
(3) (3) Complete Octets and place all remaining electronsComplete Octets and place all remaining electrons::
O N OO N O
(4) (4) CheckCheck (including formal charges). (including formal charges).
= electron pair= electron pair
-1-1
-1-1
Prof. J. T. Spencer, CHE 106
48Lewis Structure: Lewis Structure:
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
(2) (2) Write symbols and include atom connectionsWrite symbols and include atom connections::
O C OO C O OO
(3) (3) Complete Octets and place all remaining electronsComplete Octets and place all remaining electrons::
O C OO C O OO
(4) (4) CheckCheck (including formal charges). (including formal charges).
= electron pair= electron pair
-2-2
-2-2
Prof. J. T. Spencer, CHE 106
49Lewis Structure: Lewis Structure:
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
(2) (2) Write symbols and include atom connectionsWrite symbols and include atom connections::
O S OO S O OO
(3) (3) Complete Octets and place all remaining electronsComplete Octets and place all remaining electrons::
O S OO S O OO
(4) (4) CheckCheck (including formal charges). (including formal charges).
= electron pair= electron pair
-1-1
-1-1
Prof. J. T. Spencer, CHE 106
50Lewis Structure: Lewis Structure:
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
(2) (2) Write symbols and include atom connectionsWrite symbols and include atom connections::
H C NH C N
(3) (3) Complete Octets and place all remaining electronsComplete Octets and place all remaining electrons::
H C NH C N
(4) (4) CheckCheck (including formal charges). (including formal charges).
= electron pair= electron pair
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
52Resonance Resonance StructuresStructures
• 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
Prof. J. T. Spencer, CHE 106
53Resonance Resonance StructuresStructures
• 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
Prof. J. T. Spencer, CHE 106
54Resonance Resonance StructuresStructures
• 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
Prof. J. T. Spencer, CHE 106
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)
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
59 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
Prof. J. T. Spencer, CHE 106
60 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
OOOO
OOXeXeOO OO
OONNHH
Prof. J. T. Spencer, CHE 106
61 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
OOOO
OOXeXeOO OO
OONNHH
ClClClClSS
FFFF
FFClCl
NNNN CCCC
Prof. J. T. Spencer, CHE 106
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
..
.. ..
.. ....
Prof. J. T. Spencer, CHE 106
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)
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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)
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
67H estimates from H estimates from
BDE’sBDE’sestimate estimate H for the reaction: H for the reaction:
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
Prof. J. T. Spencer, CHE 106
68H estimates from H estimates from
BDE’sBDE’sestimate estimate H for the reaction: H for the reaction:
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
Prof. J. T. Spencer, CHE 106
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
Prof. J. T. Spencer, CHE 106
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