3. Formal Charge-Resonance Excellent
description
Transcript of 3. Formal Charge-Resonance Excellent
Chemical Bonding I:
Lewis Theory
2008, Prentice Hall
Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro
Roy KennedyMassachusetts Bay Community College
Wellesley Hills, MA
Tro, Chemistry: A Molecular Approach 2
Bonding Theories• explain how and why atoms attach together• explain why some combinations of atoms are stable
and others are notwhy is water H2O, not HO or H3O
• one of the simplest bonding theories was developed by G.N. Lewis and is called Lewis Theory
• Lewis Theory emphasizes valence electrons to explain bonding
• using Lewis Theory, we can draw models – called Lewis structures – that allow us to predict many properties of moleculesaka Electron Dot Structuressuch as molecular shape, size, polarity
Tro, Chemistry: A Molecular Approach 3
Why Do Atoms Bond?• processes are spontaneous if they result in a system
with lower potential energy• chemical bonds form because they lower the potential
energy between the charged particles that compose atoms
• the potential energy between charged particles is directly proportional to the product of the charges
• the potential energy between charged particles is inversely proportional to the distance between the charges
Tro, Chemistry: A Molecular Approach 4
Potential Energy Between Charged Particles
• 0 is a constant = 8.85 x 10-12 C2/J∙m
• for charges with the same sign, Epotential is + and the magnitude gets less positive as the particles get farther apart
• for charges with the opposite signs, Epotential is and the magnitude gets more negative as the particles get closer together
• remember: the more negative the potential energy, the more stable the system becomes
r
qq 21
0potential 4
1E
Tro, Chemistry: A Molecular Approach 5
Potential Energy BetweenCharged Particles
The repulsion between like-charged particles increases as the particles get closer together. To bring them closer requires the addition of more energy.
The attraction between opposite-charged particles increases as the particles get closer together. Bringing them closer lowers the potential energy of the system.
Tro, Chemistry: A Molecular Approach 6
Bonding• a chemical bond forms when the potential
energy of the bonded atoms is less than the potential energy of the separate atoms
• have to consider following interactions: nucleus-to-nucleus repulsionelectron-to-electron repulsionnucleus-to-electron attraction
Tro, Chemistry: A Molecular Approach 7
Types of Bonds
Types of Atoms Type of BondBond
Characteristic
metals to nonmetals
Ionicelectronstransferred
nonmetals tononmetals
Covalentelectrons shared
metal tometal
Metallicelectronspooled
8
Types of Bonding
Tro, Chemistry: A Molecular Approach 9
Ionic Bonds
• when metals bond to nonmetals, some electrons from the metal atoms are transferred to the nonmetal atomsmetals have low ionization energy, relatively easy to
remove an electron fromnonmetals have high electron affinities, relatively
good to add electrons to
Tro, Chemistry: A Molecular Approach 10
Covalent Bonds• nonmetals have relatively high ionization energies, so it
is difficult to remove electrons from them• when nonmetals bond together, it is better in terms of
potential energy for the atoms to share valence electronspotential energy lowest when the electrons are between the
nuclei• shared electrons hold the atoms together by attracting
nuclei of both atoms
Tro, Chemistry: A Molecular Approach 11
Determining the Number of Valence Electrons in an Atom
• the column number on the Periodic Table will tell you how many valence electrons a main group atom hasTransition Elements all have 2 valence electrons; Why?
1A/1 2A/2 3A/13 4A/14 5A/15 6A/16 7A/17 8A/10
Li Be B C N O F Ne
1 e-1 2 e-1 3 e-1 4 e-1 5 e-1 6 e-1 7 e-1 8 e-1
Tro, Chemistry: A Molecular Approach 12
Lewis Symbols of Atoms• electron dot symbols• use symbol of element to represent nucleus and
inner electrons• use dots around the symbol to represent valence
electronspair first two electrons for the s orbitalput one electron on each open side for p electrons then pair rest of the p electrons
Li Be
B
C
N
O
F
Ne
Tro, Chemistry: A Molecular Approach 13
Lewis Symbols of Ions• Cations have Lewis symbols without
valence electronsLost in the cation formation
• Anions have Lewis symbols with 8 valence electronsElectrons gained in the formation of the anion
Li• Li+1
F
1
F
Tro, Chemistry: A Molecular Approach 14
What We Know
• the noble gases are the least reactive group of elements
• the alkali metals are the most reactive metals and their atoms almost always lose 1 electron when they react
• the halogens are the most reactive group of nonmetals and in a lot of reactions they gain 1 electron
Tro, Chemistry: A Molecular Approach 15
Stable Electron ArrangementsAnd Ion Charge
• Metals form cations by losing enough electrons to get the same electron configuration as the previous noble gas
• Nonmetals form anions by gaining enough electrons to get the same electron configuration as the next noble gas
• The noble gas electron configuration must be very stable
Atom Atom’s Electron Config
Ion Ion’s Electron Config
Na [Ne]3s1 Na+1 [Ne]
Mg [Ne]3s2 Mg+2 [Ne]
Al [Ne]3s23p1 Al+3 [Ne]
O [He]2s22p4 O-2 [Ne]
F [He]2s22p5 F-1 [Ne]
Tro, Chemistry: A Molecular Approach 16
Octet Rule• when atoms bond, they tend to gain, lose, or share electrons to
result in 8 valence electrons• ns2np6
noble gas configuration• many exceptions
H, Li, Be, B attain an electron configuration like He He = 2 valence electrons Li loses its one valence electron H shares or gains one electron
though it commonly loses its one electron to become H+ Be loses 2 electrons to become Be2+
though it commonly shares its two electrons in covalent bonds, resulting in 4 valence electrons
B loses 3 electrons to become B3+
though it commonly shares its three electrons in covalent bonds, resulting in 6 valence electrons
expanded octets for elements in Period 3 or below using empty valence d orbitals
Tro, Chemistry: A Molecular Approach 17
Lewis Theory• the basis of Lewis Theory is that there are
certain electron arrangements in the atom that are more stableoctet rule
• bonding occurs so atoms attain a more stable electron configurationmore stable = lower potential energyno attempt to quantify the energy as the calculation
is extremely complex
Tro, Chemistry: A Molecular Approach 18
Covalent Bonding:Bonding and Lone Pair Electrons
• Covalent bonding results when atoms share pairs of electrons to achieve an “octet”
• Electrons that are shared by atoms are called bonding pairs
• Electrons that are not shared by atoms but belong to a particular atom are called lone pairsaka nonbonding pairs
Lone PairsBonding Pairs O S O•• ••••••••
•• ••••••
Tro, Chemistry: A Molecular Approach 19
Single Covalent Bonds• two atoms share a pair of electrons
2 electrons
• one atom may have more than one single bond
F••
••
•• • F•••••••
F••
••
•• ••
••F•••• HH O
•• ••••
••
H•H• O••
• •
••
F F
Tro, Chemistry: A Molecular Approach 20
Double Covalent Bond
• two atoms sharing two pairs of electrons4 electrons
O••••O••
••••••
O••
• •
••O••
• •
••
O O······ ··
Tro, Chemistry: A Molecular Approach 21
Triple Covalent Bond
• two atoms sharing 3 pairs of electrons6 electrons
N••
• •
•N••
• •
•
N•••••••••• N
N N····
Tro, Chemistry: A Molecular Approach 22
Covalent BondingPredictions from Lewis Theory
• Lewis theory allows us to predict the formulas of molecules
• Lewis theory predicts that some combinations should be stable, while others should notbecause the stable combinations result in “octets”
• Lewis theory predicts in covalent bonding that the attractions between atoms are directional the shared electrons are most stable between the bonding atoms resulting in molecules rather than an array
Tro, Chemistry: A Molecular Approach 23
Covalent BondingModel vs. Reality
• molecular compounds have low melting points and boiling pointsMP generally < 300°Cmolecular compounds are found in all 3 states at room
temperature• melting and boiling involve breaking the attractions
between the molecules, but not the bonds between the atoms the covalent bonds are strong the attractions between the molecules are generally weak the polarity of the covalent bonds influences the strength of
the intermolecular attractions
Tro, Chemistry: A Molecular Approach 24
Intermolecular Attractions vs. Bonding
Tro, Chemistry: A Molecular Approach 25
Ionic BondingModel vs. Reality
• some molecular solids are brittle and hard, but many are soft and waxy
• the kind and strength of the intermolecular attractions varies based on many factors
• the covalent bonds are not broken, however, the polarity of the bonds has influence on these attractive forces
Tro, Chemistry: A Molecular Approach 26
Ionic BondingModel vs. Reality
• molecular compounds do not conduct electricity in the liquid state
• molecular acids conduct electricity when dissolved in water, but not in the solid state
• in molecular solids, there are no charged particles around to allow the material to conduct
• when dissolved in water, molecular acids are ionized, and have the ability to move through the structure and therefore conduct electricity
Tro, Chemistry: A Molecular Approach 27
Bond Polarity• covalent bonding between unlike atoms results in
unequal sharing of the electronsone atom pulls the electrons in the bond closer to its
sideone end of the bond has larger electron density than the
other• the result is a polar covalent bond
bond polaritythe end with the larger electron density gets a partial
negative chargethe end that is electron deficient gets a partial positive
charge
Tro, Chemistry: A Molecular Approach 28
HF
H F••+d -d
FH
EN 2.1 EN 4.0
Tro, Chemistry: A Molecular Approach 29
Electronegativity• measure of the pull an atom has on bonding
electrons• increases across period (left to right) and• decreases down group (top to bottom)
fluorine is the most electronegative elementfrancium is the least electronegative element
• the larger the difference in electronegativity, the more polar the bondnegative end toward more electronegative atom
Tro, Chemistry: A Molecular Approach 30
Electronegativity Scale
31
Electronegativity and Bond Polarity• If difference in electronegativity between bonded atoms
is 0, the bond is pure covalentequal sharing
• If difference in electronegativity between bonded atoms is 0.1 to 0.4, the bond is nonpolar covalent
• If difference in electronegativity between bonded atoms 0.5 to 1.9, the bond is polar covalent
• If difference in electronegativity between bonded atoms larger than or equal to 2.0, the bond is ionic
“100%”
0 0.4 2.0 4.0
4% 51%Percent Ionic Character
Electronegativity Difference
Tro, Chemistry: A Molecular Approach 32
Bond Polarity
ENCl = 3.03.0 - 3.0 = 0
Pure Covalent
ENCl = 3.0ENH = 2.1
3.0 – 2.1 = 0.9Polar Covalent
ENCl = 3.0ENNa = 0.9
3.0 – 0.9 = 2.1Ionic
Tro, Chemistry: A Molecular Approach 33
Tro, Chemistry: A Molecular Approach 34
Bond Dipole Moments• the dipole moment is a quantitative way of describing the
polarity of a bonda dipole is a material with positively and negatively charged endsmeasured
• dipole moment, m, is a measure of bond polarity it is directly proportional to the size of the partial charges and
directly proportional to the distance between them m = (q)(r)not Coulomb’s Lawmeasured in Debyes, D
• the percent ionic character is the percentage of a bond’s measured dipole moment to what it would be if full ions
Tro, Chemistry: A Molecular Approach 35
Dipole Moments
Tro, Chemistry: A Molecular Approach 36
Water – a Polar Molecule
stream of water attracted to a charged glass rod
stream of hexane not attracted to a charged glass rod
Tro, Chemistry: A Molecular Approach 37
Example 9.3(c) - Determine whether an N-O bond is ionic, covalent, or polar covalent.
• Determine the electronegativity of each elementN = 3.0; O = 3.5
• Subtract the electronegativities, large minus small(3.5) - (3.0) = 0.5
• If the difference is 2.0 or larger, then the bond is ionic; otherwise it’s covalent
difference (0.5) is less than 2.0, therefore covalent• If the difference is 0.5 to 1.9, then the bond is
polar covalent; otherwise it’s covalentdifference (0.5) is 0.5 to 1.9, therefore polar covalent
Tro, Chemistry: A Molecular Approach 38
Lewis Structures of Molecules
• shows pattern of valence electron distribution in the molecule
• useful for understanding the bonding in many compounds
• allows us to predict shapes of molecules• allows us to predict properties of molecules and
how they will interact together
Tro, Chemistry: A Molecular Approach 39
Lewis Structures• use common bonding patterns
C = 4 bonds & 0 lone pairs, N = 3 bonds & 1 lone pair, O= 2 bonds & 2 lone pairs, H and halogen = 1 bond, Be = 2 bonds & 0 lone pairs, B = 3 bonds & 0 lone pairs
often Lewis structures with line bonds have the lone pairs left off their presence is assumed from common bonding patterns
• structures which result in bonding patterns different from common have formal charges
B C N O F
Tro, Chemistry: A Molecular Approach 40
Writing Lewis Structures of Molecules HNO3
1) Write skeletal structure H always terminal
in oxyacid, H outside attached to O’s
make least electronegative atom central N is central
2) Count valence electrons sum the valence electrons for each
atom add 1 electron for each − charge subtract 1 electron for each + charge
ONOH
O
N = 5H = 1O3 = 3∙6 = 18Total = 24 e-
Tro, Chemistry: A Molecular Approach 41
Writing Lewis Structures of Molecules HNO3
3) Attach central atom to the surrounding atoms with pairs of electrons and subtract from the total
ONOH
O
———
ElectronsStart 24Used 8Left 16
Tro, Chemistry: A Molecular Approach 42
Writing Lewis Structures of Molecules HNO3
4) Complete octets, outside-in H is already complete with 2
1 bond
and re-count electrons
:
::
——— ONOH
O
N = 5H = 1O3 = 3∙6 = 18Total = 24 e-
ElectronsStart 24Used 8Left 16
ElectronsStart 16Used 16Left 0
Tro, Chemistry: A Molecular Approach 43
Writing Lewis Structures of Molecules HNO3
5) If all octets complete, give extra electrons to central atom.
elements with d orbitals can have more than 8 electrons Period 3 and below
6) If central atom does not have octet, bring in electrons from outside atoms to share
follow common bonding patterns if possible
:
::
—— ONOH|
O
Tro, Chemistry: A Molecular Approach 44
Practice - Lewis Structures
• CO2
• SeOF2
• NO2-1
• H3PO4
• SO3-2
• P2H4
Tro, Chemistry: A Molecular Approach 45
Practice - Lewis Structures
• CO2
• SeOF2
• NO2-1
• H3PO4
• SO3-2
• P2H4
:O::C::O:
::
O P
O
O
O
HH
H
••
••
••
••
••
••
••
••
••
F Se
O
F
••
••
•• •
•••
••
••
••
••
••
O S
O
O
••
••
•• •
•••
••
••
••
••
••
O N O ••
••
••
••
••••
16 e-
26 e-
18 e-
26 e-
32 e-
14 e-H P P H
HH
•• ••
Tro, Chemistry: A Molecular Approach 46
Formal Charge• during bonding, atoms may wind up with more
or less electrons in order to fulfill octets - this results in atoms having a formal charge
FC = valence e- - nonbonding e- - ½ bonding e-
left O FC = 6 - 4 - ½ (4) = 0
S FC = 6 - 2 - ½ (6) = +1
right O FC = 6 - 6 - ½ (2) = -1• sum of all the formal charges in a molecule = 0
in an ion, total equals the charge
•• •• ••••••••
••O S O••••
Tro, Chemistry: A Molecular Approach 47
Writing Lewis Formulas of Molecules (cont’d)
7) Assign formal charges to the atoms a) formal charge = valence e- - lone pair e- - ½ bonding e-
b) follow the common bonding patterns
OSO
H
|
HOCCH
|||
OH
0 +1 -1
all 0
Tro, Chemistry: A Molecular Approach 48
Common Bonding Patterns
B C N O
C+
N+
O+
C-
N-
O-
B-
F
F+
-F
Tro, Chemistry: A Molecular Approach 49
Practice - Assign Formal Charges
• CO2
• SeOF2
• NO2-1
• H3PO4
• SO3-2
• P2H4
O P
O
O
O
HH
H
••
••
••
••
••
••
••
••
••
F Se
O
F
••
••
•• •
•••
••
••
••
••
••
O S
O
O
••
••
•• •
•••
••
••
••
••
••
O N O ••
••
••
••
••••H P P H
HH
•• ••
Tro, Chemistry: A Molecular Approach 50
Practice - Assign Formal Charges
• CO2
• SeOF2
• NO2-1
• H3PO4
• SO3-2
• P2H4
O P
O
O
O
HH
H
••
••
••
••
••
••
••
••
••
F Se
O
F
••
••
•• •
•••
••
••
••
••
••
O S
O
O
••
••
•• •
•••
••
••
••
••
••
O N O ••
••
••
••
••••H P P H
HH
•• ••
all 0
-1
P = +1rest 0
S = +1Se = +1
-1
-1all 0
-1
-1-1
Tro, Chemistry: A Molecular Approach 51
Resonance• when there is more than one Lewis structure for a
molecule that differ only in the position of the electrons, they are called resonance structures
• the actual molecule is a combination of the resonance forms – a resonance hybridit does not resonate between the two forms,
though we often draw it that way• look for multiple bonds or lone pairs
•••• •• ••••••••
•• ••O S O O S O•••••• ••••
••••
••••
Tro, Chemistry: A Molecular Approach 52
Resonance
Tro, Chemistry: A Molecular Approach 53
Ozone Layer
Tro, Chemistry: A Molecular Approach 54
Rules of Resonance Structures• Resonance structures must have the same connectivity
only electron positions can change• Resonance structures must have the same number of
electrons• Second row elements have a maximum of 8 electrons
bonding and nonbonding third row can have expanded octet
• Formal charges must total same• Better structures have fewer formal charges• Better structures have smaller formal charges• Better structures have − formal charge on more
electronegative atom
Tro, Chemistry: A Molecular Approach 55
O N
O
O·· ··
········
··
··
Drawing Resonance Structures1. draw first Lewis structure that
maximizes octets2. assign formal charges3. move electron pairs from atoms
with (-) formal charge toward atoms with (+) formal charge
4. if (+) fc atom 2nd row, only move in electrons if you can move out electron pairs from multiple bond
5. if (+) fc atom 3rd row or below, keep bringing in electron pairs to reduce the formal charge, even if get expanded octet.
-1
-1
+1
O N
O
O
·· ····
····
······
-1
-1 +1
Tro, Chemistry: A Molecular Approach 56
Exceptions to the Octet Rule
• expanded octetselements with empty d orbitals can have more
than 8 electrons• odd number electron species e.g., NO
will have 1 unpaired electronfree-radicalvery reactive
• incomplete octetsB, Al
Tro, Chemistry: A Molecular Approach 57
Drawing Resonance Structures1. draw first Lewis structure that
maximizes octets2. assign formal charges3. move electron pairs from atoms
with (-) formal charge toward atoms with (+) formal charge
4. if (+) fc atom 2nd row, only move in electrons if you can move out electron pairs from multiple bond
5. if (+) fc atom 3rd row or below, keep bringing in electron pairs to reduce the formal charge, even if get expanded octet.
O S
O
O
O
HH
·· ··
········
··
······
-1
-1
+2
O S
O
O
O
HH
··
······
··
······
0
0
0
Tro, Chemistry: A Molecular Approach 58
Practice - Identify Structures with Better or Equal Resonance Forms and Draw Them
• CO2
• SeOF2
• NO2-1
• H3PO4
• SO3-2
• P2H4
O P
O
O
O
HH
H
••
••
••
••
••
••
••
••
••
F Se
O
F
••
••
•• •
•••
••
••
••
••
••
O S
O
O
••
••
•• •
•••
••
••
••
••
••
O N O ••
••
••
••
••••H P P H
HH
•• ••
all 0
-1
P = +1
S = +1Se = +1
-1
-1all 0
-1
-1-1
Tro, Chemistry: A Molecular Approach 59
Practice - Identify Structures with Better or Equal Resonance Forms and Draw Them
• CO2
• SeOF2
• NO2-1
• H3PO4
• SO3-2
• P2H4
O P
O
O
O
HH
H
••
••
••
••
••
••
••
••
••
O P
O
O
O
HH
H
••
••
••
••
••
••
••
••
F Se
O
F
••
••
•• •
•••
••
••
••
••
••
F Se
O
F
••
•• •
•••
••
••
••
••
••O S
O
O
••
••
•• •
•••
••
••
••
••
••
O S
O
O
••
••
•• •
• O S
O
O
••
••
•• •
•
O S
O
O
••
••
•• •
•
••
••
••
••
••
••
••••
••
••
••
••
••
••
••
O N O ••
••
••
••
••••O N O •
•••
••
••
••••
H P P H
HH
•• ••
none
-1
-1
-1
+1
all 0
+1
all 0
-1
none
S = 0in allres. forms
Tro, Chemistry: A Molecular Approach 60
Bond Energies• chemical reactions involve breaking bonds in reactant
molecules and making new bond to create the products• the DH°reaction can be calculated by comparing the cost
of breaking old bonds to the profit from making new bonds
• the amount of energy it takes to break one mole of a bond in a compound is called the bond energy in the gas statehomolytically – each atom gets ½ bonding electrons
Tro, Chemistry: A Molecular Approach 61
Trends in Bond Energies• the more electrons two atoms share, the stronger
the covalent bondC≡C (837 kJ) > C=C (611 kJ) > C−C (347 kJ)C≡N (891 kJ) > C=N (615 kJ) > C−N (305 kJ)
• the shorter the covalent bond, the stronger the bondBr−F (237 kJ) > Br−Cl (218 kJ) > Br−Br (193 kJ)bonds get weaker down the column
Tro, Chemistry: A Molecular Approach 62
Using Bond Energies to Estimate DH°rxn
• the actual bond energy depends on the surrounding atoms and other factors
• we often use average bond energies to estimate the DHrxn
works best when all reactants and products in gas state
• bond breaking is endothermic, DH(breaking) = +• bond making is exothermic, DH(making) = −
DHrxn = ∑ (DH(bonds broken)) + ∑ (DH(bonds formed))
63
Tro, Chemistry: A Molecular Approach 64
Using Bond Energies to Estimate DH°rxn
DH°rxn NaCl
65
Estimate the Enthalpy of the Following Reaction
H H + O O H O O H
Tro, Chemistry: A Molecular Approach 66
Estimate the Enthalpy of the Following Reaction
H2(g) + O2(g) ® H2O2(g)
reaction involves breaking 1mol H-H and 1 mol O=O and making 2 mol H-O and 1 mol O-O
bonds broken (energy cost)
(+436 kJ) + (+498 kJ) = +934 kJ
bonds made (energy release)
2(464 kJ) + (142 kJ) = -1070
DHrxn = (+934 kJ) + (-1070. kJ) = -136 kJ
(Appendix DH°f = -136.3 kJ/mol)
Tro, Chemistry: A Molecular Approach 67
Bond Lengths
• the distance between the nuclei of bonded atoms is called the bond length
• because the actual bond length depends on the other atoms around the bond we often use the average bond lengthaveraged for similar bonds from
many compounds
Tro, Chemistry: A Molecular Approach 68
Trends in Bond Lengths• the more electrons two atoms share, the shorter the
covalent bondC≡C (120 pm) < C=C (134 pm) < C−C (154 pm)C≡N (116 pm) < C=N (128 pm) < C−N (147 pm)
• decreases from left to right across periodC−C (154 pm) > C−N (147 pm) > C−O (143 pm)
• increases down the columnF−F (144 pm) > Cl−Cl (198 pm) > Br−Br (228 pm)
• in general, as bonds get longer, they also get weaker
Tro, Chemistry: A Molecular Approach 69
Bond Lengths