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Chapter 9Chapter 9Chemical Chemical Bonding I:Bonding I:Lewis Lewis TheoryTheory
Determining the Number of Determining the Number of Valence Electrons in an AtomValence Electrons in an Atom
the column number on the Periodic Table will tell you how many valence electrons a main group atom has◦ Transition Elements all have 2 valence
electrons; Why?
2
1A 2A 3A 4A 5A 6A 7A 8A
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
Lewis Symbols of AtomsLewis Symbols of Atomsaka electron dot symbolsuse symbol of element to represent nucleus
and inner electronsuse dots around the symbol to represent
valence electrons◦ pair first two electrons for the s orbital◦ put one electron on each open side for p
electrons ◦ then pair rest of the p electrons
3
Lewis Symbols of IonsLewis Symbols of IonsCations have Lewis symbols without valence
electrons◦ Lost in the cation formation
Anions have Lewis symbols with 8 valence electrons◦ Electrons gained in the formation of the
anion
4
Li• Li+1
F
1
F
Stable Electron ArrangementsStable Electron ArrangementsAnd Ion ChargeAnd 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
5
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]
RulesRules when atoms bond, they tend to gain, lose, or share
electrons to result in 8 valence electrons ns2np6
◦ noble gas configuration Duet Rule: sharing of 2 electrons
◦ E.g H2 H : H
Octet Rule: sharing of 8 electrons◦ Carbon, oxygen, nitrogen and fluorine always obey
this rule in a stable molecule◦ E.g F2, O2
6
ExceptionsExceptions 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
Lewis TheoryLewis Theory the basis of Lewis Theory is that there are certain
electron arrangements in the atom that are more stable◦ octet rule
bonding occurs so atoms attain a more stable electron configuration◦ more stable = lower potential energy◦ no attempt to quantify the energy as the calculation
is extremely complex Bonding pair: two of which are shared with other
atoms Lone pair or nonbonding pair: those that are not
used for bonding
8
Electron-Dot StructuresElectron-Dot Structures
HH O•••••
•
••H•H• O
•••
•
••
Electron-Dot StructuresElectron-Dot Structures
Covalent BondingCovalent BondingPredictions from Lewis TheoryPredictions 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 not◦ because 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
11
ElectronegativityElectronegativity 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 element◦ francium is the least electronegative element
the larger the difference in electronegativity, the more polar the bond◦ negative end toward more electronegative atom
12
Bond PolarityBond Polarity covalent bonding between unlike atoms results in
unequal sharing of the electrons◦ one atom pulls the electrons in the bond closer to its
side◦ one end of the bond has larger electron density than
the other the result is a polar covalent bond
◦ bond polarity◦ the end with the larger electron density gets a partial
negative charge◦ the end that is electron deficient gets a partial positive
charge
13
Polar Covalent Bonds: Polar Covalent Bonds: ElectronegativityElectronegativity
NaCl
HCl
Cl2
Electronegativity and Bond Electronegativity and Bond PolarityPolarity
If difference in electronegativity between bonded atoms is 0, the bond is pure covalent◦ equal 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
15
“100%”0 0.4 2.0 4.0
4% 51%Percent Ionic Character
Electronegativity Difference
Bond Dipole MomentsBond 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 polarityit is directly proportional to the size of the partial
charges and directly proportional to the distance between them m = (q)(r)
r = radius q = 1.6 x 10-19 C
1 D = 3.34 x 10-30 C•m
16
% ionic character =
actual dipole moment
dipole moment if electrons were completely transferred
x 100
Polarity and Dipole MomentPolarity and Dipole Moment Dipole moment:
◦ a vector quantity from the center of the positive charge to the center of negative charge
◦ Represents with an arrow
ExampleExample Determine whether bond formed between the following
pair is ionic, covalent, or polar covalent◦ N and O◦ Sr and F◦ N and Cl
E.g Draw the dipole moment for HF HCl OF
Lewis Structures of Lewis Structures of MoleculesMolecules 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
19
Rules for writing Dots Lewis Rules for writing Dots Lewis structuresstructures
Write the correct skeletal structure for molecule◦ Least electronegative atom will be in the center◦ Hydrogen will always be the terminal
Calculate the total number of electrons for the Lewis structure by summing the valence electrons of each atom in the molecule◦ If polyatomic ions, charges must be considered when
calculating the total valence electrons Distribute the electrons among the atoms, giving octets
(or duet for hydrogen) to as many atoms as possible If any atoms lack an octet, form double or triple bonds
as necessary to give them octets.
Lewis StructuresLewis 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
21
B C N O F
Formal ChargeFormal 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-
sum of all the formal charges in a molecule = 0◦ in an ion, total equals the charge
22
ExamplesExamples
Draw a Lewis formula then assign formal charge for the following molecules and/or ions
HBr CO2
NH4+ SO3
2-
ResonanceResonance 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 hybrid◦ it does not resonate between the two forms, though
we often draw it that way look for multiple bonds or lone pairs
24
Rules of Resonance Rules of Resonance StructuresStructures 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
25
Drawing resonanceDrawing resonance Any compound for which more than one Lewis structure
may be written is accurately described by no single structure. The actual structure is a resonance hydrid of them all (NOT “flipping back and forth” between resonance forms). The various structures are called contributing structure or resonance forms
C
H
H
N
H
H
C N
H
H
H
H
resonance forms = resonance contributors
C N
H
H
H
H
combined representationresonance hybrid
Drawing Resonance Drawing Resonance StructuresStructures
27
O N
O
O·· ··
········
··
··
1. draw first Lewis structure that maximizes octets
2. 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
Exceptions to the Octet RuleExceptions to the Octet Rule expanded octets
◦ elements with empty d orbitals can have more than 8 electrons
odd number electron species e.g., NO◦ will have 1 unpaired electron◦ free-radical◦ very reactive
incomplete octets◦ B, Al
28
Drawing Resonance Drawing Resonance StructuresStructures
29
1. draw first Lewis structure that maximizes octets
2. 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
ExamplesExamplesIdentify Structures with Better or Equal
Resonance Forms and Draw Them◦O3
◦NO2-
◦PO43-