chapter 9 chemical bonding i: lewis theory chemistry: a molecular approach, 1 st ed. nivaldo tro
TRANSCRIPT
Chapter 9Chemical
Bonding I:Lewis Theory
Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro
Question
Complete the following sentence…
Properties of substances can be explained in terms of differences in chemical __________
e.g. -salt dissolves in water better than oil
-certain substances are electrolytes
-alcohol evaporates quicker than water
-wax melts at a lower temperature than salt
Tro, Chemistry: A Molecular Approach 3
Bonding Theories
• explain how and why atoms attach together• one of the simplest bonding theories is called Lewis Theory• Lewis Theory uses valence electrons to explain bonding• explains why some combinations of atoms are stable and others
are not• using Lewis Theory, we can draw models – called Lewis
structures – that allow us to predict many properties of moleculessuch as molecular shape, size, polarity
Tro, Chemistry: A Molecular Approach 9
Types of Bonds
Types of Atoms Type of BondBond
Characteristic
metals to
nonmetalsIonic
electrons
transferred
nonmetals to
nonmetalsCovalent
electrons
shared
metal to
metalMetallic
electrons
pooled
10
Types of Bonding
Tro, Chemistry: A Molecular Approach 13
Determining the Number of Valence Electrons in an Atom
• the column number on the Periodic Table tells us the no. valence e-
1A 2A 3A 4A 5A 6A 7A 8A
Li Be B C N O F Ne
1 e- 2 e- 3 e- 4 e- 5 e- 6 e- 7 e- 8 e-
Tro, Chemistry: A Molecular Approach 14
Lewis Symbols of Atoms
• 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 15
Lewis Symbols of Ions• Cations have Lewis symbols without valence e-
e.g. lithium
• Anions have Lewis symbols with 8 valence electrons
e.g. flourine
Li• Li+
F
F
e- loss
e- gain
Question
Draw Lewis dot structures of elemental magnesium and magneisum ion
Draw Lewis dot structures of elemental nitrogen and the nitride ion
Tro, Chemistry: A Molecular Approach 18
Stable Electron ArrangementsAnd Ion Charge
• Metals form cations by losing e- to become isoelectric to the previous noble gas
• Nonmetals form anions by gaining enough e- to become isoelectric to the previous noble gas
[Ne] = 1s22s22p6
Atom Atom’s Electron Config
Ion Ion’s Electron Config
Na [Ne]3s1 Na+ [Ne]
Mg [Ne]3s2 Mg+2 [Ne]
Al [Ne]3s23p1 Al+3 [Ne]
O [He]2s22p4 O-2 [Ne]
F [He]2s22p5 F- [Ne]
Tro, Chemistry: A Molecular Approach 19
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 configuration
Tro, Chemistry: A Molecular Approach 20
Octet Rule• when atoms bond, they tend to gain, lose, or share e- to result in
8 valence e-
• ns2np6
noble gas configuration
• many exceptions H, Li, Be, B attain an electron configuration like He
He = 2 valence e- Li loses its one valence e-
H shares or gains one e-
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 22
Lewis Theory and Ionic Bonding
• Transfer of e- from metal atom to nonmetal atom, resulting in ions that are attracted to each other and therefore bond, e.g. NaCl
ClNa +
ClNa + NaCl
Tro, Chemistry: A Molecular Approach 23
Tro, Chemistry: A Molecular Approach 24
Predicting Ionic FormulasUsing Lewis Symbols
• e- are transferred until the metal loses all its valence e- and the nonmetal obtains an octet
O
Li
Li
2
O2 Li + Li2O
Tro, Chemistry: A Molecular Approach 25
Crystal Lattice
• Ionic substances exist as crystal lattices of repeating unit cells
Model of NaCl
Tro, Chemistry: A Molecular Approach 37
Ionic BondingModel vs. Reality
• ionic compounds have high melting points and boiling pointsMP generally > 300°Call ionic compounds are solids at room temperature
• because the attractions between ions are strong, breaking down the crystal requires a lot of energy the stronger the attraction (larger the lattice energy), the
higher the melting point
Properties
• Describe the general properties of ionic compounds
• Metals react with non-metals:Crystalline solids3-D units extendedhigh mp/bp (all solids)brittleaqueous solutions conduct electricity
Tro, Chemistry: A Molecular Approach 40
Ionic Bonding
• When ionic compounds are dissolved in water, they dissociate to form aqueous ions:
NaCl(s) → Na+(aq) + Cl-(aq)
• The resulting solution conducts electricity and is called an electrolyte
Tro, Chemistry: A Molecular Approach 41
Conductivity of NaCl
in NaCl(s), the ions are stuck in position and not allowed to move to the charged rods
in NaCl(aq), the ions are separated and allowed to move to the charged rods
Question
Use Lewis dot structures to represent the formation of aluminum bromide
Use Lewis dot structures to represent the formation of lithium hydride
Tro, Chemistry: A Molecular Approach 43
Tro, Chemistry: A Molecular Approach 44
Types of Bonds
Types of Atoms Type of Bond Bond Characteristic
metals to nonmetals Ionic e- transferred
nonmetals to nonmetals Covalent e- shared
metal to metal Metallic e- pooled
45
Types of Bonding
Tro, Chemistry: A Molecular Approach 47
Single Covalent Bonds• two atoms share a pair of electrons
F••
•••• • F
•••••••
F••
••
•• ••••F••••
F F
e.g. fluorine
Tro, Chemistry: A Molecular Approach 48
Single Covalent Bonds
HH O•• ••••••
H•H• O••
• •
••e.g. water
octet
duet duet
2 bonding pairs2 lone pairs
Tro, Chemistry: A Molecular Approach 49
Double Covalent Bond• two atoms sharing two pairs of electrons
O••••O••
••••••
O••
• •
••O••
• •••
O O······ ··
e.g. oxygen
Tro, Chemistry: A Molecular Approach 50
Triple Covalent Bond• two atoms sharing 3 pairs of electrons
N••
• •
•N••
• ••
N•••••••••• N
N N····
e.g. nitrogen
Tro, Chemistry: A Molecular Approach 51
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 not because the stable combinations result in “octets”
• Lewis theory also shows that covalent bonds are highly directional the shared electrons are most stable between the bonding atoms resulting in molecules rather than an array
Tro, Chemistry: A Molecular Approach 55
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 56
Bond Polarity• covalent bonding between unlike atoms results in unequal sharing
of the e-
one 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 polarity the end with the larger electron density gets a partial negative
charge the end that is electron deficient gets a partial positive charge
Tro, Chemistry: A Molecular Approach 57
HF
FH
EN 2.1 EN 4.0EN 2.1
Tro, Chemistry: A Molecular Approach 58
Electronegativity
• Ability of an atom to attract e- to itself in a chemical bond• increases across period (left to right) and• decreases down group (top to bottom)
59
Electronegativity and Bond Polarity• If ΔE.N. between bonded atoms is 0, the bond is pure covalent
equal sharing• If ΔE.N. between bonded atoms is 0.1 - 0.4, the bond is nonpolar covalent• If ΔE.N. between bonded atoms 0.5 - 1.9, the bond is polar covalent• If ΔE.N. between bonded atoms ≥ 2.0, the bond is ionic
“100%”
0 0.4 2.0 4.0
4% 51%
Percent Ionic Character
Electronegativity Difference
IONICPCNP
Tro, Chemistry: A Molecular Approach 60
Bond Polarity
ENCl = 3.0ΔEN = 3.0 - 3.0 = 0
Pure Covalent
ENCl = 3.0ENH = 2.1
ΔEN = 3.0 – 2.1 = 0.9Polar Covalent
ENCl = 3.0ENNa = 1.0
ΔEN = 3.0 – 0.9 = 2.1Ionic
Tro, Chemistry: A Molecular Approach 66
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 68
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 e-
sum the valence electrons for each atom
add 1 e- for each −ve charge subtract 1 e- for each +ve charge
ONOH
O
N = 5H = 1O3 = 3(6) = 18Total = 24 e-
Tro, Chemistry: A Molecular Approach 69
Writing Lewis Structures of Molecules HNO3
3) Attach central atom to the surrounding atoms with pairs of e- and subtract from the total
ONOH
O
———
e-
Start 24Used 8Left 16
Tro, Chemistry: A Molecular Approach 70
Writing Lewis Structures of Molecules HNO3
4) Complete octets, outside-in H is already complete with 2
1 bond
and re-count e-
:
::
——— ONOH
O
N = 5H = 1O3 = 3(6) = 18Total = 24 e-
e-
Start 24Used 8Left 16
e-
Start 16Used 16 (8 pairs)Left 0
Tro, Chemistry: A Molecular Approach 71
Writing Lewis Structures of Molecules HNO35) 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 72
Practice - Lewis Structures
• CO2
• NO2-
• NH3
Draw Lewis structures for the following:
Tro, Chemistry: A Molecular ApproacH74
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 0sum of all the formal charges in a molecule = 0in an ion, total equals the charge
Tro, Chemistry: A Molecular Approach 76
Practice - Assign Formal Charges
• CO2
• NO2-
• NH3
O N O ••
••
••
••
••••-
Tro, Chemistry: A Molecular Approach 77
Practice - Assign Formal Charges
• CO2
• NO2-
• NH3
O N O ••
••
••
••
••••
all 0
-1
all 0
-
Tro, Chemistry: A Molecular Approach 78
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 hybrid it 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 81
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 82
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
0
-
-
Tro, Chemistry: A Molecular Approach 83
Exceptions to the Octet Rule
• expanded octetselements with empty d orbitals can have more
than 8 electrons
• odd number electron species e.g., NOwill have 1 unpaired electronfree-radicalvery reactive
• incomplete octetsB, Al
Tro, Chemistry: A Molecular Approach 84
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
Question
Draw Lewis structures with assigned formal charges of HCl, H2O2 and SF6
Tro, Chemistry: A Molecular Approach 97
Metallic Bonds• low ionization energy of metals allows them to lose electrons
easily• the simplest theory of metallic bonding involves the metals
atoms releasing their valence electrons to be shared by all to atoms/ions in the metalan organization of metal cation islands in a sea of electronselectrons delocalized throughout the metal structure
• bonding results from attraction of cation for the delocalized electrons
Tro, Chemistry: A Molecular Approach 98
Metallic Bonding
Tro, Chemistry: A Molecular Approach 99
Metallic BondingModel vs. Reality
• metallic solids conduct electricity• because the free electrons are mobile, it allows the
electrons to move through the metallic crystal and conduct electricity
• as temperature increases, electrical conductivity decreases • heating causes the metal ions to vibrate faster, making it
harder for electrons to make their way through the crystal
Tro, Chemistry: A Molecular Approach 100
Metallic BondingModel vs. Reality
• metallic solids conduct heat
• the movement of the small, light electrons through the solid can transfer kinetic energy quicker than larger particles
• metallic solids reflect light
• the mobile electrons on the surface absorb the outside light and then emit it at the same frequency
Tro, Chemistry: A Molecular Approach 101
Metallic BondingModel vs. Reality
• metallic solids are malleable and ductile• because the free electrons are mobile, the direction of the
attractive force between the metal cation and free electrons is adjustable
• this allows the position of the metal cation islands to move around in the sea of electrons without breaking the attractions and the crystal structure
Tro, Chemistry: A Molecular Approach 102
Metallic BondingModel vs. Reality
• metals generally have high melting points and boiling pointsall but Hg are solids at room temperature
• the attractions of the metal cations for the free electrons is strong and hard to overcome
• melting points generally increase to right across period• the charge on the metal cation increases across the period,
causing stronger attractions• melting points generally decrease down column• the cations get larger down the column, resulting in a larger
distance from the nucleus to the free electrons