new bonding: part two · 2014. 10. 30. · •metallic bond mobile valence e-(fe) •covalent bond...
TRANSCRIPT
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Bonding: Part TwoThree types of bonds:
•Ionic Bond transfer valence e- (NaCl)
•Metallic bondmobile valence e- (Fe)
•Covalent bond shared valence e- (H2O)
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Single Covalent Bond
+H H
H-atoms
H H
H2 molecule
Electrons are sharedby the two H atoms
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Single Covalent Bond
“Structural” Formula
H—H
Single covalent bond (2 shared e-)
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Ionic vs. Covalent Compounds
Covalent: discrete molecules
H2O, CH4, CO2
Ionic: cations and anions (occupy crystal lattice points)
”formula unit” NaCl, MgI2
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Covalent BondCovalent bonds usually form between nonmetal atoms (Groups 14, 15, 16, and 17).
Each atom tries to attain the e-
configuration of a noble gas by sharing electrons. (“octet rule”)
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Back to H2
+H H
H-atoms
H H
Each H has e-
config. of He
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Fluorine: F2 or F—F
F F+ F F
Each F has e-
config of [Ne]
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Lone Pairs
F F Each F has an octet
“lone pairs” or “nonbonding pairs” of e-
Paired valence e- not in the bond
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Water (H2O)
H
H
O O H
H
Each atom has Noble Gas configuration
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You Try It!
1.Ammonia (NH3)
2.Chlorine gas (Cl2)
3.Methane (CH4)
Draw dot structures for:
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Multiple Bonds
Sometimes more than one pair of bonding electrons are needed in a bond to attain a noble gas configuration.
Double bond: two pairs of e-
Triple bond: three pairs of e-
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Double Bond: O2
O+O
Still no octet, so form a double bond
O O
OO OOor
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Triple Bond
N N N N
Try nitrogen (N2)
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Coordinate Covalent Bond
Sometimes both e- in the bond come from just one of the atoms.
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Coordinate Covalent Bond
CO + O C
no octet
O C
e.g. CO
e- pair came from oxygen
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Coordinate Covalent Bond
The ammonium ion, NH4+
NH+ +
H
H
H
N
H
H
H
H
+
no e-
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Dot Structure Rules
e.g. NF31.Arrange the atoms with least
electronegative element in center.The central atom is never hydrogen.
F N FF
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Dot Structure Rules2.Count total valence electrons.
Account for charges in polyatomic ions.
F N FF
5 + 3(7) = 26
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Dot Structure Rules3.Connect atoms with single
covalent bonds. Then complete octets (H has only 2 not 8).
F N FF
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Dot Structure Rules4.If octet rule is not satisfied for the
central atom, try double or triple bonds, using lone pairs from surrounding atoms.
OKF N FF
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Dot Structure HNO3
Step 1. Skeletal structure
O N O H
O
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HNO3Step 2. Number of valence e-
O N O H
O
N, O, H 5 + 3(6) + 1 = 24
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HNO3Step 3. Add bonds and
complete octets
Out of e-, but no octet for N
O N O H
O
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HNO3Step 4. Add multiple bonds
O N O H
O
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You Try It !!!
Draw the e- dot structures for:1.Hydrogen chloride HCl
2.Hydrogen peroxide H2O2
3.Hydronium ion H3O+
4.Ozone O3
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Lewis Structures
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Resonance: Ozone O3
O O O
O O OEach is called a “resonance structure”. The bonds are equal (~1.5 bond)
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Resonance: Try It !!!
Draw resonance structures for SO2
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Exception to the Octet Rule: Odd # e-
Try to write the dot structure of nitrogen monoxide.
With an odd number of valence electrons (11), it is impossible to have octets around both atoms.
NO is “paramagnetic.”
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Predicting Molecular Shapes
linear triatomic trigonal planar
bent triatomic
trigonalpyramid tetrahedral others
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VSEPR Theory
“Valence-shell e- pair repulsion”
•All valence electron pairs
(bonding & nonbonding pairs)
repel each other.
• Predicts geometry.
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VSEPR Theory
Methane (CH4) is drawn as:
H—C—H
H
H
or C
H
H
H
H
Actually CH4 is 3-D
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Maximum
repulsion of
e- pairs
tetrahedron
C
109.5o
VSEPR Theory
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VSEPR Theory
Ammonia (NH3)
H—N—H
H 3 bonding e- pairs
1 nonbonding e- pair
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Ammonia
tetrahedral
electrons
trigonal pyramidal
atoms
VSEPR Theory
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VSEPR Theory
Water (H2O)
H—O—H
2 bonding e- pairs
2 nonbonding e- pairs
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37e-: tetrahedral atoms: bent linear
VSEPR Theory
Water
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1.Draw the Lewis dot structure.
2.Move e- pairs (bonding and lone pairs) as far apart as possible.
3.Treat double and triple bonds as if they were single bonds.
4.Distinguish between shape of e-
pairs and molecular shape
VSEPR: Rules
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Possible Molecular Shapes
See problem set.
linear bent
tetrahedron trigonal pyramid trigonal planar
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Predict the Molecular Shape Hydrogen sulfide: H2S
Carbon tetrachloride: CCl4
Sulfur dioxide: SO2
Sulfur trioxide: SO3
Nitrogen tribromide: NBr3
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Is Breaking a Bond Endo- or Exothermic?
Endothermic! It takes energy to break a bond.
Energy is given off when bonds form (exothermic).
molecule
atoms
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Bond Energy
DH = +435 kJ (per mol)
H—H H + H
The energy needed to break a bond is called “bond dissociation energy” or “bond energy”.
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Bond Bond Energy (kJ)
Bond Length (pm)
H—H 435 74
C—H 393 109
C—C 347 154
C C 657 133
C C 908 121
Bond Dissociation Energy
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Bond Strength vs. Length
Multiple bonds have shorter bond lengths! Why?
C—C C C C Cweakest, strongest,longest shortest
Multiple bonds are stronger than single bonds.
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Bond PolarityCovalent bonds involve sharing e-, however the two bonded atoms don’t always share equally.
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Bond PolarityHow do you know which atom wins the “tug-of war” for the bonding electrons?
The more electronegative one.
Regents Table ‘S’
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Bond PolarityIn some cases neither atom wins.Both atoms have same electronegativity.
Hydrogen
Nitrogen
Oxygen
Chlorine
“Nonpolar”
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Polar Molecules
FH
Bonding electrons shift toward F.
Thus HF is polar.
d+ d-
EN = 2.2 EN = 4.0
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Water
O
H HEN = 2.2
EN = 3.4Each bond in water is polar, and the overall molecule is polar because of its shape(nonsymmetrical).
Distinguish between bond polarity and molecular polarity.
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Nonpolar MoleculesO C O
Even though CO2 has polar bonds,
it is a nonpolar molecule.
Where is the average center of positive charge, and where is the center of negative charge?
CO2
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Summary: Polar Molecules
If only 2 atoms in the molecule: the molecule is polar if the atoms have different electronegativities.
H-Clpolar
Br-Brnonpolar
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Summary: Polar MoleculesIf more than 2 atoms in molecule:
Draw dot structure
If the central atom has• a lone pair or• if the outside atoms are
different, the molecule is polar (not symmetrical).
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Summary: Polar Molecules
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CH4 is nonpolar (symmetrical)
CH2F2 is polar (nonsymmetrical)
NH3 is polar (nonsymmetrical)
CH4 CH2F2 NH3
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Polar Molecule: Yes or No?
CHCl3 HI
NI3 Br2
SO3 CI4
Symmetrical nonpolarNonsymmetrical polar
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Bond Polarity & Bond TypeIf the difference in electronegativityis greater than ~2.0, one atom takes all the bonding e- and the compound is ionic.
What types of elements would have a big difference in EN?
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Difference in Electronegativity
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0 1.0 2.0 3.0
more ionic
more covalent
H2 HCl NaCl CsFnonpolar polar
covalent ionic
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Identify Bond Types nonpolar covalent polar covalent ionic for these pairs of atoms:
Cs & F P & O H & Br
http://www.wimp.com/chemistrydogs/
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Ionic vs. Covalent Compounds
Ionic Covalent
Unit formula unit molecule
Bond e- transfer e- sharing
Elements metal + nonmetal
nonmetals
D EN > 2.0 < 2.0
State @STP solid S, L, G
M. P. high low
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Warm-up
60
Write the e- dot symbol of calcium chloride.
Define the metallic bond. How does this bonding explain why metals conduct electricity?
Draw dot structure for methane, CH4.
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Warm-up
61
What is the Lewis dot structure of PH3 ?
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Warm-up:
62
Write dot structure:
• ammonium ion
• nitrate ion
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Warm-up
63
Draw the e- dot structure for NO2and draw its resonance structures.
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Warm-up
64
Predict the shape of: SeO3 CS2 PCl3 NO2
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Warm-up
65
What is the molecular shape of:NBr3
SCl2
CO3-2
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Warm-up
66
Write the dot formula for the bromine atom and the bromide ion.
What is the shape of the nitrate ion?
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Warm-up
67
Draw the correct dot structure and state whether the molecule is polar or nonpolar?
• SO2
• PBr3
• CH2Br2
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Warm-up
68
Draw a diagram showing at least 12 atoms of metallic potassium in the solid state.
Write the dot formula for sodium oxide.