chapter 5. covalent compounds (molecular compounds)
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Chapter 5. Covalent Compounds (Molecular Compounds)
NaCl (ionic compound) heat
Na+ + Cl- (gas)
H2O (liquid) Heat
H2O molecules (gas)
NH3 NH3 (molecules)
etc.
A molecular formula tells the # of atoms of each element in a molecule of the compound
C2F4 C2H6O
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A. Covalent bonds
Example H + H H2
H H Sharing of electrons H H
or H H
2
H + Cl HCl
H Cl Lewis structureClHCl
Octet rule
Draw the Lewis structures of
H2O
3
NH3
CH4
4
Draw Lewis structures for the following compounds
Consider O2
Consider N2
CH2O
C2H4
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B. Coordinate Covalent Bonds (less common)
B
H
H
H
B
H
H
H
N
H
H
HB
H
H
H
= N
H
H
HB
H
H
H
Electron deficient compound
Coordinate covalent bond
BH3
NH3
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Draw Lewis structure for each of the following molecular formulas in the most stable form (by pure sharing of electrons).
a) PCl3 b) C2F6
c) CH2O2 d) CH3N
e) C2H2Cl2 f) N2O2
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Common elements in covalent compounds: C, O, N
C C C C
N N N
O O
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For compounds containing C, H, O, N (the big 4), and F, try this
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HCN
CO2
C3H4
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C. Compounds not following the Octet Rule
NO
PCl5
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E. Lewis structures of Polyatomic ions or molecules with a central atom.
1. Calculate the total number of valence electrons.2 Draw a single bond between the central atom and each of the surrounding atoms.
3. Add nonbonding electrons to surrounding atoms such that each has an octet of electrons (2 on H).4. Place the remaining electrons on the central atom.5. If the central atom does not have octet of electrons, use one or two pairs of nonbonding e’s from the surround atoms to form double or triple bonds with the central atom.6. Check the total number of electrons.
NO2-
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Resonance
ON
O ON
O
Resonance structuresor resonance contributors
NO2-
The real molecule or ion is a resonance hybrid of the resonance structures.Each resonance structure is less stable than the resonance hybrid.
- -
O N O-
??
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Neutral molecules with a central atom
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Examples:
NO3-
SO32-
Polyatomic ions
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Lewis dot structures of ionic compounds:
K2SO3
Ca(NO3)2
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F. Electronegativity (EN)
Electronegativity of an element = the relative tendency of its atoms to attract the bonding electron pair.
H Cl H Clor
EN of Cl > EN of H
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Fig.5.11 Pauling Electronegativity Values
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-
H Cl H Cl
G. Polar covalent bond
Polar covalent bond
Figure 5.12:
(a) (b) Nonpolar and Polar Covalent Bond
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The relative E.N. determines the bond type
Examples:
Bond Type Electronegativity Difference
Nonpolar Covalent 0.4 or less
Polar Covalent Greater than 0.4 to 1.5
Polar CovalentBetween 1.5 and 2.0 (between nonmetals)
IonicBetween 1.5 and 2.0 (metal and nonmetal)
Ionic Greater than 2.0
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ExerciseArrange the following bonds from most to least polar:
a) N-F O-F C-F
b) C-F N-O Si-F
c) Cl-Cl B-Cl S-Cl
H. Molecular Geometry Valence shell electron pair repulsion (VSEPR) theory
CH4 H C
H
H
H
All 4 bonds are equivalent
H
C
HHH
H
C
HHH
Tetrahedral
Cs p p p
sp3 sp3 sp3 sp3
Four sp3 hybrid orbits
109.5o
Lewis structure
hybr
idize
Electron pair arrangement
Molecular geometry
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o180o120o 109.5o
hybrid orbitals: sp sp2 sp3
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NH3H N H
H
N
HHH
Electron pair arrangement: tetrahedralMolecular geometry: trigonal pyramidal
sp3
H2O H O H O
H
H
sp3
Electron pair arrangement: tetrahedralMolecular geometry: angular
Lewis structure
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BH3
Lewis structure
H B H
H
H
B
H H
s p p pB
s p p p
sp2 sp2 sp2
Three sp2 hybrid orbitals
sp2
Trigonal planar
Electron pair arrangement: trigonal planarMolecular geometry: trigonal planar 25
SO2 O S OS
O O
Electron pair arrangement: trigonal planarMolecular geometry: bent
BeH2
s ps p
sp2
Two sp hybrid orbitalsH Be H sp
Electron pair arrangement: linearMolecular geometry: linear
sp sp
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hybrid orbitals: sp sp2 sp3
o180o
120o 109o
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The Shape (Geometry) of Molecules
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Summary# of groups electron pair makeup molecular hybridof electrons (density) of e- groups geometry orbitalsaround arrangementcentral atom 4 tetrahedral 4 bonding tetrahedral
3 bonding trigonal1 nonbonding pyramidal sp3
2 bonding angular2 nonbonding (bent)
3 Trigonal 3 bonding Trigonal planar planar sp2
2 bonding Angular1 nonbonding
2 Linear 2 bonding Linear sp29
Examples:
HCN
Electron pairarrangement
Moleculargeometry
Hybirdorbitals
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I. Polarity of Molecules
H Cl One polar bondPolar molecule
O
H Hnet
O C O
Polar molecule
Non polar moleculeThe 2 polar bonds cancel each other
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Summary: • Draw the Lewis structure• If all electron groups around the central atom are connected to the same atom – nonplar otherwise - polar
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J. Naming of binary molecular compounds
mono- 1di- 2tri- 3tetra- 4penta- 5hexa- 6hepta- 7octa- 8ennea-(neno) 9deca- 10
P2O5 diphosphorus pentaoxide
N2O4 dinitrogen tetraoxide
CO2
SO2
NO
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