Chapter 5. Covalent Compounds (Molecular Compounds)

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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