Chapter 8

1
Chapter 8Covalent Bonding
2
Covalent Bonds

• covalent combination of prefix co- (Latin -
  together),
• valere, - to be strong
• 2 e-s shared have strength to hold 2 atoms
  together
• particle called a molecule

3
Molecules

• Some elements in nature are molecules
• neutral group of atoms covalently bonded
• Ex. - air contains O molecules, 2 O atoms joined
  covalently
• Called diatomic molecule (O2)

4
How does H2 form?

• The nuclei repel each other, (both have charge)

(diatomic hydrogen molecule)
5
How does H2 form?

• nuclei attraction to e-s stronger than repulsion
  of nuclei
• e-s shared
• covalent bond
• Only NONMETALS!

6
Covalent bonds

• Nonmetals hold valence e-s
• dont give away e-s
• still want NGC
• share valence e-s with each other covalent
  bonding
• both atoms count e-s for NGC

Covalent bonding w/ Fluorine atoms
7
Covalent bonding

• Fluorine has seven valence electrons
• A second atom also has seven
• By sharing electrons
• both end with full orbitals

F
F
8 Valence electrons
8
Covalent bonding

• Fluorine has seven valence electrons
• A second atom also has seven
• By sharing electrons
• both end with full orbitals

F
F
8 Valence electrons
single covalent bond between 2 H atoms
9
Molecular Compounds

• Compounds bonded covalently called molecular
  compounds
• Molecular compounds have
• lower melting and boiling points
• Weaker bond than ionic
• gases or liquids at room temperature
• a molecular formula
• Shows how many atoms of each element a molecule
  contains

10
Reminder from Ch. 7

• No molecule of sodium chloride
• Ionic cmpds exist as collection of - charged
  ions arranged in repeating 3D patterns.

11
Molecular Compounds

• The formula for water is written as H2O
• The subscript 2 behind hydrogen means 2 atoms
  of hydrogen
• subscript 1 omitted
• Molecular formulas do not tell any information
  about structure (arrangement of various atoms).

12
- Page 215
3. The ball and stick model is BEST, because it
shows 3D arrangement.
These are some of the different ways to represent
ammonia
1. The molecular formula shows how many atoms of
each element are present
2. The structural formula ALSO shows the
arrangement of these atoms!
13
Section 8.2The Nature of Covalent Bonding
14
A Single Covalent Bond is...

• sharing 2 valence e-s
• Only nonmetals and hydrogen.
• Different from ionic bond b/c they actually form
  molecules.
• Two specific atoms joined
• In an ionic solid, you cant tell which atom e-s
  moved from or to

15
How to show the formation

• Its like a jigsaw puzzle.
• You put the pieces together to end up with the
  right formula.
• Carbon is a special example - can it really share
  4 electrons 1s22s22p2?
• 2p
• 1s 2s
  
• C
• Yes, due to electron promotion!

16
How to show the formation

• Its like a jigsaw puzzle.
• You put the pieces together to end up with the
  right formula.
• Carbon is a special example - can it really share
  4 electrons 1s22s22p2?
• 2p
• 1s 2s
  

17
Water
Another example water is formed with covalent
bonds, by using an electron dot diagram

• Each hydrogen has 1 valence electron
• - Each hydrogen wants 1 more
• The oxygen has 6 valence electrons
• - The oxygen wants 2 more
• They share to make each other complete

18
Water

• Put the pieces together
• The first hydrogen is happy
• The oxygen still needs one more

H
19
Water

• So, a second hydrogen attaches
• Every atom has full energy levels

Note the two unshared pairs of electrons
H
H
20
(No Transcript)
21

• Examples
• Conceptual Problem 8.1 on page 220
• Well do 7 8

22
Double and Triple Covalent Bonds

• Sometimes atoms share more than one pair of
  valence e-s
• double bond atoms share 2 pairs of e-s (4
  total)
• triple bond atoms share 3 pairs of e-s (6
  total)
• Table 8.1, p.222 - Know these 7 elements as
  diatomic
• Br2 I2 N2 Cl2 H2 O2 F2

23
Dot diagram for Carbon dioxide

• CO2 - Carbon is central atom ( more metallic )
• Carbon has 4 valence e-s
• Wants 4 more
• Oxygen has 6 valence e-s
• Wants 2 more

C
The chemistry of CO2 644
24
Carbon dioxide

• Attaching 1 oxygen leaves the oxygen 1 short, and
  the carbon 3 short

C
25
Carbon dioxide

• Attaching the second oxygen leaves both of the
  oxygen 1 short, and the carbon 2 short

C
26
Carbon dioxide

• only solution ? share more

C
27
Carbon dioxide

• The only solution is to share more

C
28
Carbon dioxide

• The only solution is to share more

C
O
29
Carbon dioxide

• The only solution is to share more

C
O
30
Carbon dioxide

• The only solution is to share more

C
O
31
Carbon dioxide

• The only solution is to share more

C
O
O
32
Carbon dioxide

• The only solution is to share more
• Requires 2 double bonds
• Each atom can count all the electrons in the bond

C
O
O
33
Carbon dioxide

• The only solution is to share more
• Requires two double bonds
• Each atom can count all the electrons in the bond

8 valence electrons
C
O
O
34
Carbon dioxide

• The only solution is to share more
• Requires two double bonds
• Each atom can count all the electrons in the bond

8 valence electrons
C
O
O
35
Carbon dioxide

• The only solution is to share more
• Requires two double bonds
• Each atom can count all the electrons in the bond

8 valence electrons
C
O
O
How covalent bonds form - Mark Rosengarden
36
How to draw them?

• Use the handout guidelines
• Add up all valence e-s
• Count total e-s needed to make all atoms happy
  (stable)
• Subtract Divide by 2 (tells you how many bonds
  to draw)
• Choose central atom (least electronegative)
• Start w/ most electronegative atom, fill in
  remaining valence e-s to fill atoms up

37
Examples

• NH3, which is ammonia
• N central atom has 5 valence electrons, wants
  8
• H - has 1 (x3) valence electrons, wants 2 (x3)
• NH3 has 53 8
• NH3 wants 86 14
• (14-8)/2 3 bonds
• 4 atoms with 3 bonds

N
H
38
Examples

• Draw in the bonds start with singles
• All 8 e- accounted for
• Everything full DONE!

H
N
H
H
39
Example HCN

• HCN C is central atom
• N - has 5 valence electrons, wants 8
• C - has 4 valence electrons, wants 8
• H - has 1 valence electron, wants 2
• HCN has 541 10
• HCN wants 882 18
• (18-10)/2 4 bonds
• 3 atoms with 4 bonds this will require multiple
  bonds - not to H however

40
HCN

• Put single bond between each atom
• Need to add 2 more bonds
• Must go between C and N (Hydrogen is full)

N
H
C
41
HCN

• Put in single bonds
• Needs 2 more bonds
• Must go between C and N, not the H
• Uses 8 electrons need 2 more to equal the 10 it
  has

N
H
C
42
HCN

• Put in single bonds
• Need 2 more bonds
• Must go between C and N
• Uses 8 electrons - 2 more to add
• Must go on the N to fill its octet

N
H
C
43
Another way of indicating bonds

• Often use a line to indicate a bond
• Called a structural formula
• Each line 2 valence e-s

H
H
O
H
H
O

44
Other Structural Examples
H C N
H
C O
H
45
A Coordinate Covalent Bond...

• When one atom donates both electrons in a
  covalent bond.
• Carbon monoxide (CO) is a good example

Both the carbon and oxygen give another single
electron to share
46
Coordinate Covalent Bond

• When one atom donates both electrons in a
  covalent bond.
• Carbon monoxide (CO) is a good example

Oxygen gives both of these electrons, since it
has no more singles to share.
This carbon electron moves to make a pair with
the other single.
O
C
47
Coordinate Covalent Bond

• When one atom donates both electrons in a
  covalent bond.
• Carbon monoxide (CO)

The coordinate covalent bond is shown with an
arrow as
O
C
C O
48
Coordinate covalent bond

• Most polyatomic cations and anions contain
  covalent coordinate covalent bonds
• Table 8.2, p.224
• Sample Problem 8.2, p.225
• The ammonium ion (NH4) can be shown as another
  example

49
Bond Dissociation Energies...

• Total energy required to break bond btwn 2
  covalently bonded atoms
• High dissociation energy usually means compound
  relatively unreactive, b/c it takes hi energy to
  break bond

50
Resonance is...

• When more than one valid dot diagram is possible.
  
• Consider the two ways to draw ozone (O3)
• Which one is it? Does it go back and forth?
• Its hybrid of both, shown by double-headed arrow
• found in double-bond structures!

51
Resonance in Ozone
Note the different location of the double bond
Neither single structure is correct, actually a
hybrid of the two. To show it, draw all possible
structures, and join them with a double-headed
arrow.
52
Resonance

• Occurs when more than one valid Lewis structure
  can be written for particular molecule (due to
  position of double bond)

• resonance structures of carbonate ion (used in
  production of carbonated beverages).
• The actual structure is an avg (or hybrid) of
  these structures.

53
Polyatomic ions note the different positions of
the double bond.
Resonance in a carbonate ion (CO32-)
Resonance in an acetate ion (C2H3O21-)
54
The 3 Exceptions to Octet rule

• For some molecules, it is impossible to satisfy
  the octet rule
• 1. usually when there is an odd number of
  valence electrons
• NO2 has 17 valence electrons, because the N has
  5, and each O contributes 6. Note N page 228
• It is impossible to satisfy octet rule, yet the
  stable molecule does exist

55
Exceptions to Octet rule

• Another exception Boron
• Page 228 shows boron trifluoride, and note that
  one of the fluorides might be able to make a
  coordinate covalent bond to fulfill the boron
• 2 -But fluorine has a high electronegativity (it
  is greedy), so this coordinate bond does not form
• 3 -Top page 229 examples exist because they are
  in period 3 or below

56
Section 8.3Bonding Theories

• OBJECTIVES
• Describe the relationship between atomic and
  molecular orbitals.

57
Section 8.3Bonding Theories

• OBJECTIVES
• Describe how VSEPR theory helps predict the
  shapes of molecules.

58
Molecular Orbitals are...

• The model for covalent bonding assumes orbitals
  are those of individual atoms atomic orbital
• Orbitals that apply to overall molecule, due to
  atomic orbital overlap are molecular orbitals
• bonding orbital is molecular orbital occupied by
  2 e-s of covalent bond

59
Molecular Orbitals - definitions

• Sigma bond- 2 atomic orbitals combine to form
  molecular orbital symmetrical along axis
  connecting nuclei
• Pi bond- bonding e-s likely above and below bond
  axis (weaker than sigma)
• Note pictures - next slide

Hybridization video 136
60
- Pages 230 and 231
Sigma bond is symmetrical along axis between 2
nuclei.
Pi bond is above and below bond axis - weaker
than sigma
61
Attractive repulsive forces in H2 bond

• nuclei repel
• e-s repel
• Nuclei and e-s attract
• Attractive forces stronger than repulsion
• As nuclei distances decrease, PE decreases
• If distance decreases more, PE increases b/c
  increased repulsion
• Bond forms w/ bond length interatomic distance
  (PE minimum)

62
VSEPR stands for...

• Valence Shell Electron Pair Repulsion
• Predicts 3D shape of molecules
• The name tells you the theory
• Valence shell outside e-s
• Electron Pair repulsion e- pairs try to get as
  far away as possible from each other.
• determines angles of bonds.

63
VSEPR

• Based on of pairs of ve-s, bonded unbonded.
• Unbonded pair called lone pair.
• CH4 - draw structural formula
• Has 4 4(1) 8
• wants 8 4(2) 16
• (16-8)/2 4 bonds

64
VSEPR for methane (a gas)

• Single bonds fill all atoms.
• 4 pairs of e-s pushing away
• The furthest they can get away is 109.5

65
4 atoms bonded

• Basic shape -tetrahedral
• pyramid w/ triangular base
• Same shape for everything with 4 pairs

H
109.5º
C
H
H
H
66
Other angles, pages 232 - 233

• Ammonia (NH3) 107o
• Water (H2O) 105o
• Carbon dioxide (CO2) 180o

67
VSEPR models
VSEPR theory video 452
- Page 232
Methane has an angle of 109.5o, called tetrahedral
Ammonia has an angle of 107o, called pyramidal
Note the unshared pair that is repulsion for
other electrons.
68
VSEPR song 433
69
Hybrid Orbitals

• Provides info for molecular bonding shape

70
Hybridization w/ single bonds

• Orbitals combine
• C outer e- configuration 2s2 2p2 but one 2s e-
  promoted to 2p
• One 2s e- and 3 2p e-s
• Allows bond in methane (CH4)
• All bonds same due to orbital hybridization
• Mix to form 4 sp3 hybrid orbitals

71
Hybridization w/ double bonds

• Ethene C2H4 1 C-C double bond and 4 C-H single
  bonds
• sp2 hybrid orbitals form from one 2s and two 2p
  atomic orbitals of C

72
Section 8.4Polar Bonds and Molecules

• OBJECTIVES
• Describe how electronegativity values determine
  the distribution of charge in a polar molecule.

73
Section 8.4Polar Bonds and Molecules

• OBJECTIVES
• Describe what happens to polar molecules when
  they are placed between oppositely charged metal
  plates.

74
Section 8.4Polar Bonds and Molecules

• OBJECTIVES
• Evaluate the strength of intermolecular
  attractions compared with the strength of ionic
  and covalent bonds.

75
Section 8.4Polar Bonds and Molecules

• OBJECTIVES
• Identify the reason why network solids have high
  melting points.

76
Bond Polarity

• do covalent bonds always share equally?
• e-s pulled - tug-of-war, btwn nuclei
• In equal sharing (such as diatomic molecules),
  the bond is called nonpolar covalent bond

77
Bond Polarity

• When 2 different atoms bond covalently, unequal
  sharing
• more electronegative atom stronger attraction
• slightly negative charge
• polar bond

Lower case delta
78
Electronegativity?

• The ability of an atom in a molecule to attract
  shared electrons to itself.

Linus Pauling 1901 - 1994
79
Bond Polarity

• Refer to periodic table w/ EN
• Consider HCl
• H electronegativity of 2.1
• Cl electronegativity of 3.0
• Polar bond
• Cl slight - charge
• H slight charge

80
Bond Polarity

• Partial charges, much less than 1 or 1- in ionic
  bond
• H Cl
• Partial charges

d d-
d and d-
81
Bond Polarity

• Can also be
• arrow points to more EN atom
• KNOW Table 8.3, p.238 shows how electronegativity
  indicates bond type

H Cl
82
Polar molecules

• Sample Problem 8.3, p.239
• polar bond tends to make entire molecule polar
• areas of difference
• HCl has polar bonds, thus polar molecule
• molecule w/ 2 poles called dipole, like HCl

83
Polar molecules

• effect of polar bonds on polarity of entire
  molecule depends on molecule shape
• CO2 has 2 polar bonds
• linear
• nonpolar molecule

84
Polar molecules

• effect of polar bonds on molecule polarity
  depends on shape
• water has 2 polar bonds - bent shape
• highly electronegative O pulls e- away from H
• very polar!
• polar bond of water molecule

Chemistry of water 446
85
bonding animations
86

• Polar and non-polar covalent bonds - Mark
  Rosengarten

87
Attractions between molecules p. 240

• makes solid liquid molecular cmpds possible
• weakest called van der Waals forces - there are
  two kinds
• 1. Dispersion forces weakest of all, caused by
  motion of e- - increases as e- increases
  (momentarily more on side of molecule closest to
  neighboring molecule, neighboring molecules e-s
  move to opp side)
• halogens start as gases bromine (l) iodine (s)
  all in Group 7A

88
2. Dipole interactions

• Occurs when polar molecules attracted to each
  other
• 2. Dipole interaction happens in water Figure
  8.25, page 240
• region of one molecule attracts -region of
  another molecule

89
2. Dipole interactions

• Occur when polar molecules attracted to each
  other
• Slightly stronger than dispersion forces
• Opposites attract, but not completely hooked like
  ionic solids

90
2. Dipole Interactions
d d-
91
3. Hydrogen bonding

• is the attractive force caused by hydrogen
  bonded to N, O, F, or Cl
• N, O, F, and Cl very electronegative, so this is
  very strong dipole
• And, the hydrogen shares with lone pair in
  molecule next to it
• This is strongest of the intermolecular forces

92
Order of Intermolecular attraction strengths

1. Dispersion forces are weakest
2. Little stronger are dipole interactions
3. Strongest is H bonding
4. All are weaker than ionic bonds

93
3. Hydrogen bonding defined

• When H atom is a) covalently bonded to a highly
  EN atom, AND b) is also weakly bonded to
  unshared e- pair of nearby highly EN atom
• The H is left very e- deficient (only had 1 to
  start with!) - it shares with something nearby
• H is ONLY element with no shielding for its
  nucleus when involved in covalent bond!

94
Hydrogen Bonding(Shown in water)
This H is bonded covalently to 1) the highly
negative O, and 2) a nearby unshared pair.
95
H bonding allows H2O to be a liquid at room temp
96
Attractions and properties

• Why are some chemicals gases, some liquids, some
  solids?
• Depends on type of bonding!
• Table 8.4, page 244
• Network solids solids where all atoms
  covalently bonded to each other

97
Attractions and properties

• Figure 8.28, page 243
• Network solids melt at very high temps, or not at
  all (decomposes)
• Diamond does not really melt, but vaporizes to a
  gas at 3500 oC
• SiC, used in grinding, has melting pt of 2700 oC

98
Covalent Network Compounds
Some covalently bonded substances DO NOT form
discrete molecules.
Graphite, a network of covalently bonded carbon
atoms
Diamond, a network of covalently bonded carbon
atoms
99

• Ionic/Covalent Bond Song