1. Chapter 8 “Covalent 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. (diatomic hydrogen molecule)
The nuclei repel each other, (both have + charge)
How does H2 form?
(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 bonding w/ Fluorine atoms
Covalent bonds
Nonmetals hold valence e-’s
don’t 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. F F 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. single covalent bond between 2 H atoms
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 gases or liquids at room temperature
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
These are some of the different ways to represent ammonia:
3. The ball and stick model is BEST, because it shows 3D arrangement.
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 8.2 The 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 can’t tell which atom e-’s moved from or to

15. How to show the formation…
It’s 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 s C
Yes, due to electron promotion!

16. How to show the formation…
It’s 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 s

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

18. H O Water Put the pieces together The first hydrogen is happy
The oxygen still needs one more H O

19. H O H Water So, a second hydrogen attaches
Every atom has full energy levels
Note the two “unshared” pairs of electrons H O H

21. Examples:
Conceptual Problem 8.1 on page 220
We’ll 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 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 O
The chemistry of CO2 6:44

24. Carbon dioxide
Attaching 1 oxygen leaves the oxygen 1 short, and the carbon 3 short C O

25. Carbon dioxide
Attaching the second oxygen leaves both of the oxygen 1 short, and the carbon 2 short O C O

26. Carbon dioxide
only solution  share more O C O

27. Carbon dioxide
The only solution is to share more O C O

28. Carbon dioxide
The only solution is to share more O C O

29. Carbon dioxide
The only solution is to share more O C O

30. Carbon dioxide
The only solution is to share more O C O

31. Carbon dioxide
The only solution is to share more O C O

32. O C O Carbon dioxide The only solution is to share more
Requires 2 double bonds
Each atom can count all the electrons in the bond O C O

33. O C O 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 O C O

34. O C O 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 O C O

35. How covalent bonds form - Mark Rosengarden
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 O C 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. N H 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 5+3 = 8
NH3 wants 8+6 = 14
(14-8)/2= 3 bonds
4 atoms with 3 bonds N H

38. H H N H Examples Draw in the bonds; start with singles
All 8 e- accounted for
Everything full – DONE! H H N 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 = 10
HCN wants = 18
(18-10)/2= 4 bonds
3 atoms with 4 bonds – this will require multiple bonds - not to H however

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

41. H C N 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 H C N

42. H C N 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 H C N

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 O H H O H =

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 O C

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. C O

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: C O
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?
It’s 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 8.3 Bonding Theories
OBJECTIVES:
Describe the relationship between atomic and molecular orbitals.

57. Section 8.3 Bonding 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 – 1:36

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. H C H H H 4 atoms bonded 109.5º 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 4:52
- 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 – 4:33

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 8.4 Polar Bonds and Molecules
OBJECTIVES:
Describe how electronegativity values determine the distribution of charge in a polar molecule.

73. Section 8.4 Polar Bonds and Molecules
OBJECTIVES:
Describe what happens to polar molecules when they are placed between oppositely charged metal plates.

74. Section 8.4 Polar Bonds and Molecules
OBJECTIVES:
Evaluate the strength of intermolecular attractions compared with the strength of ionic and covalent bonds.

75. Section 8.4 Polar 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. polar bond 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? Linus Pauling 1901 - 1994
The ability of an atom in a molecule to attract shared electrons to itself.
Linus Pauling

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. Partial charges Bond Polarity d+ d- d+ and d-
Partial charges, much less than 1+ or 1- in ionic bond
H Cl
Partial charges
d d-
d+ and d-

81. Bond Polarity H Cl 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 4:46

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 Waal’s 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 molecule’s 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 H F d+ d- H F d+ d-
Occur when polar molecules attracted to each other
Slightly stronger than dispersion forces
Opposites attract, but not completely hooked like ionic solids
H F
d+ d-
H F
d+ d-

90. #2. Dipole Interactions d+ d- d+ d- d+ d- d+ d- d+ d- d+ d- 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
Dispersion forces are weakest
Little stronger are dipole interactions
Strongest is H bonding
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.
Diamond, a network of covalently bonded carbon atoms
Graphite, a network of covalently bonded carbon atoms

99. Ionic/Covalent Bond Song