1. Section 6.2 Covalent Bonding and Molecular Compounds
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2. Chapter 6 Vocabulary Reading Guide
molecule (molecular compound)
molecular formula
bond energy
electron-dot notation
Lewis structure
structural formula
single bond
resonance
Laughing Gas
Reading Guide
Section 2
write answers on same paper as Section 1
due at end of hour 2

3. Section 6.2 Covalent Bonding
Main Ideas:
Covalent bonds form from shared electrons
Bond lengths and energy vary from molecule to molecule
Atoms tend to form bonds to follow the octet rule

4. Section 6.2 Covalent Bonding
Main Ideas:
Dots placed around an element’s symbol can represent valence electrons
Electron-dot notations can represent compounds.
Some atoms can share multiple pairs of electrons.

5. Section 6.2 Covalent Bonding
Main Ideas:
Resonance structures show hybrid bonds
Some compounds are networks of bonded atoms

6. Covalent Bonds valere - “to be strong”
The word covalent is a combination of :
co- (latin) “together”
valere - “to be strong”
Two electrons shared together have the strength to hold two atoms together in a bond.

7. Covalent bonds Nonmetals hold on to their valence electrons.
They can’t give away electrons to bond.
But still want noble gas configuration.
Get it by sharing valence electrons with each other = covalent bonding
By sharing, both atoms get to count the electrons toward a noble gas configuration.

8. Molecules Many elements found in nature are in the form of molecules:
a neutral group of atoms joined together by covalent bonds.
Example: air contains oxygen molecules, consisting of two oxygen atoms joined covalently
Called a “diatomic molecule” (O2)
7 diatomics: H2, N2, O2, F2, Cl2, Br2, I2

9. Molecule

10. Molecular Compounds
Compounds that are bonded covalently (like in water, or carbon dioxide) are called molecular compounds
Molecular compounds tend to have relatively lower melting and boiling points than ionic compounds

11. Molecular Compounds
Thus, molecular compounds tend to be gases or liquids at room temperature
Ionic compounds are solids (crystals)
A molecular compound has a molecular formula:
Shows how many atoms of each element a molecule contains

12. Molecular Formulas The formula for water is written as H2O
The subscript “2” behind hydrogen means there are 2 atoms of hydrogen; if there is only one atom, the subscript 1 is omitted
Molecular formulas do not tell any information about the structure (the arrangement of the various atoms).

13. These are some of the different ways to represent ammonia:
3. The ball and stick model is the BEST, because it shows a 3-dimensional 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!

14. (diatomic hydrogen molecule)
The nuclei repel each other, since they both have a positive charge (like charges repel).
How does H2 form?
(diatomic hydrogen molecule) + +

15. How does H2 form? But, the nuclei are attracted to the electrons
They share the electrons, and this is called a “covalent bond”, and involves only NONMETALS! + +

16. Bond Dissociation Energies...
The total energy required to break the bond between 2 covalently bonded atoms
High bond energy = strong bond
High bond energy usually means the chemical is relatively unreactive, because it takes a lot of energy to break it down.

17. Section 6.2 Covalent Bonding
Main Ideas:
Covalent bonds form from shared electrons
Atoms tend to form bonds to follow the octet rule

18. Section 6.2 Covalent Bonding
Main Ideas:
Dots placed around an element’s symbol can represent valence electrons
Electron-dot notations can represent compounds.
Some atoms can share multiple pairs of electrons.

19. OBJECTIVES:
Determine the number of valence electrons in an atom of a representative element.
Explain how atoms tend to form bonds to follow the octet rule.
Use electron-dot notation to represent valence electrons and represent compounds

20. Valence Electrons are…?
The electrons responsible for the chemical properties of atoms, and are those in the outer energy level.
Valence electrons - The s and p electrons in the outer energy level
the highest occupied energy level
Core electrons – are those in the energy levels below.

21. Determining Valence Electrons
Atoms in the same column...
Have the same outer electron configuration.
Have the same valence electrons.
The number of valence electrons are easily determined. It is the group number for a representative element
Group 2: Be, Mg, Ca, etc.
have 2 valence electrons

22. Valence electron – electron in the highest occupied energy level of an atom.
*To find the number of valence e- in an atom of a representative element, simply look at its group number

23. Electron Dot diagrams…
shows valence e-’s
symbol represents nucleus & core e-’s
Each side = orbital (s or p)
dot = valence e- (8 max)
don’t pair up until they have to (Hund’s rule)
(px) X
(py)
(s)
(pz)

24. The Electron Dot diagram for Nitrogen
Nitrogen has 5 valence electrons to show.
First we write the symbol.
Then add 1 electron at a time to each side CCW.
Now they are forced to pair up.
We have now written the electron dot diagram for Nitrogen.

25. Sec 6.2 Practice problems
19. Page 200
Li e. C
Ca f. P
Cl g. Al
O h. S

26. The Octet Rule
In Chapter 5, we learned that noble gases are unreactive in chemical reactions
In 1916, Gilbert Lewis used this fact to explain why atoms form certain kinds of ions and molecules
The Octet Rule: in forming compounds, atoms tend to achieve a noble gas configuration; 8 in the outer level is stable
Each noble gas (except He, which has 2) has 8 electrons in the outer level

27. Electron dot structure– a notation that depicts
Electron dot structure– a notation that depicts valence electrons as dots around the atomic symbol of the element.
Octet rule – atoms react by gaining or losing electrons so as to acquire stable e- structure of a noble gas, usually 8 valence e-

28. Covalent bonding
Fluorine has seven valence electrons (but would like to have 8) F

29. F F Covalent bonding Fluorine has seven valence electrons
A second atom also has seven F F

30. F F Covalent bonding Fluorine has seven valence electrons
A second atom also has seven
By sharing electrons… F F

31. F Covalent bonding Fluorine has seven valence electrons
A second atom also has seven
By sharing electrons… F

32. F Covalent bonding …both end with full orbitals
Fluorine has seven valence electrons
A second atom also has seven
By sharing electrons…
…both end with full orbitals F

33. 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

34. F Covalent bonding Fluorine has seven valence electrons
A second atom also has seven
By sharing electrons…
…both end with full orbitals
8 Valence electrons F

35. Lewis Structure
Replace the two bonded electrons with a straight line… F F

36. A Single Covalent Bond is...
A sharing of two valence electrons.
Only nonmetals and hydrogen.
Different from an ionic bond because they actually form molecules.
Two specific atoms are joined.
In an ionic solid, you can’t tell which atom the electrons moved from or to

37. Carbon covalent bonding
C ….. special example - can it really share 4 electrons: 1s22s22p2? 2p
1s s C
Yes, due to e- promotion!

38. Carbon covalent bonding
C ….. special example - can it really share 4 electrons: 1s22s22p2? 2p
1s s C
Yes, due to e- promotion!

39. 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?
Yes, due to electron promotion!
Another example: lets show how water is formed with covalent bonds, by using an electron dot diagram

40. H O Water 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 H O

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

42. 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

43. Another way of indicating bonds
Often use a line to indicate a bond
Called a structural formula
Each line is 2 valence electrons H O H H O H =

44. Electron Dot/Lewis Structure Rules
Central atom is least electronegative (furthest left and down)
Count up the total number of electrons to make all atoms happy=8 except H=2 (Need).
Add up all the valence electrons (Have).
Subtract; then Divide by 2 = number of bonds
Fill in the rest of the valence electrons so each atom, except for hydrogen, beryllium, and boron, satisfies the octet rule.
Check that the number of valence electrons used is the same as added in step 3.

45. Lewis Structure for Water

46. N H Example NH3, which is ammonia
N – central atom; has 5 valence electrons, wants 8
H - has 1(x3) = 3 valence electrons, wants 2(x3) = 6
NH3 has 5+3 = 8
NH3 wants 8+6 = 14
(14-8)/2= 3 bonds
4 atoms with 3 bonds N H

47. H H N H Examples Draw in the bonds; start with singles
All 8 electrons are accounted for
Everything is full – done with this one. H H N H

48. Lewis Structural ExampleH
N H

49. Sec 6.2 Practice problems Set C p.176 #1-4
1. Draw the Lewis structure for ammonia, NH3
2. Draw the Lewis structure for hydrogen sulfide, H2S

50. 3. Draw the Lewis structure for silane(silicon tetrahydride), SiH4
4. Draw the Lewis structure for phosphorus trifluoride, PF3

51. Multiple Bonds
Sometimes atoms share more than one pair of valence electrons.
A double covalent bond is when atoms share two pairs of electrons (4 total)
A triple covalent bond is when atoms share three pairs of electrons (6 total)

52. Diatomic Elements F2 Cl2 Br2 I2 O2 N2 H2
Know these 7 elements as diatomic:
F2 Cl2 Br2 I2 O2 N2 H2
What’s the deal with the oxygen dot diagram?
halogens 52

53. Lewis Structures for Diatomics
Single, Double, and Triple Covalent Bonds

54. Electron Dot/Lewis Structure Rules
Central atom is least electronegative (furthest left and down)
Count up the total number of electrons to make all atoms happy=8 except H=2 (Need).
Add up all the valence electrons (Have).
Subtract; then Divide by 2 = number of bonds
Fill in the rest of the valence electrons so each atom, except for hydrogen, beryllium, and boron, satisfies the octet rule.
Check that the number of valence electrons used is the same as added in step 3.

55. Sec 6.2 Practice problems Set D p.178 #1-2 *Worked out on next slides
1. Draw the Lewis structure for carbon dioxide, CO2
2. Draw the Lewis structure for hydrogen cyanide, HCN
*Worked out on next slides

56. Page 178 # 1.. Draw Lewis structure for CO2 - Carbon dioxide
CO2 - Carbon is central atom ( least electronegative)
Carbon has 4 valence electrons
Wants 4 more
Oxygen has 6 valence electrons
Wants 2 more O

57. O C O Carbon dioxide The only solution is to share more
Attaching 1 oxygen leaves the oxygen 1 short, and the carbon 3 short
Attaching the second oxygen leaves both of the oxygen 1 short, and the carbon 2 short O C O
The only solution is to share more

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

59. 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 O C

60. C O Carbon dioxide 8 valence electrons
The only solution is to share more
Requires two double bonds
Each atom can count all the electrons in the bond O
8 valence electrons C

61. C O Carbon dioxide 8 valence electrons
The only solution is to share more
Requires two double bonds
Each atom can count all the electrons in the bond O C
8 valence electrons

62. C O Carbon dioxide 8 valence electrons
The only solution is to share more
Requires two double bonds
Each atom can count all the electrons in the bond O C
8 valence electrons

63. Carbon dioxide
Write as Lewis structure: O C

64. p. 178 # 2.. Draw Lewis structure for
hydrogen cyanide - 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

65. N H C HCN Put single bond between each atom Need to add 2 more bonds
Must go between C and N (Hydrogen is full)
Uses 8 electrons – need 2 more to equal the 10 it has
Must go on the N to fill its octet N H C

66. 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

67. 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

68. 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

69. Coordinate covalent bond
Most polyatomic cations and anions contain covalent and coordinate covalent bonds
The ammonium ion (NH41+) can be shown as another example

70. Lewis Structure Examples
H C N
C O
C O H

71. 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.
It is impossible to satisfy octet rule, yet the stable molecule does exist

72. # 1 – Odd number of valence electrons

73. Exceptions to Octet rule
#2 –Not enough valence electrons
Ex: B, Be
Page 173 explains boron trifluoride, and note that one of the fluorides might be able to make a coordinate covalent bond to fulfill the boron
But fluorine has a high electronegativity (it is greedy), so this coordinate bond does not form

74. #2 –Not enough valence electrons

75. Exceptions to Octet rule
#3 – More than an octet
Top page 229 examples exist because they are in period 3 or beyond
Ex: sulfur hexafluoride

76. 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 is a hybrid of both, like a mule; and shown by a double-headed arrow
found in double-bond structures!

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

78. Polyatomic ions – note the different positions of the double bond.
Resonance in a carbonate ion (CO32-):
Resonance in an acetate ion (C2H3O21-):

79. Sec 6.2 Practice problems Page 201 #24 a. OH 1- b. H3C2O2 1-
21. Draw Lewis structures for each of the following polyatomic ions. Show resonance structures, if they exist.
a. OH 1-
b. H3C2O2 1-
c. BrO3 1-
add: d. H3O 1+

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

81. Attractions and properties
Why are some chemicals gases, some liquids, some solids?
Depends on the type of bonding!
More in Section 6.5 and Chapter 7
Network solids – solids in which all the atoms are covalently bonded to each other

82. Attractions and properties
Network solids melt at very high temperatures, or not at all (decomposes)
Diamond does not really melt, but vaporizes to a gas at 3500 oC and beyond
SiC, used in grinding, has a melting point of about 2700 oC

83. Covalent Network Compounds
Some covalently bonded substances DO NOT form discrete molecules. Instead: 3-D networks of bonded atoms
Network Solids Video
Diamond, a network of covalently bonded carbon atoms
Graphite, a network of covalently bonded carbon atoms

84. Section 6.2 Practice Problems
Chapter 6 Notes
5 blocks (A-E)
Chapter 6 Vocabulary
25 words
Chapter 6 Reading Guide
Section 2
Section 6.2 Practice Problems
Page 200 # 6,7
Set C p.176 #1-4
Set D p.178 #1-2
Page 201 # 24 (add one)
6.2 Study Guide 84