1. Chapter 7: Covalent Bonding and Lewis Formula’s
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Chapter 7: Covalent Bonding and Lewis Formula’s By
Doba Jackson, Ph.D.
Copyright © 2010 Pearson Prentice Hall, Inc.

2. Electronegativity concept
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electronegativity concept
Electronegativity: The ability of an atom in a molecule to attract the shared electrons in a covalent bond.
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4. Definitions of Ionic and Covalent compounds
Ionic bonds are a result of a combination of a metal (electropositive element) and a non-metal (electronegative element).
In ionic bonds, atoms are attracted to each other by opposite charges.
Covalent bonds are a result of the combination of two non-metals (two electronegative elements).
In covalent bonds, atoms are attracted to each other by a shared pair of electrons.

5. Types & Strengths of Covalent Bonds (Dissociation energies)
Chapter 5: Covalent Bonds and Molecular Structure
Types & Strengths of Covalent Bonds (Dissociation energies)
11/11/2018 78
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6. A Comparison of Ionic and Covalent Bonds
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
A Comparison of Ionic and Covalent Bonds
Physical properties are properties we can directly see and measure.
Ionic compounds and covalent compounds are different in their physical properties (ex. NaCl and HCl)
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7. Points to consider: Ionic verses Covalent compounds
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Points to consider: Ionic verses Covalent compounds
Point 1: Ionic compounds are usually solids (when pure)
Point 2: Ionic compounds have very high boiling and melting points
Point 3: Covalent compounds can be either solids, liquids or gases
Point 4: Covalent compounds have relatively lower boiling and melting points
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8. Why does NaCl have such high boiling and melting points?
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Why does NaCl have such high boiling and melting points?
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9. Types & Strengths of Covalent Bonds
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Types & Strengths of Covalent Bonds 78
Copyright © 2010 Pearson Prentice Hall, Inc.

10. A chemical representation of a covalent compound (ethanol)
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
A chemical representation of a covalent compound (ethanol)
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11. Drawing Structural Formula’s
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Drawing Structural Formula’s
EXAMPLE 5.1
Propane, C3H8, has a structure in which the three carbon atoms are bonded in a row, each end carbon is bonded to three hydrogens, and the middle carbon is bonded to two hydrogens. Draw the structural formula, using lines between atoms to represent covalent bonds.
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12. Naming Covalent Compounds: Must know these prefixes
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Naming Covalent Compounds: Must know these prefixes
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13. Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Choose which compound is more electronegative, then change the suffix to -ide
Write the name: electropositive - electronegative
Add the suffix “ide” to the end of the electronegative atom.
- Use the prefixes to indicate the multiplicity of the both atoms.
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14. Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Example: naming N2O4
N2O4
The first element (Nitrogen) is more electropositive and takes the name of the element.
The second element (Oxygen) is more electronegative and takes the name of the element with an “ide” modification to the ending.
The prefix is added to the front of each to indicate the number of each atom.
dinitrogen tetraoxide
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15. Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
WORKED EXAMPLE 5.2
Give systematic names for the following compounds
Solution
PCl3
(b) N2O3
(c) P4O7
(d) BrF3
Phosphorus trichloride
Dinitrogen trioxide
Tetraphosphorus heptoxide
Bromine trifluoride
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16. Chemical Representation: Electron-Dot Structures
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Chemical Representation: Electron-Dot Structures
Molecular model’s: shows the 3D connectivity.
Electron-Dot Structures (Lewis Structures): A representation of an atom’s valence electrons by using dots.
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17. Lewis Electron-Dot Structures
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Lewis Electron-Dot Structures
Copyright © 2010 Pearson Prentice Hall, Inc.

18. Electron-Dot Structures
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures
Single bonds.
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19. Lewis Electron-Dot Structures
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Lewis Electron-Dot Structures
Copyright © 2010 Pearson Prentice Hall, Inc.

20. Chapter 5: Covalent Bonds and Molecular Structure
WORKED EXAMPLE 5.3 Drawing an Electron-Dot Structure
11/11/2018
Draw an electron-dot structure for phosphine, PH3.
Solution
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21. Electron-Dot Structures of Polyatomic Molecules
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures of Polyatomic Molecules
Step 1: Valence Electrons
Count the total number of valence electrons for all atoms in the molecule.
Add one additional electron for each negative charge in an anion or subtract one for each positive charge in a cation.
Copyright © 2010 Pearson Prentice Hall, Inc.

22. Electron-Dot Structures of Polyatomic Molecules
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures of Polyatomic Molecules
Step 2: Connect Atoms
Draw lines to represent bonds between atoms.
For hydrogen and second row atoms, use the number of bonds listed below.
For third row and greater atoms, they may have more bonds than predicted by the octet rule.
Copyright © 2010 Pearson Prentice Hall, Inc.

23. Electron-Dot Structures of Polyatomic Molecules
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures of Polyatomic Molecules
Step 3: Assign Electrons to the Terminal Atoms
Subtract the number of electrons used for bonding in the previous step from the total number determined in step 1.
Complete each terminal atom’s octet (except for hydrogen).
Step 4: Assign Electrons to the Central Atom
If unassigned electrons remain after step 3, place them on the central atom.
Copyright © 2010 Pearson Prentice Hall, Inc.

24. Electron-Dot Structures of Polyatomic Molecules
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures of Polyatomic Molecules
Step 5: Multiple Bonds
If no unassigned electrons remain after step 3 but the central atom does not yet have an octet, use one or more lone pairs of electrons from a neighboring atom to form a multiple bond (either a double or a triple).
Copyright © 2010 Pearson Prentice Hall, Inc.

25. Electron-Dot Structures of Polyatomic Molecules
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures of Polyatomic Molecules
Draw an electron-dot structure for H2O.
Step 1:
2(1) + 6 = 8 valence electrons
Step 4:
bonding pair of electrons
lone pair of electrons H O H O
Step 2:
Step 2: The positioning of the terminal atoms about the central is not critical as long as they simply surround the central atom.
Step 3: The terminal atom is hydrogen.
Step 5: The central atom has an octet so no multiple bonding.
Step 3:
8 – 2(2) = 4 nonbonding electrons
Copyright © 2010 Pearson Prentice Hall, Inc.

26. Electron-Dot Structures of Polyatomic Molecules
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures of Polyatomic Molecules
Draw an electron-dot structure for CCl4.
Step 1:
4 + 4(7) = 32 valence electrons Cl C Cl C
Step 2:
Step 4:
Step 4: The central atom already has an octet.
Step 5: The central atom has an octet so no multiple bonding.
Step 3: 32 – 4(2) = 24 nonbonding electrons
Copyright © 2010 Pearson Prentice Hall, Inc.

27. Electron-Dot Structures of Polyatomic Molecules
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures of Polyatomic Molecules
Draw an electron-dot structure for H3O1+.
Step 1:
3(1) = 8 valence electrons 1+ H O H O
Step 2:
Step 4:
Step 1: Subtract 1 from the total number of valence because of the 1+ charge.
Step 3: The terminal atoms are hydrogen.
Step 5: The central atom has an octet so no multiple bonding.
Don’t forget to show the charge on the ion!
Step 3: 8 – 3(2) = 2 nonbonding electrons
Copyright © 2010 Pearson Prentice Hall, Inc.

28. Draw electron-Dot formula’s for the following compounds
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Draw electron-Dot formula’s for the following compounds
AlCl3
BH3
NH3
CH4
C2H6
H2O2
C2H4
Cl2CO
CH2O
C2H4O2
Copyright © 2010 Pearson Prentice Hall, Inc.

29. Electron-Dot Structures of Polyatomic Molecules
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures of Polyatomic Molecules
Draw an electron-dot structure for CH2O.
Step 1:
4 + 2(1) + 6 = 12 valence electrons H C O H C O
Step 2:
Step 5:
Step 2: The least electronegative atom is the central one (hydrogen can’t be the central atom).
Step 3: Two of the terminal atoms are hydrogen.
Step 4: There are no remaining electrons to place on the central atom.
Step 5: “Borrow” a pair from oxygen to complete the central atom’s octet. H C O H C O
Step 3:
Copyright © 2010 Pearson Prentice Hall, Inc.

30. Electron-Dot Structures of Polyatomic Molecules
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures of Polyatomic Molecules
Draw an electron-dot structure for SF6.
Step 1:
6 + 4(7) = 34 valence electrons F S F S
Step 2:
Step 3:
Step 2: The central atom is in row 3 so there can be more than 8 electrons.
Step 4: There are no remaining electrons.
Step 5: The central atom has at least 8 electrons so no multiple bonding.
Copyright © 2010 Pearson Prentice Hall, Inc.

31. Electron-Dot Structures of Polyatomic Molecules
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures of Polyatomic Molecules
Draw an electron-dot structure for ICl3.
Step 1:
7 + 3(7) = 28 valence electrons Cl I Cl Cl Cl
Step 2:
Step 4: I
Step 4: The remaining electrons go on the central atom.
Step 5: The central atom has at least 8 electrons so no multiple bonding. Cl I
Step 3:
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32. Electron-Dot Structures and Formal Charges
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures and Formal Charges
Draw an electron-dot structure for O3.
Step 1:
3(6) = 18 valence electrons O
Step 2: O
Step 4:
Step 4: There is only 1 more pair of electrons. Thus, the central oxygen only has 3 pairs of electrons (less than an octet).
Step 5: There is a choice to be made. If all that is desired is the electron-dot structure, then either of the 2 terminal oxygen atoms could have been chosen. O O
Step 3:
Step 5:
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33. Formal Charges are charges isolated to specific atoms in an electron-dot structure
Formal Charge of an atom is a value that shows whether an atom has gained or lost electrons through bonding
The sum of all formal charges of every atom should equal the net charge of the molecule.
Formal
Charge
# of
valence e-
# of
bonding e-
# of
nonbonding e- - = -

34. Example of a molecule with a formal charge
Formal Charge = Valence electrons – Bonding electrons/2 – nonbonding electrons

35. Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Formal Charges
Formal
Charge
# of
valence e-
in free atom - 2 1
nonbonding e-
bonding =
Calculate the formal charge on each atom in O3. O
The sum of the formal charges will be equal to the overall charge of the molecule or ion. 1 2 1 2 1 2
(4) - 4 = 0
(6) - 2 = +1
(2) - 6 = -1
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36. Electron-Dot Structures and Formal Charges
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures and Formal Charges
Draw an electron-dot structure for OF2.
Step 1:
2(7) + 6 = 20 valence electrons F O
Step 2: F O
Step 4:
20 – 4 = 16 electrons F O
Step 3:
16 – 12 = 4 electrons
Copyright © 2010 Pearson Prentice Hall, Inc.

37. An alternative structure of OF2
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
An alternative structure of OF2
Draw an electron-dot structure for OF2.
Step 1:
2(7) + 6 = 20 valence electrons O F
Step 2: O F
Step 4:
20 – 4 = 16 electrons O F
Step 3:
16 – 12 = 4 electrons
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38. Formal Charges in two structures of OF2
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Formal Charges in two structures of OF2 F O 1 2 1 2 1 2
(2) - 6 = 0
(4) - 4 = 0
(2) - 6 = 0 + - O F + 1 2 1 2 1 2
(2) - 6 = 0
(4) - 4 = +1
(2) - 6 = -1
Copyright © 2010 Pearson Prentice Hall, Inc.

39. Problem 2. 7- Nitromethane has the structure indicated
Problem 2.7- Nitromethane has the structure indicated. Explain why it must have a formal charge on N and O.
Formal Charge = Valence electrons – Bonding electrons/2 – nonbonding electrons

40. Problem 2.8- Calculate the formal charges for the nonhydrogen atoms in the following molecules+1 -1 +1 -1 +1 -1
Formal Charge = Valence electrons – Bonding electrons/2 – nonbonding electrons

41. Electron-Dot Structures and Resonance
Chapter 5: Covalent Bonds and Molecular Structure
11/11/2018
Electron-Dot Structures and Resonance
Move a lone pair from this oxygen?
Step 4: O
Or, move a lone pair from this oxygen?
Either electron-dot structure suggests that ozone has a double and a single bond. A bond analysis actually shows one type of bond and it’s neither a single nor a double bond. A resonance hybrid attempts to overcome this shortcoming of electron-dot structures.
At this stage, we usually just take this simplified look at resonance structures. Organic chemistry, for example, takes a much deeper look at resonance. O O
Resonance
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42. Points to consider for Resonance forms
Rule 1: Individual resonance forms are imaginary.
Rule 2: Resonance forms differ only in their placement of their nonbonding or π electrons.
Rule 3: Different Resonance forms of a substance don’t have to be equivalent.
Rule 4: Resonance obeys normal rules of valency.

43. Only difference is where you place the π and non-bonding electrons
Resonance forms
Only difference is where you place the π and
non-bonding electrons