1) Molecular compounds are bonded covalently and form molecules joined by shared pairs of electrons. They tend to have lower melting and boiling points than ionic compounds. 2) Covalent bonds form when atoms share one or more pairs of electrons to achieve noble gas configurations. Electron dot structures can represent these shared electrons. 3) Exceptions to the octet rule exist, such as when there is an odd number of valence electrons or the atom is in period 3 or beyond. Coordinate covalent bonds also involve electron pair donation.

1. Chapter
8
Covalent Bonding

2. Section 8.1
Molecular Compounds
OBJECTIVES:
• Distinguish between the melting points and boiling points of molecular compounds
and ionic compounds.
• Describe the information provided by a molecular formula.
Bonds are…
 Forces that hold groups of atoms together and make them function as a unit. Two
types:
1) Ionic bonds – transfer of electrons (gained or lost; makes formula unit)
2) Covalent bonds – ________________________ of electrons. The resulting
particle is called a ___________________________________
Covalent Bonds
• The word covalent is a combination of the prefix co- (from Latin com, meaning
“with” or “together”), and the verb valere, meaning “to be strong”.
• Two electrons shared together have the strength to hold two atoms together in a bond.
Molecules
• Many elements found in nature are in the form of molecules:
• a ______________________________________________________________ joined
together by covalent bonds.
• For example, air contains oxygen molecules, consisting of two oxygen atoms joined
covalently
• Called a “_______________________________________________________” (O2)
How does H2 form?
(diatomic hydrogen molecule)
• The nuclei repel each other, since they both have a positive charge (like charges
repel).

3. + +
+ +
• But, the nuclei are attracted to the electrons
• They _______________________ the electrons, and this is called a
“_____________________________________________”, and involves only
____________________________________
+ +
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 _______________________________________________with each other =
___________________________________________
• By sharing, both atoms get to count the electrons toward a noble gas configuration.
• Fluorine has seven valence electrons (but would like to have 8)
• A second atom also has seven
• By sharing electrons…
• …both end with full orbitals

4. Molecular Compounds
• Compounds that are bonded covalently (like in water, or carbon dioxide) are called
________________________________________________
• Molecular compounds tend to have relatively __________________________
melting and boiling points than ionic compounds – this is not as strong a bond as
ionic
• Thus, molecular compounds tend to be _____________________________________
at room temperature
• Ionic compounds were solids
• A molecular compound have a ___________________________________________
o Shows how many atoms of each element a molecule contains
• The formula for water is written as H2O
• The ___________________________________ 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).

5. Section 8.2
The Nature of Covalent Bonding
OBJECTIVES:
• Describe how electrons are shared to form covalent bonds, and identify
exceptions to the octet rule.
• Demonstrate how electron dot structures represent shared electrons.
• Describe how atoms form double or triple covalent bonds.
• Distinguish between a covalent bond and a coordinate covalent bond, and
describe how the strength of a covalent bond is related to its bond dissociation
energy.
• Describe how oxygen atoms are bonded in ozone.
A Single Covalent Bond is...
• A sharing of two valence electrons.
• Only _____________________________ and ____________________________
• 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
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?
o Yes, due to electron promotion!
• Another example: lets show how water is formed with covalent bonds, by using
an electron dot diagram
Water
 Each hydrogen has 1 valence electron
H
- Each hydrogen wants 1 more
 The oxygen has 6 valence electrons
- The oxygen wants 2 more
 They share to make each other complete
O

6. • Put the pieces together
• The first hydrogen is happy
• The oxygen still needs one more
• So, a second hydrogen attaches
• Every atom has full energy levels
Multiple Bonds
• Sometimes atoms share more than one pair of valence electrons.
• A ____________________________________________ is when atoms share
two pairs of electrons (4 total)
• A ____________________________________________ is when atoms share
three pairs of electrons (6 total)
• Table 8.1, p.222 - Know these 7 elements as _____________________________:
Dot diagram for Carbon dioxide
• CO2 - Carbon is central atom ( more metallic )
• Carbon has 4 valence electrons
• Wants 4 more
• Oxygen has 6 valence electrons
• Wants 2 more
C
O

7. Carbon dioxide
• 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
• The only solution is to share more
• Requires two double bonds
• Each atom can count all the electrons in the bond
How to draw them?
 Use these guidelines:
1) Add up all the valence electrons.
2) Count up the total number of electrons to make all atoms happy.
3) Subtract; then Divide by 2
4) Tells you how many bonds to draw
5) Fill in the rest of the valence electrons to fill atoms up.

8. Example
• 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 = _______
• NH3 wants 8+6 = 14
• (14-8)/2= ___________________
• 4 atoms with _________ bonds
• Draw in the bonds; start with singles
• All 8 electrons are accounted for
• Everything is full – done with this one
Example: HCN
• HCN: C is central atom
• N - has _____ valence electrons, wants _____
• C - has _____ valence electrons, wants _____
• H - has _____ valence electron, wants _____
• HCN has _____+_____+_____ = _____
• HCN wants _____+_____+_____=_____
• (_____-_____)/2= _____ bonds
• 3 atoms with _____ bonds – this will require multiple bonds - not to H however
HCN
• Put single bond between each atom
• Need to add 2 more bonds
• Must go between C and N (Hydrogen is full)

9. • Uses 8 electrons – need 2 more to equal the 10 it has
• Must go on the N to fill its octet
Another way of indicating bonds
• Often use a line to indicate a bond
• Called a ________________________________________________
• Each line is ________ valence electrons
Other Structural Examples
A Coordinate Covalent Bond...
• When one atom donates both electrons in a covalent bond.
• Carbon monoxide (CO) is a good example:

10. Coordinate covalent bond
• Most polyatomic cations and anions contain covalent and coordinate covalent
bonds
• Table 16.2, p.445
• Sample Problem 16.2, p.446
• The ammonium ion (NH41+) can be shown as another example
Bond Dissociation Energies...
• _________________________________________________________________
_________________________________________________________________
__
• High dissociation energy usually means the chemical is relatively
_________________________________, because it takes a lot of energy to break
it down.
Resonance is...
• When ____________________________________ 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!
Resonance in Ozone
• 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.
Resonance

11. • Occurs when more than one valid Lewis structure can be written for a particular
molecule (due to position of double bond)
• These are resonance structures of benzene.
• The actual structure is an average (or hybrid) of these structures.
The 3 Exceptions to Octet rule
• For some molecules, it is impossible to satisfy the octet rule
#1. usually when there is an ________________________________ 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
• Another exception: Boron
• Page 451 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 ________________________________________
(it is greedy), so this coordinate bond does not form
• #3 -Top page 229 examples exist because they are in period 3 or beyond