1. Chapter 8 Covalent Bonding
Section 8.1 The Covalent Bond *
Section 8.2 Naming Molecules
Section 8.3 Molecular Structures *
Section 8.4 Molecular Shapes
Section 8.5 Electronegativity and Polarity

2. The stability of an atom, ion or compound is related to its energy: lower energy states are more stable.
Atoms achieve noble gas configuration by forming compounds (substances composed of two or more different elements chemically combined). They can be either ionic compounds or molecular compounds.
A covalent bond results from sharing electrons (usually between nonmetallic elements.
A molecule is formed when two or more atoms bond covalently and is lower in potential energy than its constituent atoms.

3. A molecule is the smallest electrically neutral unit of a substance that still has the properties of the substance and is held together by covalent bonds.
Molecules of one compound are identical, but different from molecules of other compounds.
water
H2O
2 Hydrogen atoms and
1 oxygen
atom
carbon dioxide
CO2
1 carbon
atom and
2 oxygen
atoms

4. “start at 7, make a 7, plus hydrogen”
Seven nonmetallic elements that exist as diatomic molecules
Chlorine Cl2
Bromine Br2
Iodine I2
Hydrogen H2
Fluorine F2
Oxygen O2
Nitrogen N2
“start at 7, make a 7, plus hydrogen”
forget At2, astatine! because its
radioactive, unstable, half life of 8 hrs

5. Different ways to display molecular formulas
Perspective Drawing
Molecular Formula
Lewis structure
Space-filled Molecular Model
Structural Formula
Ball-and Stick Molecular Model
Strategy to draw Lewis structure see also p. 254 5

6. is one shared pair of valence electrons “Lewis electron dot structure”
A single covalent bond
is one shared pair of valence electrons
H H or
“Lewis electron dot structure”

7. Every single dot is looking for one other single dot of another atom
Pairs of valence electrons that are not shared between atoms are called unshared, free, lone or nonbonding electron pairs.
Every single dot is looking for one other single dot of another atom
(like holding hands)

8. s s Sigma bonds are single covalent bonds.
Sigma bonds occur when the pair of shared electrons is in an area centered between the two atoms. s
s = Sigma (lower case “s” in Greek alphabet)

9. Multiple Covalent Bonds
Double bonds form when two pairs of electrons are shared between two atoms.
Triple bonds form when three pairs of electrons are shared between two atoms.

10. A multiple covalent bond consists of
one sigma bond and at least one p bond.
The pi (p) bond is formed when parallel orbitals overlap and share electrons, and it occupies the space above and below the line that represents where the two atoms are joined together.

11. (Sigma (s) bond along the line not shown) Two sigma and two pi bonds
pi (p) bond,
(Sigma (s) bond along the line not shown)
Two sigma and two pi bonds

12. Drawing Lewis dot structures: Can you find a different way
Ex. C6H6
Drawing Lewis dot structures:
Put atoms with many single dots in the center, every dot can make one bond.
Atoms with only one single dot are usually on the outside, because they form only one “regular” covalent bond (H, F, Cl, Br, I).
Leftover single dots should be used for double or triple bonds.
Often more than one formula is possible (isomers).
C2H4O2
Can you find a different way
to “connect the dots”?
Example acetic acid

13. Lewis structure of polyatomic ions
Atoms within a polyatomic ion are covalently bonded.  same procedure, but first add or remove dots corresponding to charge (-) add, (+) remove
Symmetry often stabilizes a molecule or particle

14. Coordinate covalent bond is a bond in which one atom contributes both bonding electrons.
also known as dative bond (latin: to give),
dipolar bond or semipolar bond

15. Resonance structures of NO3-
Resonance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion. (move electrons but not atoms!!)
The molecule or ion behaves as though it has only one average structure. The bond lengths are identical to each other and intermediate between single and double covalent bonds.
Resonance structures of NO3-
Electrons that can be pushed around in a molecular structure are called delocalized and stabilize the molecule or ion.

16. Lewis resonance structures for
carbonate CO3 2-
The double arrow is used between resonance structures (one line but with arrow heads on both end)
Benzene C6H6
Draw the
Lewis resonance structures for
NO2-
SO2 O3
Sulfur trioxide SO3

17. Three Exceptions to the Octet Rule
1) A small group of molecules might have an odd number of valence electrons, for example NO2 ( = 17)
Resonance structures help to stabilize the molecule

18. Exceptions to the Octet Rule
2) Few compounds form stable configurations with less than 8 electrons around the atom a suboctet.
Boron trihydride is reactive and will form coordinate bonds or “dimerize”

19. Exceptions to the Octet Rule
3) Elements in period 3 or higher can use d-orbitals to form more than 4 covalent bonds (more than 8 valence electrons), called an expanded octet. (most common exception)
Draw the expanded octet Lewis structure for ClF3
A rare compound, containing the noble gas xenon

20. The Strength of Covalent Bonds
depends on the distance between the two nuclei, or bond length. The shorter the bond the stronger it is, the greater the energy required to break it.
The amount of energy required to break a bond is called the bond dissociation energy

21. Every chemical reactions involves a change in energy
Exothermic Reaction:
Energy is being released (test tube getting hot)
more energy is released than is required to break the bonds in the initial reactants
CH4 (g) + 2 O2 (g) -- CO2 (g) + 2 H2O (g) + energy
Endothermic Reaction:
Energy is being absorbed (test tube getting cold)
2 H2O (l) + energy -- 2H2 (g) + O2 (g)
a greater amount of energy is required to break a bond in reactants than is released when the new bonds form in the products.

22. Exothermic!

23. What is it called when one or more correct Lewis structures can be drawn for a molecule?
A. suboctet
B. expanded octet
C. expanded structure
D. resonance

24. Where do atoms with expanded octets occur?
A. transition metals
B. noble gases
C. elements in period 3 or higher
D. elements in group 3 or higher