1. Covalent Properties
Polarity and IMF

2. Properties
Depend on strength of forces between “particles” or separate units.
In covalent substances, the units are molecules.

3. Properties of Covalent (Molecular) Substances
Poor conductors of heat & electricity in any phase. No charged particles!
Low melting & boiling points: easy to pull molecules apart from each other.
Majority of solids are soft
Low Hf and Hv compared to ionic & metallic substances
High vapor pressure compared to ionic & metallic substances

4. Attractions between Molecules
Intermolecular Forces
Dispersion (London) or Van der Waals
Occurs between NONPOLAR molecules
Dipole-Dipole Forces
Occurs between POLAR molecules
Hydrogen Bonding
Occurs between molecules with a H-F, H-O, H-N bond
Memory Jogger

5. Attractions and Properties
Some atoms are able to form multiple repeating covalent bonds with themselves.
Some with other atoms.
The result is a 3-D structure known as a
carbon
Si, O, B, N
network solid

6. Crystal Structure of Diamond
The ball-and-stick diagram shows the crystal structure of diamond, a covalent network solid. Each corner or intersection of lines represents a carbon atom. Notice the tetrahedral arrangement at each carbon atom.

7. Properties of Network Solids
Extremely high melting points
Very hard
Insoluble in polar and nonpolar liquids
Both solid and liquid do NOT conduct electricity….except for
GRAPHITE

8. Graphite Structure
The ball-and-stick diagram of graphite shows the that the carbon atoms form a network of hexagonal rings within each plane. The planes are stacked in layers that can slide over one another.

9. Bond Energy The strength of a Chemical Bond
Energy required to break a chemical bond
Measured in kJ/mole
A large bond dissociation energy corresponds to a strong covalent bond.
Covalent Bond Strength: Triple > Double > Single
HBr = 367 kJ/mole

10. Bond Energies

11. Resonance Structures
Sulfur Dioxide
Nitric Acid

12. Resonance
A condition that occurs when more than one valid Lewis structure can be written for a molecule or ion.
Resonance structures differ only in the position of the electron pairs, never the atom positions.
Examples include O3 and polyatomics: NO3-, NO2-, SO32-, and CO32-

13. Exceptions to the Octet Rule
Odd number of valence electrons
NO2, ClO2, NO
Coordinate Covalent Bonds
Polyatomic Ions
Some compounds from with fewer than eight electrons…example is BH3
Expanded Octet
Central atom contains more than eight electrons electrons…examples PCl5 , SF6

14. Polarity of Molecules
A molecule may contain polar bonds, but not be polar! Depends on the geometry of the molecule.
If molecule is symmetric, the “pull” of one polar bond is offset by the “pull” of another polar bond.
It’s a tug-of-war that no one can win!

15. Symmetric Molecules
Contain at least one mirror plane.

16. The green arrows do not cancel out, so water is polar!
Polarity of H2O?
The green arrows do not cancel out, so water is polar!

17. Bond vs. Molecular Polarity
Water is a polar molecule because:
O-H bond is polar
Molecule has a bent geometric shape
Polar Molecule
Negative end
Positive end

18. Polarity of Carbon Dioxide?
Carbon needs double
bonds between both
oxygen atoms to
complete its octet.
Electronegativity difference
for C O is 0.9 so bond is polar.
Is this a polar or nonpolar
MOLECULE?

19. Polarity of CO2? symmetry
Individual C-O bond may be polar but MOLECULE is nonpolar by
symmetry
Draw an arrow along each bond pointing to the more electronegative atom. If the arrows cancel out, the molecule is NONPOLAR.

20. Symmetric vs. Asymmetric
H2O is bent.
Electron cloud is lopsided.
H2O is polar.
CO2 is nonpolar.
Can’t tell the ends apart.

21. Symmetry of Larger Molecules
CF4 is fairly symmetric. Overall, it is nonpolar.
The arrows cancel out.

22. Use the structural formula to predict Molecular Polarity!
H  C  H  

23. Ethane = C2H6
Ethene = C2H4
Ethyne = C2H2
These molecules are symmetric and the electron cloud is the same on both ends. Overall, they are nonpolar.

24. Summary Molecules containing only nonpolar bonds are always nonpolar.
H2 , O2 , CH4
Molecules containing polar bonds with symmetrical charge distributions are nonpolar.
CO2, CCl4
Molecules containing polar bonds with asymmetrical charge distributions are polar.
HF , H2O , NH3

25. Perspective Drawings
Front of plane
Back of plane
Partial bond

26. Tetrahedron CH4 H C METHANE H H
Carbon’s s and p orbitals overlap with hydrogen’s s orbital to form a hybrid orbital known as a sigma bond.
Tetrahedron
CH4 H C
H H
METHANE

27. Lewis Dot Structures
Unshared
electron pairs
Shared electron pairs