1. Chemical Bonding
10/18/12

2. The Octet Rule
Combinations of elements tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level.
Monatomic chlorine
Diatomic chlorine

3. The Octet Rule and Covalent Compounds
Covalent compounds tend to form so that each atom, by sharing electrons, has an octet of electrons in its highest occupied energy level.
Covalent compounds involve atoms of nonmetals only.
The term “molecule” is used exclusively for covalent bonding

4. The Octet Rule: The Diatomic Fluorine Molecule1s 2s 2p
Each has seven valence electrons F 1s 2s 2p F F

5. The Octet Rule: The Diatomic Oxygen Molecule1s 2s 2p
Each has six valence electrons O 1s 2s 2p O O

6. The Octet Rule: The Diatomic Nitrogen Molecule1s 2s 2p
Each has five valence electrons N 1s 2s 2p N N

7. Lewis Structures
Lewis structures show how valence electrons are arranged among atoms in a molecule.
Lewis structures Reflect the central idea that stability of a compound relates to noble gas electron configuration.
Shared electrons pairs are covalent bonds and can be represented by two dots (:) or by a single line ( - )

8. Comments About the Octet Rule
2nd row elements C, N, O, F observe the octet rule (HONC rule as well).
2nd row elements B and Be often have fewer than 8 electrons around themselves - they are very reactive.
3rd row and heavier elements CAN exceed the octet rule using empty valence d orbitals.
When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals. (usually the central atom)

9. Lewis Structures
Show how valence electrons are arranged among atoms in a molecule.
Reflect the central idea that stability of a compound relates to noble gas electron configuration.

10. The HONC Rule Hydrogen (and Halogens) form one covalent bond
Oxygen (and sulfur) form two covalent bonds
One double bond, or two single bonds
Nitrogen (and phosphorus) form three covalent bonds
One triple bond, or three single bonds, or one double bond and a single bond
Carbon (and silicon) form four covalent bonds.
Two double bonds, or four single bonds, or a triple and a single, or a double and two singles

11. Completing a Lewis Structure -CH3Cl
Make the atom wanting the most bonds central
Add up available valence electrons:
C = 4, H = (3)(1), Cl = 7 Total = 14
Join peripheral atoms
to the central atom
with electron pairs. H .. ..
Complete octets on
atoms other than
hydrogen with remaining
electrons H .. C .. Cl .. .. .. H

12. Multiple Covalent Bonds: Double bonds
Ethene
Two pairs of shared electrons

13. Multiple Covalent Bonds: Triple bonds
Ethyne
Three pairs of shared electrons

14. A Note About Bond #, Strength, and Length
The more covalent bonds (single, double, triple)
The shorter the bond length
The stronger the bond

15. Bond
Bond type
Bond length
(pm)
Bond Energy
(kJ/mol)
C - C
Single
154
347
C = C
Double
134
614
C  C
Triple
120
839
C - O
143
358
C = O
123
745
C - N
305
C = N
138
615
C  N
116
891

16. Acetic Acid Two electrons (one bond) per hydrogen
Eight electrons (four bonds) per carbon
Eight electrons (two bonds, two unshared pairs) per oxygen

17. Resonance
Resonance is invoked when more than one valid Lewis structure can be written for a particular molecule.
Benzene, C6H6
The actual structure is an average of the resonance
structures.
The bond lengths in the ring are identical, and
between those of single and double bonds.

18. Resonance Bond Length and Bond Energy
Resonance bonds are shorter and stronger than single bonds.
Resonance bonds are longer and weaker than double
bonds.

19. Resonance in Ozone, O3 Neither structure is correct.
Oxygen bond lengths are identical, and intermediate to single and double bonds

20. Resonance in Polyatomic Ions
Resonance in a carbonate ion:
Resonance in an acetate ion:

21. 1. Covalent Bonding and Polarity 2. Intermolecular Forces
10/22/12

22. Two Ways of Sharing NONPOLAR
Equal Sharing
Unequal Sharing
NONPOLAR
Difference in electronegativity of <0.5
POLAR
Difference in electronegativity
More electronegative atoms has larger share of electrons

23. “Partial Positive”
“Partial Negative”
(larger share of the electrons)

24. Polar Bonds vs. Polar Molecules
A molecule can be nonpolar, even if it has polar bonds
It’s like a tug-of-war: If all arms (bonds) are pulling equally in opposite directions, overall there is no gain or net difference.
(We will see more of this in the coming days)

25. How are molecules held together
How are molecules held together?? There must be something, or materials would never stick together.

26. Intermolecular Forces
Inter: Between, Molecular: Molecules
Forces that attract molecules to other molecules. These include:
Hydrogen bonding
Dipole-dipole attraction
London dispersion forces

27. NOTE: Intermolecular Forces are NOT bonds
NOTE: Intermolecular Forces are NOT bonds!!!! Friends don’t let friends make this mistake!!

28. Dipole-Dipole Attraction
Attraction between oppositely charged regions of neighboring molecules.
Dipole-dipole attraction in hydrogen chloride, a gas that is used to make hydrochloric acid

30. Hydrogen Bonding: A Special Type of Dipole-Dipole
Attractions between the semi-positive H atoms with the semi-negative N, O, and F atoms.
Ex) Base pairing in DNA by hydrogen bonding

31. Hydrogen Bonding in Water

32. Hydrogen bonds are very strong.
Evidence: High boiling point of water and high surface tension

33. London Dispersion Forces
Temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles
Electrons are in constant motion, so atoms can have areas that are temporarily + or -
This type of force is always present for all categories of molecules (polar or not)

35. The strength of LDFs is proportional to a molecule’s size (weight)
These are the only forces of attraction between completely nonpolar molecules
The strength of LDFs is proportional to a molecule’s size (weight)
Large nonpolar molecules may have substantial dispersion forces
Small nonpolar molecules have weak dispersion forces and exist almost exclusively as gases

36. More LDFs

37. Relative Magnitudes of Forces
The types of bonding forces vary in their strength as measured by average bond energy.
Strongest
Weakest
Covalent bonds (400 kcal)
Hydrogen bonding (12-16 kcal )
Dipole-dipole interactions (2-0.5 kcal)
London forces (less than 1 kcal)

38. Effect of Intermolecular Forces
Higher strengths/amounts of IMFs leads to:
High boiling points
Higher melting points
High surface tension
High viscosity (thickness)

39. London Forces in Hydrocarbons

40. IMFs affect solubility: what will and won’t dissolve in a solution
Things with similar IMFs will dissolve in each other
Ex) Sugar dissolved in water: both allow for hydrogen bonding