1. Fundamentals of General, Organic and Biological Chemistry
7th Edition
Chapter Four
Covalent Compounds
James E. Mayhugh
Copyright © 2010 Pearson Education, Inc.

2. 4. 1 Covalent Bonds A covalent bond: A bond between 2 nonmetal atoms
They work together by sharing electrons with each other such that all atoms get their “octet”.
A group of atoms held together by covalent bonds is called a covalent compound or molecule.

3. Example: Chlorine gas
Example: water (H2O)

4. 4.5 Molecular Formulas and Lewis Structures
Molecular formula: A formula that shows the numbers and kinds of atoms in one molecule of a compound.
Lewis structure: A drawing that shows both the connections among atoms and the locations of non-bonding (lone pairs) valence electrons.

5. 4.6 Drawing Lewis Structures
Method #1: Pairing up dots from electron dot symbols until every atom has its “octet”
Exceptions:
Hydrogen only needs a “duet”
Boron and aluminum are often stuck with a “sextet”

6. 4.6 Drawing Lewis Structures
Lots-o-examples!

7. MEMORIZE the 7 diatomic elements:

8. 4.6 Drawing Lewis Structures
But, if you run across a molecule where the dots just won’t “pair up” correctly, a different method of electron sharing called “coordinate covalent” bonding must be occurring. In THOSE cases, you should employ…

9. 4.6 Drawing Lewis Structures
Method #2 - A Guess-n-Check method
Lay the atoms out on the paper as usual, but then put single bonds between each pair of neighboring atoms
Add lone pairs to any atoms that don’t have an “octet” (or “doublet” for Hydrogen) until they all do.
Then…

10. 4.6 Drawing Lewis Structures
Method #2
Count up how many valence electrons you’ve shown.
Count up how many valence electrons you’re actually allowed (from the periodic table)
For every 2 electrons that you’re over the allowed number, remove 2 neighboring lone pairs and replace with a multiple bond.
This drops your electron count by two, while still leaving every atom with its “octet”

11. 4.6 Drawing Lewis Structures
“Method #2” Examples

12. 4.6 Lewis Structures of Polyatomic Ions
Polyatomic ions are covalent compounds that have an overall charge.
Same technique as before for drawing Lewis structures, but take into account
the extra electron(s) for anions
the lack of electron(s) for cations

13. 4.6 Drawing Lewis Structures
Polyatomic Ion Lewis structure examples:

14. 4.11 Naming “Binary” Covalent (Molecular) Compounds
Write the element with the lower ionization energy first.
Name the first element in the formula, using the Greek prefix that corresponds to the number of atoms. (“mono” is optional for 1st element)

16. 4.11 Naming Simple Covalent (Molecular) Compounds
Name the 2nd element in the formula, using the ending -ide along with the appropriate Greek prefix (“mono” is NOT optional on 2nd element)

18. 4.8 The Shapes of Molecules
Molecular shapes can be predicted by the valence-shell electron-pair repulsion (VSEPR) model.
the negatively charged clouds of electrons in bonds and lone pairs repel each other, and keep as far apart as possible.
These central atom electron charged clouds are also called “electron domains”

19. There are three step to applying the VSEPR model:
Step 1: Draw a Lewis structure of the molecule.
Step 2: Count the number of electron charge clouds surrounding the central atom.
There can be more than one “central atom”… treat each separately
Step 3: Predict molecular shape by assuming that the charge clouds orient in space so that they are as far away from one another as possible.

20. 4.8 The Shapes of Molecules
You must MEMORIZE the names and bond angles of each shape… summarized on pgs of your text.

21. 4.8 The Shapes of Molecules
2 electron clouds on the central atom

22. 4.8 The Shapes of Molecules
3 electron clouds on the central atom

23. 4.8 The Shapes of Molecules
3 electron clouds on the central atom (continued)
Key concepts on this example:
Atoms don’t repel, electron clouds do
lone pairs of electrons repel more than bonded pairs do

24. 4.8 The Shapes of Molecules
3 electron clouds on the central atom (continued)

25. 4.8 The Shapes of Molecules
4 electron clouds on the central atom

26. 4.8 The Shapes of Molecules
4 electron clouds on the central atom (continued)
(3 bonded pairs, one lone pair on Cent. Atom)

27. 4.8 The Shapes of Molecules
4 electron clouds on the central atom (continued)
(2 bonded pairs, 2 lone pairs on cent. atom)

28. Pg. 115 in text
Trigonal Planar

29. 4.9 Polar Covalent Bonds and Electronegativity
Electronegativity: The amount of attraction that an atom has for a SHARED pair of electrons.

30. Electronegativity increases going up and to the right on the periodic table… the same as ionization energy and electron affinity

31. 4.9 Polar Covalent Bonds and Electronegativity
In H2 and Cl2, electrons are shared equally by both atoms.

32. 4.9 Polar Covalent Bonds and Electronegativity
If the atoms are different, one atom will attract the electron pair more than the other.
Example: H-Cl

33. 4.9 Polar Covalent Bonds and Electronegativity
Chlorine has a greater attraction for the shared pair of electrons…
…giving Cl a partial negative charge, and leaving H with a partial positive charge.
δ = partial d+ d-

34. 4.9 Polar Covalent Bonds and Electronegativity
… this results in a bond with 2 poles… a δ+ pole and a δ- pole
We call that kind of bond a polar covalent bond
What would you call the bond between the hydrogens in H2?

35. 4.9 The “Ultimate” Polar Bond?
… would be an ionic bond!!
If EN difference >2.0, the atoms will form an ionic bond.

36. 4.10 Polar Molecules
But knowing if the BONDS of a molecule are polar or not is only the means to the end…
Whether the WHOLE MOLECULE is polar or not.
It’s the polarity of the whole molecule that gives the molecule many of it’s properties.

37. 4.10 Polar Molecules
A molecule is polar if it has a δ- side and a δ+ side (a δ- pole and a δ+ pole).
If the bonds are polar, the molecule will be polar… UNLESS…
All the poles in the molecule cancel each other out!
If all the partial charges cancel each other out, the whole molecule is nonpolar.

38. 4.10 Polar Molecules
Determining whether a given molecule is polar or nonpolar will be an EXTREMELY important task for the rest of the course.

39. 4.10 Criteria for a Molecule to be Nonpolar
a) there are NO lone pairs on the central atom
AND
b) all the outside atoms are the same.

40. 4.10 Criteria for a Molecule to be Nonpolar
if either “a” or “b” is a “fail”, then the molecule is POLAR… it’s polar bonds did NOT cancel

41. 4.10 Determining if a Molecule is Polar or Nonpolar:
Examples:

42. 4.5 Characteristics of Molecular Compounds (vs. Ionic)
Usually nonmetal-nonmetal compounds
Ionic: metal - nonmetal
Some are solids, some are liquids and some are gases at room temperature
Ionic: all are solids
Low melting and boiling points
Ionic: very high melting and boiling pts.
Do not conduct electricity, even when dissolved or melted
Ionic: conduct if melted or dissolved in water.

43. End of Chapter 4

44. Chapter Summary
A covalent bond is formed by the sharing of electrons between atoms rather than by the complete transfer of electrons from one atom to another.
Two shared electrons are a single bond , four are a double bond, and six are a triple bond.
The group of atoms held together by covalent bonds is called a molecule.
When a lone pair of electrons on one atom overlaps a vacant orbital on another atom a coordinate covalent bond is formed.
An atom shares enough electrons to reach a noble gas configuration.

45. Chapter Summary Contd.
Molecular formulas show the numbers and kinds of atoms in a molecule.
Lewis structures show how atoms are connected in molecules.
Covalent bonds are indicated as lines between atoms, and valence electron lone pairs are shown as dots.
Molecules have shapes that can be predicted using the VSEPR model.
The electronic geometry of atoms with 2 electron charge clouds is linear, with 3 it is planar triangular, and with 4 it is tetrahedral.

46. Chapter Summary Contd.
Bonds between atoms are polar if the bonding electrons are not shared equally between the atoms.
The ability of an atom to attract electrons is electronegativity. It is highest on the upper right of the periodic table and lowest on the lower left.
Molecular polarity is the sum of all individual bond polarities and lone pair contributions in a molecule.
Molecular compounds usually have lower melting points and boiling points than ionic compounds, many are water insoluble, and they do not conduct electricity when melted or dissolved.

47. Key Words Binary compound Bond angle Bond length Condensed structure
Coordinate covalent bond
Covalent bond
Double bond
Electronegativity
Lewis structure
Lone pair
Molecular compound
Molecular formula
Molecule
Polar covalent bond
Regular tetrahedron
Single bond
Structural formula
Triple bond
Valence-shell electron-pair repulsion (VSEPR) model