Chemistry 10050 - Covalent Bonding
Electron Dot Symbols
These can only be done for main group elements. First step
is to count the number of electrons in outermost (largest 'n') shell. Only
count e- in 's' and 'p' orbitals.
Ex. P = ... 3s2 3p3 (5 valence-)
Types of bonds
| Composition |
Bonding |
| Metal + Metal → Metal |
Delocalized electrons throughout solid. |
| Non-metal + Non-metal → Non-metal |
Covalent bonds between molecules. |
| Metal + Non-metal → Salt |
Ionic "bonds" (electrostatic
attraction) between ions. |
Definitions
- Covalent Bond:
- Sharing of two or more electrons between two atoms. If 2 e-
are shared, it is a single bond. If 4 e- shared, it is a double
bond, and if 6 e- are shared, it is a triple bond. If the
electronegativities of the two atoms differ, the bond is a polar covalent
bond. If both atoms are the same, it is a non-polar covalent bond.
- Charge Cloud:
- Region of space occupied by two or more e- (Lone pairs or
bonds). Note that single, double, and triple bonds all occupy only one
region of space and therefore only count as one charge cloud.
- Octet Rule:
- In covalent compounds, all atoms (except H) prefer to have access to
eight valence shell electrons. The common exceptions to this rule are:
- B: typically has only has 6 e-
- P: can have up to 10 e- (in compounds such as PCl5)
- S: can have up to 12 e- (in compounds such as SF6)
- Electronegativity:
- Measure of the ability of an atom to attract electrons. Greatest for F
(4.0), lowest for Fr. Know the following trends: Going across row,
E.N. increases. Going up column, E.N. increases. (Note that values for
noble gases not defined, since these compounds don't form covalent bonds.)
- Molecular Formula:
- Indicates the number of each type of atom in a molecule (ex. H2O).
- Structural Formula:
- Shows how atoms in a molecule are connected. (Lewis structure).
"Rules" for Drawing Lewis Structures:
| Element |
H |
|
C |
N |
O |
F |
| # Valence e- |
1 |
|
4 |
5 |
6 |
7 |
| 'Ideal' # of bonds |
1 |
|
4 |
3 |
2 |
1 |
- Count number of valence shell e- on each atom, and determine the
total # of e-.
- Determine the number of bonds each atom "wants". (8 - column #).
- Place atom that wants the most bonds in "center".
- Arrange all remaining atoms except H atoms and connect with
single bonds.
- Arrange H atoms, looking at the ideal number of bonds each atom
"wants".
- Add multiple bonds (if necessary) so that each atom has as close
to its 'ideal' number of bonds as possible.
- Add lone pairs so that each atom obeys the octet rule.
- Check to be sure that the total number of electrons in the structure
you have drawn is correct.
Bond Polarity
If covalently bonded atoms are different, the sharing of electrons between
these atoms may be unequal. If the difference in electronegativities is large
enough, this bond is classified as a polar bond. In a polar bond, the more
electronegative atom has a partial negative charge (δ-), and
the less electronegative atom has a partial positive charge
(δ+).
For most organic compounds, the following guidelines provide a
reasonable estimate of the polarity of bonds.
- C-H bonds are non-polar.
- If atoms share an edge on the periodic table, then a covalent bond
between these atoms will be non-polar. (Ex. C-N).
- If atoms don't share an edge, then a covalent bond between these
atoms is polar, and the more electronegative atom (the atom
closest to F) will have a partial negative charge.
(Ex. δ+C-Oδ-)
- The most common exception to this rule is that C-S bonds are
non-polar.
VSEPR Model:
Draw Lewis structure for molecule. Count the number of "charge clouds"
immediately surrounding the atom of interest (central atom). Use following table to
determine geometry. This table must be memorized.
| # Charge Clouds |
Angle |
# Bonds |
# Lone Pairs |
Molecular Geometry |
| 2 |
180° |
2 |
0 |
Linear |
| 3 |
120° |
3 |
0 |
Trigonal Planar |
| 2 |
1 |
Bent |
| 4 |
110° |
4 |
0 |
Tetrahedral |
| 3 |
1 |
Pyramidal |
| 2 |
2 |
Bent |
Nomenclature for Binary Covalent Compounds: (Naming compounds)
Name least electronegative element first, with name unchanged. Name most
electronegative element last, changing suffix to -ide. Use the
appropriate prefix to indicate number of each atom type.
| NI3 |
Nitrogen triiodide |
| CCl4 |
Carbon tetrachloride |
| P2O5 |
Diphosphorus pentoxide |