1. Covalent bonding & Lewis Structures

2. Remember…
In your notes: Compare and contrast the similarities and differences of ionic and covalent bonding. You may use illustrations.

3. Covalent compounds Also known as molecular compounds
Usually liquids or gases at room temperature
Lower melting and boiling points than ionic bonds
Consist of bonding pairs and lone pairs (unshared pairs)

4. Purpose of Lewis Structures
Give us a visual representation of the structure of a molecule or compound.
Important in medicine and how drugs interact with the body, and more.
Ex: Thalidomide

5. Steps to follow
Add up the total number of valence electrons in the compound.
CH4
If there is a charge, add or subtract electrons from your total according to the charge.
Connect all of the atoms with bonds.

6. Steps Continued
4. Count how many electrons were used in your connecting bonds (two electrons per bond), and subtract from your total.
5. Add any remaining electrons with bonds or lone pairs.
6. If there is a charge, add brackets around the entire structure and write the charge to the upper right of the brackets.

7. Practice
SiF4 O2
NO2-

8. Bonds before lone pairs
If the middle atom does not have 8 electrons, make the outer atoms share their electrons with the middle atom.
CO2
HCN

9. Exceeding the Octet
A nonmetal central atom that is in Period 3 or above, with empty d-orbitals, can accommodate more than 8 electrons.
PCl5, SF4, CIF3

10. Less than an octet
Few elements can have less than an octet as a central atom:
Be (only needs 4 electrons)
B, Pb, Sn (only need 6 electrons. To remember, think of a 6 inch BLT)
BeCl2
BF3
(These tend to bond with other molecules that have a lone pair on their central atom.)

11. Bond Strength Strength of bonding pairs:
Triple bonds (3 shared pairs of electrons) > double bonds (2 shared pairs) > single bonds (1 shared pair)
Bond dissociation energy: the energy required to break the bond between two covalently bonded atoms.
C-C bonds and C-H bonds have high bond dissociation energies

12. Resonance
When the Lewis structure of a molecule has two or more possibilities with the same number of electron pairs. Ozone: O3