1. Nature of Covalent Bonding

2. Objectives
Describe how electrons are shared to form covalent bonds and identify exceptions to the octet rule
Demonstrate how electron dot structures represent shared electrons
Describe how atoms form double or triple covalent bonds
Distinguish between a covalent bond and a coordinate covalent bond and describe how the strength of a covalent bond is related to its bond dissociation energy
Describe how oxygen atoms are bonded in ozone

3. Important Vocabulary Coordinate covalent bond Polyatomic ion
Single covalent bond
Structural formula
Unshared pair
Double covalent bond
Triple covalent bond
Coordinate covalent bond
Polyatomic ion
Bond dissociation energy
Resonance structure

4. Octet Rule in Covalent Bonding
In forming covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases
Combinations of atoms of nonmetallic elements in groups 4A, 5A, 6A, & 7A are likely to form covalent bonds
For example, hydrogen has 1 electron
Thus it will form a covalent bond with itself to share 2 electrons and attain the noble gas configuration of helium

5. Single Covalent Bonds
Form when two atoms are held together by sharing a pair of electrons
Of the diatomic molecules: hydrogen, chlorine, fluorine, bromine & iodine form single covalent bonds
An electron dot structure such as H:H represents the shared pair of electrons of the covalent bond by two dots
The pair of shared electrons forming the covalent bond is also represented as a dash H‒H

6. Single Covalent Bonds

7. Structural Formulas Represent the covalent bonds by dashes
Shows the arrangement of covalently bonded atoms

8. Unshared Pair
An unshared pair is a pair of valence electrons that are not shared between atoms
It is also called a lone pair of a nonbonding pair
For example: in the water molecule oxygen has 2 unshared pairs of electrons and forms 2 single covalent bonds

9. Unshared Pair Examples
Identify the unshared pairs in the pictures below

10. Drawing Lewis Structure for Simple Molecules
Step 1: draw the symbols for the molecule side by side, leaving space for the dots
Step 2: place two dots, one on top of the other, between the two symbols to represent the single covalent bond
Step 3: Add remaining number electrons for both symbols!

11. Examples
H Cl H-F

12. Practice Problems #1
F I H-Cl

13. Multiple Bonds
Atoms can share more than one pair of electrons in a covalent bond
Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing two or three pairs of electrons
Sharing two pairs of electrons results in a ___________________
Sharing three pairs of electrons results in a ___________________

14. Double Covalent Bonds

15. Triple Covalent Bonds

16. Drawing Lewis Structures with Multiple Elements
Draw a Lewis structure for each atom in the compound
Determine the number of valence electrons in the compound
Arrange the Lewis structure to show how the atoms bond in the molecule
Halogen and hydrogen atoms often bind to only one other atom and are usually at the end of the molecule
Carbon is often placed in the center of the molecule
The atom with the lowest electronegativity is usually placed in the center

17. Drawing Lewis Structures with Multiple Elements
Distribute the dots so that each atom except for hydrogen, beryllium, and boron, satisfies the octet rule
Change each pair of dots that represents a shared pair of electrons to a long dash
Count the number of electrons surrounding each atom making sure you satisfy the octet rule for each one (including hydrogen, beryllium, & boron) and that you have the same number of valence electrons from step 1

18. Examples
CH3I H2S CH2Cl2

19. Practice Problems #2
SCl2 SiH4 H2O CHF3

21. Coordinate Covalent Bonds
Is a covalent bond in which one atom contributes both bonding electrons
In a coordinate covalent bond, the shared electron pair comes from one of the bonding atoms
For example: carbon monoxide (CO)

22. Polyatomic Ions
A polyatomic ion is a group of 2 or more atoms joined by covalent bonds that has a positive or negative charge and behaves as a unit
Most polyatomic cations and anions contain both covalent and coordinate covalent bonds
Compounds containing polyatomic ions also include both _____________________

23. Ammonium Ion Formation
Forms when a positively charged hydrogen ion (H+) attaches to the unshared electron pair of an ammonia molecule (NH3)

24. Drawing Lewis Structures for Polyatomic Ions
Is different in that we must account for the charges on them
For example: SO32-
Step 2
Step 1
Step 4
Step 3

25. Examples
ClO3‒ H3O+

26. Practice Problems #3
NO3- PO43- NH4+ CO32-

28. Bond Dissociation Energy
Is the energy required to break a bond between two covalently bonded atoms
The ___________ the bond the ______ energy is required to break it
Bond energy is measured in ____________
Examples:
H2 requires 435 kJ/mol to break
C‒C requires 347 kJ/mol

29. Resonance
Some molecules can not be represented by a single Lewis structure
Resonance structures are two or more possible configurations for the same compound that differ in the arrangement of electrons
For example: ozone, O3
The actual bonding of oxygen atoms in ozone is a hybrid, or mixture, of the extremes represented by the resonance forms

30. Exceptions to the Octet Rule
The octet rule cannot be satisfied in molecules whose total number of valence electrons is an _________
There are also molecules in which an atom has fewer, or more, than a complete octet of valence electrons
For example: nitrogen dioxide has a total of 17 valence electrons

31. Exceptions to the Octet Rule
Several molecules with an even number of electrons fail to follow the octet rule
For example: boron trifluoride (BF3)
The boron atom within this compound is deficient by two electrons

32. Exceptions to the Octet Rule
A few atoms, especially phosphorus and sulfur, can expand the octet to include 10 or 12 electrons