Atypical bonding, stability, and reactivity
Major concepts
 Ionic compounds occur when metals and nonmetals are in the same compound
 Formal charge occurs when an atom in a Lewis dot structure “owns” more or less valence
electrons in a molecule than the atom would have by itself
 Formal charges and incomplete octets indicate that the structure is relatively less stable than a
typical molecule
 Less stable molecules are more reactive; more stable molecules are less reactive
 During a reaction, changes in bonding are controlled by how that bonding affect stability
Vocabulary
 Formal charge
 More or less stable
 More or less reactive
Students should be able to:
 Quickly recognize atoms with formal charge in any structural representation
 Determine the formal charge of any atom in a molecule
 Given a structure with formal charge, draw in all hydrogen atoms and lone pairs
 Begin to recognize whether a reaction increased or decreased the stability of a system
Daily Problems
1. Problems 34-68 in chapter 1 of Organic as a Second Language
2. Fill in the chart below with bonding patterns that are common for each of these situations:
Positive formal charge
No formal charge
Negative formal charge
Carbon
Nitrogen
Oxygen
Halogens (fluorine)
3. Draw Lewis structures of these IONIC molecules. (How do you recognize an ionic molecule?)
a. NaOH
Solution: Remember to draw the cationic and anionic portions separately.
Na
O
H
b. KOH
C. NaNH2
d. LiH
e. Mg(OH)2
f. NH4Cl (hint: in this compound, the cation is the polyatomic ion.)
4. For each reaction, consider what took place in terms of bond making and breaking.
A. Some electrons have been indicated in the starting material. Where are they in the
products?
B. Did this reaction lead to greater stability or less stability?
5. Fill in all implicit lone pairs and hydrogen atoms in these ions.
Cumulative Problems
6. Draw two different Lewis structures for each of these ionic compounds. Be sure to draw in all lone
pairs.
A. CH3OB. C3H7+
In each structure you drew, which atom is the least stable? Which atom is the most reactive?
7. The structures below do not have their formal charges drawn--fill in any necessary formal charges. In
the reactions below, did the compound become more or less reactive? (Later on we will learn about
reagents—don’t worry about how this happened, just consider the structures.)
8. In each of these reactions, something was added or removed from the compound. What was it? It
might help to keep track of all hydrogens and electrons by drawing them in explicitly.
Example:
Answer: It is tempting to say that the starting material lost a bond, but that is not true. The bond just
changed. The molecule actually gained two hydrogen atoms. It becomes more obvious when they are
drawn in:
9. Which atom(s) in each of the structures below is/are Lewis basic?
Extension problems
9. A carboxylic acid that has one less hydrogen atom than typical, but the same number of electrons,
has a formal charge on one of its oxygen atoms. Draw the formal charge on the correct atom in the
structure below:
The way that Lewis structure was drawn was a bit arbitrary, though. It could have been draw with one
bond to the top oxygen and two bonds to the bottom oxygen, giving a formal charge on the opposite
oxygen atom. Draw this alternative Lewis structure.