AP Chemistry Questions on Formal Charge and Lewis Structures

🧪 Quick Study Guide

• ⚛️ Lewis Structure Basics: Represents valence electrons as dots around an atom. Single, double, and triple bonds are shown as lines connecting atoms.
• 🔎 Formal Charge Formula: Formal Charge = (Valence Electrons) - (Non-bonding Electrons) - (1/2 Bonding Electrons).
• 📝 Steps to Calculate Formal Charge:
  1. Draw the Lewis structure.
  2. Count valence electrons of the atom.
  3. Count non-bonding electrons around the atom.
  4. Count bonding electrons around the atom and divide by 2.
  5. Apply the formula.
• 💡 Optimal Lewis Structures: Minimize formal charges on atoms. Negative formal charges should be on more electronegative atoms.
• 📌 Resonance Structures: Multiple valid Lewis structures can be drawn. The actual structure is a hybrid of all resonance structures.
• 🔑 Common Elements: Carbon (4 bonds), Nitrogen (3 bonds, 1 lone pair), Oxygen (2 bonds, 2 lone pairs), Halogens (1 bond, 3 lone pairs).

Practice Quiz

1. Which of the following is the correct formula for calculating formal charge?
   1. A) Valence Electrons + Non-bonding Electrons + (1/2 Bonding Electrons)
   2. B) Valence Electrons - Non-bonding Electrons - (1/2 Bonding Electrons)
   3. C) Valence Electrons - Non-bonding Electrons + (1/2 Bonding Electrons)
   4. D) Valence Electrons + Non-bonding Electrons - (1/2 Bonding Electrons)
2. What is the formal charge on the central nitrogen atom in $NO_2^+$?
   1. A) +1
   2. B) -1
   3. C) 0
   4. D) +2
3. What is the formal charge on the oxygen atom in $H_3O^+$?
   1. A) 0
   2. B) +1
   3. C) -1
   4. D) +2
4. Which of the following molecules has a central atom with a formal charge of +1?
   1. A) $CO_2$
   2. B) $SO_2$
   3. C) $BF_3$
   4. D) $NH_4^+$
5. In the best Lewis structure for $SCN^-$, what is the formal charge on carbon?
   1. A) -1
   2. B) 0
   3. C) +1
   4. D) -2
6. Which of the following Lewis structures is the most stable based on formal charge considerations?
   1. A) A structure with large formal charges
   2. B) A structure with minimal formal charges
   3. C) A structure with positive formal charges on electronegative atoms
   4. D) All Lewis structures are equally stable
7. What is the formal charge on the central atom in $PCl_5$?
   1. A) +1
   2. B) -1
   3. C) 0
   4. D) +2