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AP Chemistry Practice Quiz: Lewis Diagrams

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 7 questions to check your progress.

Question 1 of 7

According to the established principles for constructing Lewis diagrams, what is the total number of valence electrons that must be accounted for in the structure of the carbonate ion, CO₃²⁻?

All Questions (7)

According to the established principles for constructing Lewis diagrams, what is the total number of valence electrons that must be accounted for in the structure of the carbonate ion, CO₃²⁻?

A) 22

B) 23

C) 24

D) 32

Correct Answer: C

To calculate the total valence electrons, sum the valence electrons for each atom and add the number of electrons corresponding to the negative charge. Carbon (Group 14) has 4, each Oxygen (Group 16) has 6. The 2- charge indicates an additional 2 electrons. Total = 4 + 3(6) + 2 = 24.

The principles for constructing Lewis diagrams must sometimes account for exceptions to the octet rule. Which of the following molecules is best represented by a Lewis diagram that includes a central atom with an expanded octet?

A) BF₃

B) CH₄

C) NH₃

D) SF₆

Correct Answer: D

In the Lewis diagram for SF₆, the central sulfur atom forms single bonds with six fluorine atoms. This results in the sulfur atom being surrounded by 12 valence electrons, which is an 'expanded octet'. This is possible because sulfur is in the third period and has accessible d-orbitals. BF₃ has an incomplete octet, while CH₄ and NH₃ obey the octet rule.

When representing the cyanate ion, OCN⁻, with a Lewis diagram, multiple resonance structures can be drawn. In the resonance structure that features a double bond between carbon and oxygen and a double bond between carbon and nitrogen, what is the formal charge on the oxygen atom?

A) -2

B) -1

C) 0

D) +1

Correct Answer: C

The structure is [:O=C=N:]⁻. Formal Charge = (Valence Electrons) - (Non-bonding Electrons) - (1/2 * Bonding Electrons). For Oxygen (Group 16), this is 6 - 4 - (1/2 * 4) = 6 - 4 - 2 = 0. The formal charge on the oxygen atom is zero in this specific representation.

Which of the following Lewis diagrams most accurately represents a molecule of hydrogen cyanide, HCN?

A) A diagram showing H double-bonded to C, and C single-bonded to N.

B) A diagram showing H single-bonded to N, and N triple-bonded to C.

C) A diagram showing H single-bonded to C, and C triple-bonded to N.

D) A diagram showing H single-bonded to C, and C single-bonded to N.

Correct Answer: C

The correct Lewis diagram for HCN has a single bond between H and C and a triple bond between C and N. This representation uses the correct total of 10 valence electrons (H=1, C=4, N=5) and satisfies the octet rule for both carbon and nitrogen, while hydrogen has its stable duet.

The principle of formal charge is used to determine the most plausible Lewis diagram among different possibilities. For the molecule N₂O (dinitrogen monoxide, with a N-N-O skeletal structure), the most stable representation is the one that:

A) Places a triple bond between the two nitrogen atoms.

B) Places a triple bond between the nitrogen and oxygen atoms.

C) Minimizes the magnitude of formal charges and places the negative charge on the most electronegative atom.

D) Maximizes the number of atoms that satisfy the octet rule, regardless of formal charge.

Correct Answer: C

While multiple resonance structures for N₂O satisfy the octet rule, the most stable (and thus best) representation is the one that minimizes the magnitude of the formal charges on the atoms. Furthermore, any necessary negative formal charge should be placed on the most electronegative atom, which is oxygen in this case. The structure N≡N-O accomplishes this best.

A correct Lewis diagram for which of the following molecules would contain at least one double bond?

A) F₂

B) C₂H₆

C) H₂O

D) C₂H₄

Correct Answer: D

To construct the Lewis diagram for C₂H₄ (ethene), a double bond must be placed between the two carbon atoms to satisfy the octet rule for both carbons while using the correct total of 12 valence electrons. F₂, C₂H₆, and H₂O are all correctly represented using only single bonds.

The Lewis diagram for the nitrate ion, NO₃⁻, is best represented as a resonance hybrid of three equivalent structures. This representation is necessary because the principles of Lewis diagrams indicate that:

A) Nitrogen cannot form more than three bonds, so one oxygen must have a different bond order.

B) All three N-O bonds are experimentally shown to be identical in length and strength, which a single Lewis structure with one double and two single bonds cannot represent.

C) The ion has a negative charge, which must be delocalized across all three oxygen atoms equally.

D) The central nitrogen atom has an expanded octet that shifts between the three oxygen atoms.

Correct Answer: B

A single Lewis diagram for NO₃⁻ would show one N=O double bond and two N-O single bonds. However, experimental evidence shows that all three N-O bonds are identical, intermediate in length and strength between a single and a double bond. The concept of resonance, represented by drawing all three possible structures, is used to reconcile the Lewis model with this physical reality.