AP Chemistry Practice Quiz: Resonance and Formal Charge
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Test your understanding with short quizzes. This quiz has 10 questions to check your progress.
Question 1 of 10
All Questions (10)
A) When a molecule contains atoms from the third period or below.
B) When the central atom has an incomplete octet.
C) When more than one equivalent Lewis structure can be constructed for the molecule.
D) When the calculation of formal charges results in non-zero values.
Correct Answer: C
The provided content states, 'Resonance must be included as a refinement to the Lewis structure when more than one equivalent structure can be constructed.' This indicates that the existence of multiple, equivalent Lewis diagrams is the specific condition that requires the use of resonance.
A) To determine the overall charge of a polyatomic ion.
B) To select the most plausible model among several nonequivalent Lewis structures.
C) To predict the polarity of the bonds within the molecule.
D) To confirm that all atoms have a complete octet of electrons.
Correct Answer: B
The content specifies that formal charge is used 'to select between nonequivalent structures' and 'to determine which of several possible valid Lewis diagrams best models the molecule's structure and properties.' This highlights its role as a tool for evaluating and choosing the best representation from multiple possibilities.
A) O₃ (Ozone)
B) CH₄ (Methane)
C) H₂O (Water)
D) BeCl₂ (Beryllium Chloride)
Correct Answer: A
Ozone (O₃) can be drawn with a double bond to one terminal oxygen and a single bond to the other. An equivalent structure can be drawn by switching the positions of the double and single bonds. According to the content, when 'more than one equivalent structure can be constructed,' resonance must be included. The other molecules have single, unambiguous Lewis structures.
A) Molecules with highly polar covalent bonds.
B) Molecules with an odd number of valence electrons.
C) Molecules containing noble gas atoms.
D) Molecules that form large network covalent solids.
Correct Answer: B
The content explicitly states, 'The Lewis structure model has limitations, particularly for molecules with an odd number of valence electrons.' This is because it is impossible to pair all electrons to satisfy the octet rule for all atoms.
A) One bond is a double bond, and the other two are single bonds.
B) The ion rapidly alternates between the three different Lewis structures.
C) All three N-O bonds are identical in length and strength.
D) Two bonds are covalent, and one bond is ionic.
Correct Answer: C
The use of 'equivalent structures' in a resonance model implies that the true structure is a hybrid, or average, of all contributing structures. Therefore, all the bonds are indistinguishable from one another, having characteristics intermediate between a single and a double bond. This results in identical bond lengths and strengths.
A) Always choose the structure that strictly adheres to the octet rule.
B) Always choose the structure that minimizes formal charges, regardless of the octet rule.
C) Use both the octet rule and formal charge to determine which diagram best models the molecule.
D) Conclude that neither structure is a valid representation of the molecule.
Correct Answer: C
The content states, 'The octet rule and formal charge can be used to determine which of several possible valid Lewis diagrams best models the molecule's structure and properties.' This indicates that both principles are tools to be used in conjunction to evaluate and select the best possible representation, involving a trade-off rather than a rigid adherence to one rule over the other.
A) They are distinct isomers of a molecule that can be isolated in a lab.
B) They are a set of valid Lewis diagrams used collectively to represent a single, more stable hybrid structure.
C) They represent a molecule that is constantly flipping between different electronic configurations.
D) They are only necessary for molecules that do not obey the octet rule.
Correct Answer: B
The content describes resonance as a 'refinement to the Lewis structure' needed when multiple 'equivalent structures' can be drawn. This implies that the individual structures are not real but are components of a single, blended (hybrid) model that more accurately predicts the molecule's properties.
A) CO₂
B) NH₃
C) NO₂
D) SO₄²⁻
Correct Answer: C
The content highlights a key limitation of the Lewis model for 'molecules with an odd number of valence electrons.' Nitrogen dioxide (NO₂) has 5 + 2(6) = 17 valence electrons, which is an odd number. This makes it impossible to draw a single Lewis structure where all atoms satisfy the octet rule. The other options have even numbers of valence electrons.
A) Resonance between equivalent structures.
B) The limitation of the Lewis model for odd-electron species.
C) Using formal charge to select between nonequivalent structures.
D) The universal application of the octet rule for all atoms.
Correct Answer: C
The provided text states that one can 'use formal charge to select between nonequivalent structures.' Since the chemist is comparing different possible structures for OCN⁻ and using formal charge to make a selection, this is a direct application of that principle.
A) is the simplest to draw.
B) contains the maximum possible number of bonds.
C) best models the molecule's actual structure and properties.
D) places all valence electrons on the most electronegative atoms.
Correct Answer: C
The content states that the octet rule and formal charge are used to find the Lewis diagram that 'best models the molecule's structure and properties,' and that resonance is 'often needed for accurate predictions.' This shows that the goal of these tools is to improve the accuracy and predictive power of the model.