Free Practice Quiz Question List

08 Lewis Structures and Resonance Online Quiz Questions

Use this free practice quiz with 20 questions to review 08 Lewis Structures and Resonance, test your knowledge, and prepare for your next test or exam.

20 questions

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1 point

When selecting a skeleton for a typical molecule, which rule best guides the choice of the central atom?

A
Choose the most electronegative atom other than hydrogen as the central atom.
B
Make hydrogen terminal; usually place the least electronegative remaining atom at the center.
C
Choose the atom with the most valence electrons as the central atom.
D
Place the atom with the largest formal charge at the center.

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1 point

You are counting valence electrons for a species with an overall charge of $+2$ . What adjustment should you make to the sum of the neutral atoms' valence electrons?

A
Add two electrons to the neutral-atom total.
B
Subtract one electron from the neutral-atom total.
C
Subtract two electrons from the neutral-atom total.
D
Leave the neutral-atom total unchanged.

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1 point

In Lewis electron bookkeeping, what is the consequence of connecting two atoms with a single bond?

A
It uses two electrons as one shared pair.
B
It uses one electron from each atom but counts as one electron total.
C
It uses four electrons because both atoms contribute a pair.
D
It does not use electrons from the available total.

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1 point

A $\mathrm{CO_2}$ skeleton has one single bond from carbon to each oxygen. After completing both oxygen octets, carbon still lacks an octet. Which next step yields the structure with octets and zero formal charges on all three atoms?

A
Keep the single bonds and place all remaining electrons as lone pairs on carbon.
B
Add one electron to each oxygen so carbon reaches an octet.
C
Remove a lone pair from each oxygen without using those electrons in bonding.
D
Convert one lone pair from each oxygen into a bonding pair, forming two double bonds.

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1 point

In the Lewis structure $\mathrm{O=C=O}$ , carbon has four bonds and no lone-pair electrons. What is its formal charge using the equivalent formula based on lone-pair electrons and number of bonds?

A
+2
B
0
C
−2
D
+4

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1 point

A proposed Lewis structure is for a polyatomic ion with an overall charge of $-1$ . Which check must its atom-by-atom formal charges pass?

A
The sum must be zero for every species.
B
The sum must equal the number of bonds in the structure.
C
The sum must equal the overall charge of the molecule or ion.
D
The sum must equal the number of lone pairs.

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1 point

Two Lewis structures are plausible and have the same total electron count. Which principle best supports choosing one contributor as more favorable?

A
Favor structures that satisfy second-period octets, keep formal-charge magnitudes small, and place negative charge on more electronegative atoms when other factors are similar.
B
Favor structures with the greatest possible formal-charge magnitudes, regardless of where the charges occur.
C
Always place negative formal charge on the least electronegative atom.
D
Ignore formal charges if the total electron count is correct.

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1 point

Which statement correctly distinguishes the incomplete-octet examples $\mathrm{BF_3}$ and $\mathrm{BeH_2}$ ?

A
BF₃ and BeH₂ are both odd-electron species.
B
Boron in BF₃ has four shared electrons, while beryllium in BeH₂ has six.
C
Both molecules must be redrawn to force an octet on the central atom.
D
BF₃ is commonly shown with six shared electrons around boron, while BeH₂ is shown with four around beryllium.

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1 point

Why is nitric oxide, $\mathrm{NO}$ , classified as an odd-electron radical in Lewis electron counting?

A
NO has an even electron count, so it must be diamagnetic.
B
NO has 11 valence electrons, so one electron remains unpaired.
C
NO has 10 valence electrons, so it has a complete octet on every atom.
D
NO has 12 valence electrons because the nitrogen and oxygen counts are both doubled.

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1 point

A student rejects the familiar Lewis representations of $\mathrm{PCl_5}$ and $\mathrm{SF_6}$ solely because the central atoms have more than eight electrons. What is the best correction to the student's reasoning?

A
They are familiar hypervalent Lewis representations; the octet model is not universal, so the electron count should not be changed merely to force an octet.
B
They prove that the total valence-electron count should be reduced until the central atom has eight electrons.
C
They are invalid because every atom in every Lewis structure must have exactly eight electrons.
D
They are odd-electron radicals because having more than eight electrons requires one unpaired electron.

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1 point

Which set of conditions must two drawings satisfy to represent resonance forms of the same species?

A
They have different atom arrangements but the same placement of electrons.
B
They have the same atom arrangement but different total electron counts.
C
They have the same atom arrangement, total electron count, and overall charge, but differ in electron placement.
D
They have the same electron placement but different overall charges.

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1 point

For nitrite, $\mathrm{NO_2^-}$ , the two resonance contributors are equivalent and differ in which N–O bond is double. Which interpretation best describes the real species?

A
One N–O bond is permanently single and the other permanently double, with the molecule switching places over time.
B
The two contributors represent separate nitrite ions that rapidly collide.
C
Both N–O bonds are pure single bonds because resonance removes all π bonding.
D
The two N–O bonds in the hybrid are equivalent, with bonding intermediate between single and double; the species is not rapidly switching between contributors.

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1 point

For neutral $\mathrm{CO_2}$ , which bonding arrangement gives a Lewis structure that uses 16 valence electrons, gives every atom an octet, and has zero formal charge on every atom?

A
$\mathrm{O=C=O}$
B
$\mathrm{O-C-O}$ , with only single bonds
C
$\mathrm{O\equiv C-O}$ , with a triple bond to one oxygen and a single bond to the other
D
$\mathrm{O=C-O}$ , with one double bond and one single bond

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1 point

How many available valence electrons must be included when drawing $\mathrm{NO_2^-}$ ?

A
17
B
18
C
19
D
20

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1 point

Nitrogen contributes 5 valence electrons and oxygen contributes 6. Which conclusion follows for a Lewis structure of $\mathrm{NO}$ ?

A
It has 10 valence electrons and no unpaired electron.
B
It has 12 valence electrons and two unpaired electrons.
C
It has 11 valence electrons and one unpaired electron.
D
It has 11 valence electrons, but all can be paired in a Lewis structure.

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1 point

In the common Lewis structure of $\mathrm{BeH_2}$ , each H is terminal and forms one single bond to Be. How should the electron count around Be be interpreted?

A
Be has eight electrons, so $\mathrm{BeH_2}$ is an ordinary octet structure.
B
Be has four shared electrons, illustrating an incomplete-octet exception.
C
Be has more than eight electrons, illustrating a hypervalent structure.
D
Be has two electrons, because each hydrogen contributes a duet to the central atom.

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1 point

A polyatomic ion has an overall charge of $-2$ . What must the sum of the formal charges on all its atoms equal in any valid Lewis structure?

A
The formal charges must sum to zero because the structure is a Lewis diagram.
B
The formal charges must sum to $+2$ , the opposite of the ion’s charge.
C
The formal charges must sum to $-2$ , the ion’s overall charge.
D
The formal charges must each equal $-2$ .

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1 point

Starting from either nitrite contributor, which change in electron placement can produce the other equivalent resonance contributor while preserving the atom positions, total electron count, and overall charge?

A
Move a lone pair on the single-bonded oxygen to form an N=O π bond, and move the existing N=O π pair onto the other oxygen.
B
Exchange the positions of the two oxygen atoms while leaving every electron pair unchanged.
C
Move one electron from outside the structure onto nitrogen to make the two N–O bonds equivalent.
D
Remove the lone pair on the single-bonded oxygen without changing any bonds.

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1 point

Two equivalent Lewis contributors for a species differ only in electron placement. Which statement best describes the actual species?

A
The species rapidly alternates between separate structures, each with its own localized electrons.
B
Only one contributor exists at a time, and the resonance arrow shows the rate of switching.
C
The atoms move between contributor arrangements, while the electron positions remain fixed.
D
The contributors represent one delocalized species whose bonds can have properties intermediate between single and double bonds.

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1 point

Two plausible Lewis contributors for the same species satisfy octets for all second-period atoms and have equally small formal-charge magnitudes. Candidate A places the negative formal charge on a less electronegative atom; candidate B places it on a more electronegative atom. When the other factors are similar, which candidate is favored?

A
Favor A, because a negative formal charge is more favorable on a less electronegative atom.
B
Favor B, because it places the negative formal charge on the more electronegative atom.
C
Favor A, because any formal-charge separation is preferred over having a negative charge on an electronegative atom.
D
Neither can be favored: electronegativity is never relevant when comparing Lewis structures.