Define Lewis diagram .

Lewis diagram is a representation of a molecular compound that shows all valence electrons around atoms as bonding pairs (shared) or nonbonding lone pairs .

The octet rule states that atoms are most stable when they have .......... electrons in their valence shell.

The octet rule states that atoms are most stable when they have eight electrons in their valence shell.

True or False? Hydrogen follows the octet rule and requires 8 electrons to complete its valence shell.

False. Hydrogen completes its valence shell with only 2 electrons (a duet), not 8, because it has only one electron shell.

Why must Lewis diagrams show both bonding pairs and lone pairs?

Lewis diagrams must show all electron pairs — bonding and nonbonding — because both affect molecular shape , polarity and reactivity. Omitting lone pairs gives an incomplete and potentially incorrect picture of the molecule.

In a Lewis diagram, a double bond is represented by .......... lines connecting the two bonded atoms, indicating that 4 electrons are shared.

In a Lewis diagram, a double bond is represented by two lines connecting the two bonded atoms, indicating that 4 electrons are shared.

True or False? Boron completes a full octet of 8 electrons in its Lewis structure.

False. Boron completes a sextet — only 6 electrons in its valence shell — because it has only three valence electrons available for bonding.

What is the first step in drawing a Lewis diagram for any molecule?

Calculate the total number of valence electrons in the molecule using the periodic table, adding one electron for each negative charge and removing one for each positive charge on an ion.

Define resonance structures .

Resonance structures are two or more valid Lewis structures for the same molecule that differ only in the arrangement of electrons, not in the arrangement of atoms.

Why is the resonance hybrid more stable than any individual resonance structure?

The resonance hybrid is more stable because electrons are delocalized across the molecule rather than fixed to one bond, spreading electron density and lowering the overall energy of the structure.

In a resonance hybrid, delocalized electrons are shown using .......... lines to indicate they are spread across the molecule.

In a resonance hybrid, delocalized electrons are shown using dotted lines to indicate they are spread across the molecule.

True or False? The nitrate ion (NO 3 - ) has three equivalent resonance structures.

True. The nitrate ion has three resonance structures, each with one N=O double bond and two N–O single bonds, where the double bond position rotates between the three oxygen atoms.

What causes resonance in molecules such as ozone and the carbonate ion?

Resonance arises because electrons in these molecules are delocalized — they are not fixed to a single bond but spread across multiple equivalent positions, so no single Lewis structure accurately represents the true electron distribution .

True or False? A molecule's actual structure is identical to one of its resonance structures.

False. The actual structure is the resonance hybrid — a blend of all resonance structures — not any single one of them. The hybrid has intermediate bond lengths and orders.

The resonance hybrid of benzene (C 6 H 6 ) shows .......... bond order for all C–C bonds, between a single and a double bond.

The resonance hybrid of benzene (C 6 H 6 ) shows intermediate bond order for all C–C bonds, between a single and a double bond.

Define formal charge .

Formal charge is a charge assigned to an atom in a molecule by assuming all bonding electrons are shared equally between atoms, regardless of electronegativity differences.

The formula for formal charge is: FC = V − ½B − N, where V = valence electrons, B = .......... electrons, and N = nonbonding electrons.

The formula for formal charge is: FC = V − ½B − N, where V = valence electrons, B = bonding electrons, and N = nonbonding electrons.

Why is a resonance structure with formal charges of zero on all atoms preferred over one with non-zero formal charges?

A structure with zero formal charges on all atoms more accurately represents the true electron distribution , as large formal charges are energetically unfavorable and indicate a less realistic electron arrangement.

True or False? In a neutral molecule, the sum of all formal charges must equal zero.

True. For a neutral molecule, all formal charges must sum to zero. For an ion, the formal charges must sum to the overall charge of the ion.

When comparing two resonance structures, why should a negative formal charge be placed on the most electronegative atom?

A negative formal charge on the most electronegative atom is preferred because electronegative atoms have a greater ability to hold extra electron density , making the structure more stable and energetically realistic.

For the BH 4 - ion, boron has 3 valence electrons, 8 bonding electrons, and 0 nonbonding electrons, giving a formal charge of .......... .

For the BH 4 - ion, boron has 3 valence electrons, 8 bonding electrons, and 0 nonbonding electrons, giving a formal charge of −1 . FC = V − ½B − N = 3 − ½(8) − 0 = 3 − 4 = −1

True or False? Large formal charges on atoms in a Lewis structure indicate that the structure is a stable and accurate representation.

False. Large formal charges are energetically unfavorable, so structures with smaller or zero formal charges are preferred as they better represent the actual electron distribution.

Define free radical .

Free radical is a molecule or atom with an odd number of electrons, leaving one electron unpaired , which makes it highly reactive and unstable.

Why do molecules with an odd number of electrons pose a limitation for Lewis diagrams?

Lewis diagrams are built on the principle of electron pairs , so an odd number of electrons means one electron must remain unpaired. This violates the standard pairing assumption and cannot be accurately represented using conventional Lewis notation .

True or False? Expanded octets can only occur in elements from the second period (e.g., C, N, O).

False. Expanded octets occur in third-period elements and below (such as S and P), because only these elements have empty d-orbitals available to accommodate extra electrons beyond eight.

In PCl 5 , the central phosphorus atom holds .......... electrons in its valence shell, exceeding the normal octet.

In PCl 5 , the central phosphorus atom holds ten electrons in its valence shell, exceeding the normal octet.

What structural condition tends to favour the formation of an expanded octet?

Expanded octets are favoured when a non-metal central atom from the third period or below is surrounded by terminal atoms with high electronegativity values , such as F, Cl or O, which pull electron density away and stabilize the extra bonding.

True or False? SF 6 is an example of a molecule with an expanded octet in which sulfur holds 12 electrons.

True. In SF 6 , sulfur forms six bonds with six fluorine atoms, giving it 12 electrons in its valence shell — an expanded octet made possible by available d-orbitals in the third period.

AP®ChemistryCollege BoardFlashcardsUnit 2: Compound Structure & PropertiesLewis Diagrams

Lewis Diagrams (College Board AP® Chemistry): Flashcards

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Cards in this collection (27)

Define Lewis diagram.
Lewis diagram is a representation of a molecular compound that shows all valence electrons around atoms as bonding pairs (shared) or nonbonding lone pairs.
The octet rule states that atoms are most stable when they have .......... electrons in their valence shell.
The octet rule states that atoms are most stable when they have eight electrons in their valence shell.
True or False?
Hydrogen follows the octet rule and requires 8 electrons to complete its valence shell.
False.
Hydrogen completes its valence shell with only 2 electrons (a duet), not 8, because it has only one electron shell.
Why must Lewis diagrams show both bonding pairs and lone pairs?
Lewis diagrams must show all electron pairs — bonding and nonbonding — because both affect molecular shape, polarity and reactivity. Omitting lone pairs gives an incomplete and potentially incorrect picture of the molecule.
In a Lewis diagram, a double bond is represented by .......... lines connecting the two bonded atoms, indicating that 4 electrons are shared.
In a Lewis diagram, a double bond is represented by two lines connecting the two bonded atoms, indicating that 4 electrons are shared.
True or False?
Boron completes a full octet of 8 electrons in its Lewis structure.
False.
Boron completes a sextet — only 6 electrons in its valence shell — because it has only three valence electrons available for bonding.
What is the first step in drawing a Lewis diagram for any molecule?
Calculate the total number of valence electrons in the molecule using the periodic table, adding one electron for each negative charge and removing one for each positive charge on an ion.
Define resonance structures.
Resonance structures are two or more valid Lewis structures for the same molecule that differ only in the arrangement of electrons, not in the arrangement of atoms.
Why is the resonance hybrid more stable than any individual resonance structure?
The resonance hybrid is more stable because electrons are delocalized across the molecule rather than fixed to one bond, spreading electron density and lowering the overall energy of the structure.
In a resonance hybrid, delocalized electrons are shown using .......... lines to indicate they are spread across the molecule.
In a resonance hybrid, delocalized electrons are shown using dotted lines to indicate they are spread across the molecule.
True or False?
The nitrate ion (NO₃^-) has three equivalent resonance structures.
True.
The nitrate ion has three resonance structures, each with one N=O double bond and two N–O single bonds, where the double bond position rotates between the three oxygen atoms.
What causes resonance in molecules such as ozone and the carbonate ion?
Resonance arises because electrons in these molecules are delocalized — they are not fixed to a single bond but spread across multiple equivalent positions, so no single Lewis structure accurately represents the true electron distribution.
True or False?
A molecule's actual structure is identical to one of its resonance structures.
False.
The actual structure is the resonance hybrid — a blend of all resonance structures — not any single one of them. The hybrid has intermediate bond lengths and orders.
The resonance hybrid of benzene (C₆H₆) shows .......... bond order for all C–C bonds, between a single and a double bond.
The resonance hybrid of benzene (C₆H₆) shows intermediate bond order for all C–C bonds, between a single and a double bond.
Define formal charge.
Formal charge is a charge assigned to an atom in a molecule by assuming all bonding electrons are shared equally between atoms, regardless of electronegativity differences.
The formula for formal charge is: FC = V − ½B − N, where V = valence electrons, B = .......... electrons, and N = nonbonding electrons.
The formula for formal charge is: FC = V − ½B − N, where V = valence electrons, B = bonding electrons, and N = nonbonding electrons.
Why is a resonance structure with formal charges of zero on all atoms preferred over one with non-zero formal charges?
A structure with zero formal charges on all atoms more accurately represents the true electron distribution, as large formal charges are energetically unfavorable and indicate a less realistic electron arrangement.
True or False?
In a neutral molecule, the sum of all formal charges must equal zero.
True.
For a neutral molecule, all formal charges must sum to zero. For an ion, the formal charges must sum to the overall charge of the ion.
When comparing two resonance structures, why should a negative formal charge be placed on the most electronegative atom?
A negative formal charge on the most electronegative atom is preferred because electronegative atoms have a greater ability to hold extra electron density, making the structure more stable and energetically realistic.
For the BH₄^- ion, boron has 3 valence electrons, 8 bonding electrons, and 0 nonbonding electrons, giving a formal charge of .......... .
For the BH₄^- ion, boron has 3 valence electrons, 8 bonding electrons, and 0 nonbonding electrons, giving a formal charge of −1.
FC = V − ½B − N = 3 − ½(8) − 0 = 3 − 4 = −1
True or False?
Large formal charges on atoms in a Lewis structure indicate that the structure is a stable and accurate representation.
False.
Large formal charges are energetically unfavorable, so structures with smaller or zero formal charges are preferred as they better represent the actual electron distribution.
Define free radical.
Free radical is a molecule or atom with an odd number of electrons, leaving one electron unpaired, which makes it highly reactive and unstable.
Why do molecules with an odd number of electrons pose a limitation for Lewis diagrams?
Lewis diagrams are built on the principle of electron pairs, so an odd number of electrons means one electron must remain unpaired. This violates the standard pairing assumption and cannot be accurately represented using conventional Lewis notation.
True or False?
Expanded octets can only occur in elements from the second period (e.g., C, N, O).
False.
Expanded octets occur in third-period elements and below (such as S and P), because only these elements have empty d-orbitals available to accommodate extra electrons beyond eight.
In PCl₅, the central phosphorus atom holds .......... electrons in its valence shell, exceeding the normal octet.
In PCl₅, the central phosphorus atom holds ten electrons in its valence shell, exceeding the normal octet.
What structural condition tends to favour the formation of an expanded octet?
Expanded octets are favoured when a non-metal central atom from the third period or below is surrounded by terminal atoms with high electronegativity values, such as F, Cl or O, which pull electron density away and stabilize the extra bonding.
True or False?
SF₆ is an example of a molecule with an expanded octet in which sulfur holds 12 electrons.
True.
In SF₆, sulfur forms six bonds with six fluorine atoms, giving it 12 electrons in its valence shell — an expanded octet made possible by available d-orbitals in the third period.

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