Draw the best Lewis dot structure for each of the following substances (including any resonance structures): a. I3-1 b) CO32- c) N2F2
The Correct Answer and Explanation is:
Here are the best Lewis structures for each of the given substances, including resonance structures where applicable:
a) I₃⁻ (Triiodide ion)
Best Lewis structure:
I – I – I
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:
⁻
- The central iodine atom has three lone pairs.
- The terminal iodine atoms have three lone pairs each.
- One of the terminal iodines bears a negative formal charge.
- The molecule is linear.
This structure shows one single bond on either side of the central iodine. There are no resonance structures because all iodines are large and capable of holding lone pairs, but the structure already distributes the charge optimally.
b) CO₃²⁻ (Carbonate ion)
Best Lewis structure with resonance:
Resonance structures (3 total):
Structure 1:
O⁻ O O
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C – C – C
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O O⁻ O⁻
Each resonance structure has:
- One C=O double bond.
- Two C–O⁻ single bonds.
- Each oxygen with single bond has three lone pairs and a formal charge of −1.
- The actual structure is a resonance hybrid with all three C–O bonds equivalent in length and strength.
c) N₂F₂ (Dinitrogen difluoride)
Best Lewis structure:
F – N = N – F
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: :
- The molecule has a planar or cis/trans configuration.
- The nitrogen atoms share a double bond.
- Each nitrogen has one fluorine attached via a single bond.
- All fluorines have three lone pairs.
- Each nitrogen has one lone pair.
There are no significant resonance structures, because moving electrons would break the N=N double bond or cause unstable formal charges.
Explanation (Approx. 300 Words):
Lewis structures represent how valence electrons are distributed in a molecule. The structures help predict shape, polarity, and reactivity.
In I₃⁻, three iodine atoms are involved. Iodine can expand its octet. The central iodine forms single bonds with the two terminal iodines. A lone pair on the central iodine allows the structure to maintain a linear shape. The extra electron is placed on one terminal iodine, giving the ion its −1 charge.
CO₃²⁻ displays resonance. Carbon forms three bonds, one double bond with an oxygen and two single bonds with negatively charged oxygen atoms. Because the position of the double bond can shift among the three oxygens, three resonance structures are drawn. These structures depict equal bond lengths between carbon and oxygen due to electron delocalization.
In N₂F₂, the nitrogen atoms are double bonded. Each nitrogen is also bonded to a fluorine. Fluorine can only form one bond, and it carries three lone pairs. Each nitrogen has a lone pair to complete its octet. The molecule shows geometric isomerism — existing in cis and trans forms — but not resonance, since the bond arrangement is fixed and stable.
These structures ensure formal charges are minimized, octets are fulfilled, and overall charges are correct. This method allows accurate predictions of molecular properties.
