Lewis Structure Worksheet | Lecture notes Geometry | Docsity - Free Printable

The task involves drawing Lewis structures for a variety of molecules and ions, including resonance structures where applicable. Additionally, formal charges should be used to determine the most preferred Lewis structure when multiple options exist. Below, I will outline the general steps for solving these problems and provide examples for a few molecules/ions.

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General Steps for Drawing Lewis Structures

1. Calculate the Total Number of Valence Electrons:
- Sum the valence electrons from all atoms.
- Add extra electrons for negative charges or subtract electrons for positive charges.

2. Determine the Central Atom:
- The central atom is typically the least electronegative atom (except for hydrogen, which is never central).
- For polyatomic ions, the central atom is often the one that can accommodate more than an octet.

3. Draw a Skeletal Structure:
- Connect atoms with single bonds to form a skeleton.
- Ensure each atom has at least one bond unless it is a noble gas.

4. Distribute Remaining Electrons:
- Place lone pairs on terminal atoms first to satisfy the octet rule.
- If the central atom does not have an octet, use multiple bonds (double or triple) to complete its octet.

5. Check Formal Charges:
- Calculate formal charges for each atom:
\[
\text{Formal Charge} = \text{Valence Electrons} - \text{Non-bonding Electrons} - \frac{\text{Bonding Electrons}}{2}
\]
- The best Lewis structure minimizes formal charges and keeps them as close to zero as possible.

6. Identify Resonance Structures:
- Draw alternative Lewis structures if electrons can be rearranged while maintaining the same connectivity.
- Use formal charges to determine the most stable resonance structure.

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Examples of Lewis Structures

#### Example 1: CH₄ (Methane)
1. Total Valence Electrons:
\( C \): 4 valence electrons
\( H \): 1 valence electron × 4 = 4 valence electrons
Total = 4 + 4 = 8

2. Skeletal Structure:
Carbon is the central atom. Bond each hydrogen to carbon with a single bond.

3. Final Structure:
Each hydrogen satisfies its duet, and carbon satisfies its octet. No lone pairs are needed.
\[
\ce{H-C-H}
\]
(All hydrogens are bonded to carbon in a tetrahedral arrangement.)

#### Example 2: BF₃ (Boron Trifluoride)
1. Total Valence Electrons:
\( B \): 3 valence electrons
\( F \): 7 valence electrons × 3 = 21 valence electrons
Total = 3 + 21 = 24

2. Skeletal Structure:
Boron is the central atom. Bond each fluorine to boron with a single bond.

3. Final Structure:
Each fluorine satisfies its octet, but boron only has 6 electrons. This is acceptable for boron, as it can have an incomplete octet.
\[
\ce{F-B-F}
\]
(Fluorines are bonded to boron in a trigonal planar arrangement.)

#### Example 3: N₃⁻ (Azide Ion)
1. Total Valence Electrons:
\( N \): 5 valence electrons × 3 = 15 valence electrons
Extra electron for the negative charge = 1
Total = 15 + 1 = 16

2. Skeletal Structure:
Nitrogen atoms can form chains. Try different arrangements and calculate formal charges.

3. Resonance Structures:
There are three equivalent resonance structures for N₃⁻.
\[
\ce{N=N#N-}, \quad \ce{-N#N=N-}, \quad \ce{-N=N#N}
\]
In each structure, the formal charges are minimized.

4. Preferred Structure:
All three structures are equally preferred due to their identical formal charges.

#### Example 4: H₂O (Water)
1. Total Valence Electrons:
\( O \): 6 valence electrons
\( H \): 1 valence electron × 2 = 2 valence electrons
Total = 6 + 2 = 8

2. Skeletal Structure:
Oxygen is the central atom. Bond each hydrogen to oxygen with a single bond.

3. Final Structure:
Oxygen has two lone pairs to satisfy its octet.
\[
\ce{H-O-H}
\]
(Hydrogens are bonded to oxygen in a bent arrangement.)

#### Example 5: NO₂⁻ (Nitrite Ion)
1. Total Valence Electrons:
\( N \): 5 valence electrons
\( O \): 6 valence electrons × 2 = 12 valence electrons
Extra electron for the negative charge = 1
Total = 5 + 12 + 1 = 18

2. Skeletal Structure:
Nitrogen is the central atom. Bond each oxygen to nitrogen with a single bond.

3. Resonance Structures:
Two resonance structures can be drawn:
\[
\ce{O=N-O^-}, \quad \ce{O^-=N-O}
\]
In both structures, formal charges are minimized.

4. Preferred Structure:
Both structures are equally preferred.

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Summary

For each molecule/ion:
1. Calculate total valence electrons.
2. Determine the central atom.
3. Draw a skeletal structure.
4. Distribute electrons to satisfy octets.
5. Check formal charges.
6. Identify resonance structures if applicable.

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Final Answer

\[
\boxed{\text{Follow the steps above to draw Lewis structures for all given molecules and ions.}}
\]