Solved Lewis Structures and Molecular Geometry Practice | Chegg.com - Free Printable

Let's solve each molecule step by step using Lewis structures, VSEPR theory, and determine the electron geometry and molecular geometry.

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🔹 1. CH₄ (Methane)

#### Step 1: Lewis Structure
- Carbon (C) is the central atom.
- C has 4 valence electrons, each H has 1.
- Total valence electrons:
$ 4 + 4(1) = 8 $
- C forms single bonds with 4 H atoms → 4 single bonds (uses all 8 electrons).
- No lone pairs on carbon.

```
H
|
H--C--H
|
H
```

#### Step 2: Fill in the table:
- Number of bonds: 4 (each bond is a single bond)
- Lone pairs on central atom: 0
- Total electron groups: 4 (bonds only)
- Electron geometry: Tetrahedral (4 electron groups)
- Molecular geometry: Tetrahedral (no lone pairs)

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🔹 2. CO₂ (Carbon dioxide)

#### Step 1: Lewis Structure
- Central atom: C
- O has 6 valence electrons, C has 4.
- Total valence electrons:
$ 4 + 2(6) = 16 $
- To satisfy octets: C double-bonded to each O: O=C=O
- Each double bond uses 4 electrons → two double bonds = 8 electrons used.
- Remaining 8 electrons go as lone pairs on O atoms (2 lone pairs per O).

```
O = C = O
```
Each O has two lone pairs.

#### Step 2: Fill in the table:
- Number of bonds: 2 (two double bonds → count as 2 bonding groups)
- Lone pairs on central atom: 0
- Total electron groups: 2 (both are bonding groups)
- Electron geometry: Linear
- Molecular geometry: Linear

> Note: Double bonds count as one electron group in VSEPR.

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🔹 3. SCl₂ (Sulfur dichloride)

#### Step 1: Lewis Structure
- Central atom: S (sulfur)
- S has 6 valence electrons, each Cl has 7.
- Total valence electrons:
$ 6 + 2(7) = 20 $
- S bonded to two Cl atoms with single bonds → 2 bonds = 4 electrons used.
- Remaining 16 electrons → 3 lone pairs on each Cl (3 × 2 = 6 lone pairs = 12 electrons), and 4 electrons left → 2 lone pairs on S.
- So: S has 2 bonds and 2 lone pairs.

```
Cl
\
S
/
Cl
```
With two lone pairs on S.

#### Step 2: Fill in the table:
- Number of bonds: 2
- Lone pairs on central atom: 2
- Total electron groups: 4 (2 bonds + 2 lone pairs)
- Electron geometry: Tetrahedral
- Molecular geometry: Bent (or angular) — because 2 lone pairs push the bonding pairs closer.

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🔹 4. CCl₄ (Carbon tetrachloride)

#### Step 1: Lewis Structure
- Central atom: C
- C has 4 valence electrons, each Cl has 7.
- Total valence electrons:
$ 4 + 4(7) = 32 $
- C forms single bonds with 4 Cl atoms → 4 single bonds (8 electrons used).
- Remaining 24 electrons → 3 lone pairs on each Cl (6 lone pairs per Cl → 3 × 4 = 12 lone pairs = 24 electrons).
- No lone pairs on C.

```
Cl
|
Cl--C--Cl
|
Cl
```

#### Step 2: Fill in the table:
- Number of bonds: 4
- Lone pairs on central atom: 0
- Total electron groups: 4
- Electron geometry: Tetrahedral
- Molecular geometry: Tetrahedral

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✔ Final Answer Table:

| Molecular formula | Lewis structure | Number of bonds | Lone pairs on central atom | Total electron groups | Electron geometry | Molecular geometry |
|-------------------|------------------|------------------|-------------------------------|------------------------|--------------------|----------------------|
| CH₄ | H–C–H (tetrahedral) | 4 | 0 | 4 | Tetrahedral | Tetrahedral |
| CO₂ | O=C=O | 2 | 0 | 2 | Linear | Linear |
| SCl₂ | Cl–S–Cl (with 2 lone pairs on S) | 2 | 2 | 4 | Tetrahedral | Bent |
| CCl₄ | Cl–C–Cl (tetrahedral) | 4 | 0 | 4 | Tetrahedral | Tetrahedral |

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📝 Summary of Key Concepts:

- Electron geometry depends on total number of electron groups (bonds + lone pairs).
- Molecular geometry depends on arrangement of atoms only (ignoring lone pairs).
- Double/triple bonds count as one electron group.
- Common geometries:
- 2 groups → Linear
- 3 groups → Trigonal planar
- 4 groups → Tetrahedral
- 5 groups → Trigonal bipyramidal
- 6 groups → Octahedral

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