Practice worksheet for drawing Lewis structures of common chemical compounds.

Let's go through each compound and draw the Lewis structures step by step. A Lewis structure shows the valence electrons of atoms in a molecule, including bonding pairs and lone pairs.

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1) PBr₃ (Phosphorus tribromide)

- Phosphorus (P) is in group 15 → 5 valence electrons
- Each Bromine (Br) is in group 17 → 7 valence electrons
- Total valence electrons:
P: 5
3 × Br: 3 × 7 = 21
Total = 5 + 21 = 26 electrons

- Phosphorus is the central atom (less electronegative than Br).
- Form three single bonds between P and each Br → uses 3 × 2 = 6 electrons
- Remaining electrons: 26 - 6 = 20 electrons → place as lone pairs
- Each Br needs 3 lone pairs (6 electrons), so 3 Br × 6 = 18 electrons used
- Remaining: 20 - 18 = 2 electrons → go on P as one lone pair

✔ Structure:
```
Br
|
Br--P--Br
|
:
```
Each Br has 3 lone pairs (not shown here for brevity). P has one lone pair.

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2) NH₂⁻ (Amide ion)

- Nitrogen (N): group 15 → 5 valence electrons
- Each H: 1 electron → 2 × 1 = 2
- Add 1 electron for the negative charge → total = 5 + 2 + 1 = 8 electrons

- Nitrogen is central.
- Two N–H bonds → 2 × 2 = 4 electrons used
- Remaining: 8 - 4 = 4 electrons → two lone pairs on N

✔ Structure:
```
H
|
:N:⁻
|
H
```
Nitrogen has two lone pairs and a negative charge.

---

3) CH₃OH (Methanol)

- C: 4 valence e⁻
- H: 1 each → 4 H’s = 4 e⁻
- O: 6 valence e⁻
- Total: 4 + 4 + 6 = 14 electrons

- Carbon is central; bonded to 3 H and 1 O
- Oxygen bonded to C and 1 H

Structure:
- C–O single bond
- O–H single bond
- C–H × 3

Electron count:
- Bonds: 4 single bonds → 4 × 2 = 8 electrons
- Remaining: 14 - 8 = 6 electrons → 3 lone pairs on oxygen

✔ Structure:
```
H
|
H--C--O--H
|
H
```
With two lone pairs on oxygen (since O has 6 electrons: 2 bonds = 4 electrons, need 2 more lone pairs → 4 electrons, total 8)

Wait — correction: oxygen has two lone pairs (4 electrons) + 2 bonds = 8 electrons.

So:
```
H
|
H--C--O--H
| ..
H
```
The two dots represent lone pairs on oxygen.

---

4) NO₂⁻ (Nitrite ion)

- N: 5 valence e⁻
- O: 6 × 2 = 12
- Add 1 electron for charge → total = 5 + 12 + 1 = 18 electrons

- Nitrogen is central (bonded to two O atoms)
- Possible resonance structures

We can have:
- One N=O double bond
- One N–O⁻ single bond with extra electron on O

But both O atoms are equivalent via resonance.

Resonance structures:

Structure A:
```
O
║
O⁻--N
```
But better to show formal charges.

Better way:
- One N=O double bond
- One N–O⁻ single bond
- Negative charge on single-bonded O
- Lone pairs:
- Double-bonded O: 2 lone pairs (4 e⁻)
- Single-bonded O: 3 lone pairs (6 e⁻)
- N: 1 lone pair (2 e⁻)

Check electrons:
- Bonds: 1 double bond (4 e⁻) + 1 single bond (2 e⁻) = 6 e⁻
- Lone pairs:
- Double-bonded O: 4 e⁻
- Single-bonded O: 6 e⁻
- N: 2 e⁻
- Total: 6 + 4 + 6 + 2 = 18 ✔️

Resonance:
Two structures where the double bond switches between the two O atoms.

✔ Final Lewis Structure (resonance hybrid):
```
O
║
O⁻--N ↔ O--N=O
│ │
.. ..
```

With formal charges:
- In each resonance form:
- Double-bonded O: 0 charge
- Single-bonded O: -1
- N: +1
- Overall charge: -1

---

5) C₂H₄ (Ethene)

- C: 4 × 2 = 8
- H: 1 × 4 = 4
- Total: 12 valence electrons

- Two carbon atoms double bonded to each other
- Each C bonded to 2 H atoms

Structure:
- C=C double bond
- Each C has two single bonds to H

Electrons:
- Double bond: 4 electrons
- Four C–H bonds: 4 × 2 = 8 electrons
- Total: 4 + 8 = 12 ✔️

No lone pairs.

✔ Structure:
```
H H
| |
C=C
| |
H H
```

Or more clearly:
```
H₂C=CH₂
```

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✔ Summary of Lewis Structures:

1. PBr₃:
Central P bonded to 3 Br atoms. P has one lone pair. Each Br has three lone pairs.

2. NH₂⁻:
N bonded to two H atoms. N has two lone pairs and a negative charge.

3. CH₃OH:
C bonded to 3 H and 1 O; O bonded to H. O has two lone pairs.

4. NO₂⁻:
Resonance hybrid: N double bonded to one O, single bonded to another O⁻. Lone pairs on both O atoms. Formal charges: N⁺, O⁻.

5. C₂H₄:
C=C double bond, each C bonded to two H atoms. No lone pairs.

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Let me know if you'd like visual diagrams or help drawing them!