Free Printable Lewis Dot Structure Worksheets - Free Printable

Let’s go step by step for each molecule. We’ll figure out:

1. Lewis Structure – how atoms are connected and where the electrons (dots) go.
2. Molecular Structure – what shape the molecule actually has in 3D space.
3. Molecular Geometry – the name of that shape.

We’ll use simple rules:
- Count total valence electrons.
- Put least electronegative atom in center (except H).
- Connect with single bonds first.
- Fill octets (8 electrons around each atom, except H which wants 2).
- If needed, make double or triple bonds to satisfy octets.
- Then look at electron groups around central atom to find geometry.

---

1. H₂O (Water)

Valence electrons:
- H: 1 × 2 = 2
- O: 6 → Total = 8

Lewis Structure:
- O in center, bonded to two H atoms.
- Each bond uses 2 electrons → 4 used.
- Remaining 4 electrons go on O as two lone pairs.
→ So: H–O–H with two lone pairs on O.

Electron groups around O:
- 2 bonding pairs + 2 lone pairs = 4 groups → tetrahedral electron geometry.
- But molecular geometry only cares about atom positions → bent (or angular).

✔ Molecular Geometry: Bent

---

2. SO₃ (Sulfur Trioxide)

Valence electrons:
- S: 6
- O: 6 × 3 = 18 → Total = 24

Lewis Structure:
- S in center, bonded to three O atoms.
- Try single bonds first: 3 bonds × 2 = 6 electrons used.
- Remaining 18 electrons → give each O 6 more (to complete octet): 3 O × 6 = 18 → perfect!
But wait — sulfur can have expanded octet. Actually, better structure is with double bonds.

Better version:
- Make one double bond and two single bonds? No — resonance makes all bonds equal.
Actually, best Lewis structure has S double-bonded to all three O atoms (with no lone pairs on S), and each O has two lone pairs.

Total electrons:
- 3 double bonds = 6 bonds × 2 = 12 electrons
- Each O has 2 lone pairs → 3 O × 4 electrons = 12 → total 24 ✔️

Electron groups around S:
- 3 bonding domains (all double bonds count as 1 group each) → trigonal planar.

✔ Molecular Geometry: Trigonal Planar

*(Note: Some might draw it with one double bond and two single bonds with formal charges — but the most stable/resonance hybrid is symmetric with 3 double bonds. Either way, geometry is same.)*

---

3. HCN (Hydrogen Cyanide)

Valence electrons:
- H: 1
- C: 4
- N: 5 → Total = 10

Lewis Structure:
- H must be on end. C in middle? Or N? Usually C is central.
Try: H–C≡N
- Single bond H–C: 2 electrons
- Triple bond C≡N: 6 electrons → total 8 used
- Remaining 2 electrons → put on N as a lone pair.
Check octets:
- H: 2 ✔️
- C: 4 bonds = 8 electrons ✔️
- N: triple bond (6) + lone pair (2) = 8 ✔️

Perfect.

Electron groups around C:
- Two groups: one single bond (to H), one triple bond (to N) → linear arrangement.

✔ Molecular Geometry: Linear

---

4. SiCl₄ (Silicon Tetrachloride)

Valence electrons:
- Si: 4
- Cl: 7 × 4 = 28 → Total = 32

Lewis Structure:
- Si in center, bonded to four Cl atoms.
- Four single bonds → 8 electrons used.
- Remaining 24 electrons → give each Cl 6 more (3 lone pairs per Cl) → 4 Cl × 6 = 24 ✔️

Each Cl has 3 lone pairs, Si has no lone pairs.

Electron groups around Si:
- 4 bonding pairs → tetrahedral.

✔ Molecular Geometry: Tetrahedral

---

5. CO₃²⁻ (Carbonate Ion)

Valence electrons:
- C: 4
- O: 6 × 3 = 18
- Plus 2 extra because of 2– charge → Total = 4 + 18 + 2 = 24

Lewis Structure:
- C in center, bonded to three O atoms.
- Try single bonds: 3 bonds × 2 = 6 electrons used.
- Remaining 18 electrons → give each O 6 more → 3 × 6 = 18 → total 24.
But now C only has 6 electrons → not enough.

So we need a double bond.

Best structure: One double bond between C and one O, and two single bonds to other O atoms. The single-bonded O atoms get negative charges (each has 3 lone pairs). Double-bonded O has 2 lone pairs.

Total electrons:
- Bonds: 1 double (4e) + 2 single (4e) = 8e
- Lone pairs:
- Double-bonded O: 2 pairs = 4e
- Two single-bonded O: each 3 pairs = 6e × 2 = 12e
→ Total lone pairs electrons: 4 + 12 = 16e
- Total: 8 + 16 = 24 ✔️

Also, resonance — the double bond rotates among the three O atoms.

Electron groups around C:
- 3 bonding domains (one double bond counts as 1 group, two single bonds) → trigonal planar.

✔ Molecular Geometry: Trigonal Planar

---

Final Answer:

| Molecular Formula | Lewis Structure | Molecular Structure | Molecular Geometry |
|-------------------|-------------------------------------|----------------------------|------------------------|
| H₂O | H–O–H with two lone pairs on O | Bent | Bent |
| SO₃ | S double-bonded to three O atoms | Flat triangle | Trigonal Planar |
| HCN | H–C≡N with lone pair on N | Straight line | Linear |
| SiCl₄ | Si bonded to four Cl atoms | Pyramid-like (tetrahedron) | Tetrahedral |
| CO₃²⁻ | C with one double bond and two single bonds to O (resonance) | Flat triangle | Trigonal Planar |