Period 3 Elements and Their Oxides
Period 3 Elements and Their Oxides
Period 3 runs from sodium (Na) to argon (Ar). The elements show clear trends in their physical properties, and their oxides reveal the transition from ionic to covalent bonding across the period.
Trends in Properties of Period 3 Elements
Atomic radius decreases across the period:
- Each element has one more proton and one more electron in the same shell
- Shielding remains roughly constant, but nuclear charge increases
- Electrons are pulled closer to the nucleus
First ionisation energy generally increases (with the two dips at Al and S discussed in the Ionisation Energies note).
Electronegativity increases across the period as nuclear charge increases.
Electrical conductivity:
- Na, Mg, Al are metallic conductors (delocalised electrons)
- Si is a semiconductor (small band gap)
- P, S, Cl, Ar are non-conductors (covalent molecular or monatomic)
Melting and boiling points:
- Na, Mg, Al: increase (metallic bonding strengthens — more delocalised electrons and higher charge density: Na has 1, Mg has 2, Al has 3 delocalised electrons per atom, and ionic radius decreases)
- Si: very high (giant covalent structure, strong Si–Si covalent bonds throughout)
- P₄, S₈: low (simple molecular, only weak London forces between molecules; S₈ higher than P₄ because it has more electrons)
- Cl₂, Ar: very low (small simple molecules / monatomic; very weak London forces)
Period 3 Oxides — Structures and Bonding
| Oxide | Formula | Bonding | Structure | Melting point |
|---|---|---|---|---|
| Sodium oxide | Na₂O | Ionic | Giant ionic lattice | High |
| Magnesium oxide | MgO | Ionic (strong) | Giant ionic lattice | Very high |
| Aluminium oxide | Al₂O₃ | Ionic with covalent character | Giant ionic lattice | Very high |
| Silicon dioxide | SiO₂ | Covalent | Giant covalent (macromolecular) | Very high |
| Phosphorus(V) oxide | P₄O₁₀ | Covalent | Simple molecular | Low |
| Sulfur trioxide | SO₃ | Covalent | Simple molecular | Low |
| Dichlorine monoxide / Chlorine(I) oxide | Cl₂O | Covalent | Simple molecular | Very low |
Key pattern: Na₂O and MgO are ionic with high melting points. MgO has a higher melting point than Na₂O because Mg²⁺ and O²⁻ have higher charges than Na⁺ and O²⁻, and Mg²⁺ is smaller. Al₂O₃ is ionic but with significant covalent character (Al³⁺ is small and highly charged, so it polarises the O²⁻ ion). SiO₂ is giant covalent. P₄O₁₀, SO₃, and Cl₂O are simple molecular.
Reactions of Period 3 Oxides with Water
The oxides show a clear trend from basic (left) through amphoteric (Al₂O₃) to acidic (right):
Na₂O — reacts vigorously, forming a strongly alkaline solution:
Na₂O(s) + H₂O(l) → 2NaOH(aq) pH ≈ 13–14
MgO — reacts slowly (low solubility), forming a weakly alkaline solution:
MgO(s) + H₂O(l) → Mg(OH)₂(aq) pH ≈ 9–10
Al₂O₃ — amphoteric; does not dissolve in water but reacts with both acids and bases:
- With acid: Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)
- With base: Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄(aq)
SiO₂ — weakly acidic; does not react with water but reacts with hot concentrated NaOH:
SiO₂(s) + 2NaOH(aq) → Na₂SiO₃(aq) + H₂O(l)
P₄O₁₀ — reacts vigorously, forming phosphoric acid (strongly acidic):
P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq) pH ≈ 1–2
SO₃ — reacts vigorously, forming sulfuric acid:
SO₃(g) + H₂O(l) → H₂SO₄(aq) pH ≈ 0–1
SO₂ — dissolves to form sulfurous acid (weakly acidic):
SO₂(g) + H₂O(l) → H₂SO₃(aq) pH ≈ 2–3
Why the Acid-Base Trend Exists
- Metal oxides are ionic. The oxide ion (O²⁻) is a strong base — it accepts H⁺ from water to form OH⁻ ions
- Non-metal oxides are covalent. They react with water by forming oxoacids where the non-metal atom is the central atom, bonded to OH groups and additional O atoms; these release H⁺ ions
- The higher the oxidation state of the non-metal, the more acidic the oxide (e.g. SO₃/+6 is more acidic than SO₂/+4)
Reactions with Acids and Bases
| Oxide | With HCl | With NaOH |
|---|---|---|
| Na₂O | Reacts (basic) | No reaction |
| MgO | Reacts (basic) | No reaction |
| Al₂O₃ | Reacts | Reacts (amphoteric) |
| SiO₂ | No reaction | Reacts (acidic, needs hot conc.) |
| P₄O₁₀ | No reaction | Reacts (acidic) |
| SO₃ | No reaction | Reacts (acidic) |
Environmental Link
Sulfur dioxide and nitrogen oxides from burning fossil fuels dissolve in rainwater to form acid rain (H₂SO₃ / HNO₃). This corrodes limestone buildings (CaCO₃ reacts with acid) and acidifies lakes, killing aquatic life.
Exam Tips
- Learn the equations for oxide + water reactions — they are frequently examined
- When explaining melting point trends, always link structure and bonding to the strength of forces that must be overcome
- For amphoteric Al₂O₃, write balanced equations for reactions with BOTH acid and base