Properties of Period 3 Elements and Their Oxides
Quick Notes
- Sodium and magnesium react with water to form hydroxides and hydrogen gas.
- Period 3 elements react with oxygen to form oxides with different bonding and properties.
- Melting points of oxides vary due to their structure:
- Ionic oxides (Na2O, MgO, Al2O3) have high melting points.
- Giant covalent oxides (SiO2) have the highest melting points.
- Molecular oxides (P4O10, SO2, SO3) have low melting points.
- Oxides react with water:
- Ionic oxides form alkaline solutions.
- Covalent (molecular) oxides form acidic solutions.
- Al2O3 and SiO2 are insoluble.
- Acids formed when P4O10, SO2, and SO3 react with water include H3PO4, H2SO3, and H2SO4.
Full Notes
Period 3 elements include Sodium (Na), Magnesium (Mg), Aluminium (Al), Silicon (Si), Phosphorus (P), Sulfur (S), Chlorine (Cl) and Argon (Ar).
There are some key reactions, trends and properties of the period 3 elements you need to know.
Reactions of Sodium and Magnesium with Water
Sodium reacts vigorously with cold water, producing NaOH and hydrogen gas:
2Na + 2H2O → 2NaOH + H2
Magnesium reacts very slowly with cold water but reacts with steam to form MgO:
Mg + 2H2O (cold) → Mg(OH)2 + H2 (very slow)
Mg + H2O (steam) → MgO + H2
Sodium forms a strong alkaline solution (pH 13–14).
Magnesium hydroxide is only sparingly soluble, so it forms a weakly alkaline solution (pH 9–10).
Reactions of Period 3 Elements with Oxygen
- Metals (Na, Mg, Al) react with oxygen to form basic oxides.
- Non-metals (Si, P, S) react to form acidic oxides.
- Al2O3 has both ionic and covalent character, making it amphoteric.
| Element | Oxide formed | Typical equation | Nature of oxide | Acid/base character |
|---|---|---|---|---|
| Na | Na2O | 4Na + O2 → 2Na2O | Ionic | Basic |
| Mg | MgO | 2Mg + O2 → 2MgO | Ionic | Basic |
| Al | Al2O3 | 4Al + 3O2 → 2Al2O3 | Ionic/covalent | Amphoteric |
| Si | SiO2 | Si + O2 → SiO2 | Giant covalent | Acidic |
| P | P4O10 | P4 + 5O2 → P4O10 | Molecular | Acidic |
| S | SO2, SO3 | S + O2 → SO2; 2SO2 + O2 → 2SO3 | Molecular | Acidic |
Trends in Melting Points of Period 3 Oxides
| Oxide | Structure | Relative melting point |
|---|---|---|
| Na2O | Ionic lattice | High |
| MgO | Ionic lattice | Very high |
| Al2O3 | Ionic/covalent | Very high |
| SiO2 | Giant covalent | Highest (very high) |
| P4O10 | Simple molecular | Low |
| SO2, SO3 | Simple molecular | Low |
Explanation of the trend:
- Ionic oxides (Na2O, MgO, Al2O3) have high melting points due to strong electrostatic forces (ionic bonding) holding the solid lattice together.
- SiO2 has a very high melting point due to its giant covalent structure – lots of strong covalent bonds need to be broken to melt the structure, meaning high amounts of energy required.
- Molecular oxides (P4O10, SO2, SO3) have low melting points due to weak intermolecular forces between molecules.
Reactions of Period 3 Oxides with Water and pH of Solutions
| Oxide | Reaction with water | Resulting solution | Approx. pH |
|---|---|---|---|
| Na2O | Na2O + H2O → 2NaOH | Alkaline (basic oxide) | 13–14 |
| MgO | MgO + H2O ⇌ Mg(OH)2 (sparingly soluble) | Weakly alkaline | 9–10 |
| Al2O3 | Insoluble | — | — |
| SiO2 | Insoluble | — | — |
| P4O10 | P4O10 + 6H2O → 4H3PO4 | Acidic (strong acid solution) | ~1–2 |
| SO2 | SO2 + H2O ⇌ H2SO3 | Acidic | ~2–3 |
| SO3 | SO3 + H2O → H2SO4 | Acidic (strong acid solution) | ~0–1 |
- Na2O and MgO dissolve in water to form alkaline solutions; they are basic oxides.
- The ionic structure means O2− ions are released into solution when the oxide dissolves.
- O2− ions readily accept H+ ions from H2O molecules, forming OH− ions and making the solution basic.
- Al2O3 and SiO2 are insoluble in water.
- P4O10, SO2 and SO3 dissolve in water to form acidic solutions; they are acidic oxides.
- Because these oxides are covalently bonded, no O2− ions get released into solution, hence they don’t cause OH− ions to be released.
Structures of Acids and Anions Formed in Water
Phosphoric acid (H3PO4) from P4O10:
P4O10 + 6H2O → 4H3PO4
- The H3PO4 can be a source of phosphate anions (PO43−).
Sulfurous acid (H2SO3) from SO2:
SO2 + H2O → H2SO3
- The H2SO3 can be a source of sulfite anions (SO32−).
Sulfuric acid (H2SO4) from SO3:
SO3 + H2O → H2SO4
- The H2SO4 can be a source of sulfate anions (SO42−).
Reactions of Period 3 Oxides with Acids and Bases
Basic Oxides, Na2O and MgO (React with Acids Only)
| Oxide | Reaction with Hydrochloric Acid HCl(aq) |
|---|---|
| Na2O | Na2O + 2HCl → 2NaCl + H2O |
| MgO | MgO + 2HCl → MgCl2 + H2O |
Amphoteric Oxide, Al2O3 (Reacts with Both Acids and Bases)
| Reaction Type | Equation |
|---|---|
| With Acid (HCl) |
Al2O3 + 6HCl → 2AlCl3 + 3H2O |
| With Base (NaOH) | Al2O3 + 2NaOH + 3H2O → 2Na[Al(OH)4] |
Acidic Oxides, SiO2, P4O10, SO2 and SO3 (React with Bases Only)
| Oxide | Reaction with Sodium Hydroxide NaOH(aq) |
|---|---|
| SiO2 | SiO2 + 2NaOH → Na2SiO3 + H2O |
| P4O10 | P4O10 + 12NaOH → 4Na3PO4 + 6H2O |
| SO2 | SO2 + 2NaOH → Na2SO3 + H2O |
| SO3 | SO3 + 2NaOH → Na2SO4 + H2O |