1. Period 3 Oxide Formulas, Structures & Bonding
Across Period 3 from left to right, elements increase in electronegativity. Consequently, their bonding with oxygen transitions from ionic with electropositive metals (sodium and magnesium) to macromolecular covalent (silicon) to simple molecular covalent with non-metals (phosphorus and sulfur).
| Oxide | Oxidation State of Period 3 Element | Melting Point / K | Structure & Bonding | Explanation of Melting Point |
|---|---|---|---|---|
| Na2O | +1 | 1548 | Giant ionic lattice | Strong electrostatic attractions between Na+ and O2- ions. |
| MgO | +2 | 3125 | Giant ionic lattice | Extremely high: Mg2+ has higher charge and smaller radius than Na+, creating stronger ionic bonds. |
| Al2O3 | +3 | 2345 | Ionic with covalent character | High lattice enthalpy; small, highly charged Al3+ polarises O2- electron clouds. |
| SiO2 | +4 | 1883 | Macromolecular (giant covalent) | Very high: immense energy required to break rigid 3D tetrahedral network of Si-O covalent bonds. |
| P4O10 | +5 | 573 (sublimes) | Simple molecular | Low: weak van der Waals London dispersion forces between discrete P4O10 molecules. |
| SO2 | +4 | 200 (gas at RT) | Simple molecular | Very low: weak intermolecular dispersion and dipole-dipole forces between discrete SO2 molecules. |
| SO3 | +6 | 290 (liquid at RT) | Simple molecular | Low: discrete SO3 molecules held together by weak intermolecular dispersion forces. |
2. Reactions of Period 3 Oxides with Water & Resulting pH
The behavior of Period 3 oxides in water mirrors their chemical bonding:
1. Sodium Oxide (Na2O)
Reacts vigorously and exothermically with water to form a strongly alkaline solution of sodium hydroxide:
Na2O(s) + H2O(l) -> 2NaOH(aq) [pH = 13 to 14]
Ionic equation: O2-(s) + H2O(l) -> 2OH-(aq). The oxide ion is a powerful Bronsted-Lowry base that accepts a proton from water.
2. Magnesium Oxide (MgO)
Reacts slowly with water to form sparingly soluble magnesium hydroxide:
MgO(s) + H2O(l) <=> Mg(OH)2(aq/s) [pH = 9 to 10]
Because Mg(OH)2 is only slightly soluble, few OH- ions are released into solution, producing a weakly alkaline pH.
3. Aluminum Oxide (Al2O3) and Silicon Dioxide (SiO2)
Both oxides are completely insoluble in water and produce an unreactive neutral suspension (pH = 7). Al2O3 has an extremely high lattice enthalpy that water cannot overcome, while SiO2 is an immense macromolecular covalent network.
4. Phosphorus(V) Oxide (P4O10)
Reacts violently and exothermically with water to form an acidic solution of phosphoric(V) acid:
P4O10(s) + 6H2O(l) -> 4H3PO4(aq) [pH = 1 to 2]
5. Sulfur Dioxide (SO2) and Sulfur Trioxide (SO3)
Sulfur dioxide dissolves readily to form weak sulfurous / sulfuric(IV) acid:
SO2(g) + H2O(l) <=> H2SO3(aq) [pH = 2 to 3]
Sulfur trioxide reacts violently with water in a dense mist of sulfuric(VI) acid droplets:
SO3(l) + H2O(l) -> H2SO4(aq) [pH = 0 to 1]
3. Period 3 Oxide Structure & pH Continuum Diagram
4. Acid-Base Reactions of Period 3 Oxides
Basic Oxides (Na2O and MgO)
React with acids to form a simple salt and water:
- Na2O(s) + 2HCl(aq) -> 2NaCl(aq) + H2O(l) [Ionic: O2- + 2H+ -> H2O]
- MgO(s) + 2HCl(aq) -> MgCl2(aq) + H2O(l)
- MgO(s) + H2SO4(aq) -> MgSO4(aq) + H2O(l)
Acidic Oxides (SiO2, P4O10, SO2, SO3)
React with aqueous alkalis / bases to form salts and water:
- Silicon dioxide: Reacts with hot concentrated sodium hydroxide:
SiO2(s) + 2NaOH(aq) -> Na2SiO3(aq) + H2O(l) (sodium silicate) - Phosphorus(V) oxide: Reacts with sodium hydroxide:
P4O10(s) + 12NaOH(aq) -> 4Na3PO4(aq) + 6H2O(l) (sodium phosphate) - Sulfur dioxide: Reacts with sodium hydroxide:
SO2(g) + 2NaOH(aq) -> Na2SO3(aq) + H2O(l) (sodium sulfite) - Sulfur trioxide: Reacts with sodium hydroxide:
SO3(l) + 2NaOH(aq) -> Na2SO4(aq) + H2O(l) (sodium sulfate)
5. Amphoteric Behavior of Aluminum Oxide (Al2O3)
1. Aluminum Oxide Acting as a Base (Reaction with Acids)
Al2O3 reacts with warm dilute hydrochloric acid or sulfuric acid to form aluminum salts:
Al2O3(s) + 6HCl(aq) -> 2AlCl3(aq) + 3H2O(l)
Ionic: Al2O3(s) + 6H+(aq) -> 2Al3+(aq) + 3H2O(l)
2. Aluminum Oxide Acting as an Acid (Reaction with Alkalis)
Al2O3 dissolves in hot concentrated aqueous sodium hydroxide to form sodium tetrahydroxoaluminate:
Al2O3(s) + 2NaOH(aq) + 3H2O(l) -> 2Na[Al(OH)4](aq)
Ionic: Al2O3(s) + 2OH-(aq) + 3H2O(l) -> 2[Al(OH)4]-(aq)
6. Worked Calculations & Equations
Step 1: Identify reactants and products:
P4O10 is an acidic oxide. It reacts with the alkali NaOH to form the salt sodium phosphate (Na3PO4) and water.
Step 2: Balance the phosphorus atoms:
1 mole of P4O10 produces 4 moles of Na3PO4.
Step 3: Balance sodium and oxygen/hydrogen:
4 moles of Na3PO4 require 12 moles of NaOH, yielding 6 moles of H2O:
P4O10(s) + 12NaOH(aq) -> 4Na3PO4(aq) + 6H2O(l)
Reaction type: Acid-base neutralisation.
Step 1: Describe the structure and bonding in SiO2:
SiO2 has a macromolecular (giant covalent) structure. Every silicon atom is covalently bonded to four oxygen atoms in a rigid 3D tetrahedral network. Melting requires breaking numerous strong covalent bonds, requiring vast thermal energy.
Step 2: Describe the structure and bonding in P4O10:
P4O10 consists of simple molecular units. While the covalent bonds inside each P4O10 molecule are strong, the molecules are held together in the solid state only by weak van der Waals (London dispersion) intermolecular forces. Little energy is needed to overcome these weak forces.
Exam-Style Practice Questions
Test your understanding of these core syllabus concepts with targeted questions and detailed explanations.
Question 1: Which Period 3 oxide is amphoteric?
Show Answer & Explanation
Correct Answer: B
Aluminum oxide (Al2O3) is amphoteric: it reacts with acids to form aluminum salts (e.g. AlCl3) and with alkalis to form aluminates (e.g. [Al(OH)4]-).
Question 2: What is observed when a sample of sulfur dioxide gas is bubbled into universal indicator solution?
Show Answer & Explanation
Correct Answer: B
Sulfur dioxide dissolves in water to form sulfurous acid (H2SO3), a weak acid that produces a pH of around 2 to 3, turning universal indicator orange-red.
Question 3: Why does magnesium oxide (MgO) have a significantly higher melting point than sodium oxide (Na2O)?
Show Answer & Explanation
Correct Answer: B
The Mg2+ ion has double the charge and a smaller ionic radius than Na+. By Coulomb's law, the lattice enthalpy and electrostatic attraction between Mg2+ and O2- ions in MgO is far greater than between Na+ and O2- in Na2O.
Question 4: What is the formula of the species formed when solid aluminum oxide dissolves in hot aqueous sodium hydroxide?
Show Answer & Explanation
Correct Answer: C
In alkaline solution, aluminum oxide acts as an acid to form the soluble tetrahydroxoaluminate complex ion, [Al(OH)4]- (sodium salt: Na[Al(OH)4]).