Properties of Period 3 Elements & their Oxides

Na & Mg with Water

Sodium & Magnesium

• Sodium (Na) and Magnesium (Mg) are metals in Group 1 and 2, respectively.
• Both have high melting points, but Mg has a higher melting point due to its 2+ charge, increasing charge density.
• Both are silvery metals; sodium is softer and tarnishes quickly, while magnesium is harder and often seen as a ribbon.

Reactions with Water

• Sodium with Cold Water:
  • Reaction: 2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
  • Vigorous and exothermic, produces alkaline solution (pH 13-14).
  • Sodium's oxidation state changes from 0 to +1.
• Magnesium with Cold Water:
  • Reaction: Mg (s) + 2H2O (l) → Mg(OH)2 (aq) + H2 (g)
  • Slow reaction, produces weakly alkaline solution (pH ~10).
  • Magnesium's oxidation state changes from 0 to +2.
• Heated Magnesium with Steam:
  • Reaction: Mg (s) + H2O (g) → MgO (s) + H2 (g)
  • Faster reaction, magnesium burns with a bright, white flame.

Reaction with Oxygen

Oxide Formation

• Period 3 elements (excluding Cl and Ar) form oxides with oxygen.
• Reactions involve elements reaching their highest oxidation state:
  • 4Na + O2 → 2Na2O
  • 2Mg + O2 → 2MgO
  • 4Al + 3O2 → 2Al2O3
  • Si + O2 → SiO2
  • 4P4 + 5O2 → P4O10
  • S + O2 → SO2
  • SO2 formation requires a catalyst and high temperature for SO3.
• Physical properties vary by structure and bonding:
  • Sodium, magnesium, and aluminium oxides are ionic.
  • Silicon oxide has a giant covalent structure.
  • Phosphorus oxide and sulfur dioxide are simple covalent.

Melting Point Trend

Melting Points of Oxides

• Melting points indicate forces of attraction in structures.
• Ionic Oxides: Sodium, magnesium, aluminium oxides have high melting points due to giant lattice structures.
• Giant Covalent Oxides: Silicon dioxide has a high melting point due to strong covalent bonds.
• Simple Covalent Oxides: Phosphorus(V) oxide, sulfur dioxide, and sulfur trioxide have low melting points due to weak intermolecular forces.

Oxides Reacting with Water

Chemical Properties

• Na and Mg oxides form alkaline solutions due to oxide ions forming hydroxide ions.
• P and S oxides form acidic solutions by donating H+ ions to water.
• Al and Si are insoluble; Al can react with both acids and bases, showing amphoteric behavior.

Participant Reactions

• Na2O with Water: Forms strongly alkaline solution (pH 14).
• MgO with Water: Forms weakly alkaline solution (pH 10).
• No Reaction: Al2O3 and SiO2 do not react with water.
• P4O10 with Water: Forms strongly acidic solution (pH 2).
• SO2 with Water: Forms strongly acidic solution (pH 1).

Acid-Base Reactions of the Oxides

Nature of Oxides

• Basic: Na2O, MgO
• Amphoteric: Al2O3 (reacts as both acid and base)
• Acidic: SiO2, P4O10, SO2, SO3

Reactions with Acids/Bases

• Na2O and MgO: React with HCl to form salts and water.
• Al2O3: Reacts with H2SO4 to form salts and water; also reacts with NaOH to form salts.
• SiO2: Reacts with NaOH to form a salt and water.
• P4O10 and SO2: Reacts with NaOH to form salts.

Examiner Tips

• Understand the relationship between oxides' physical properties and their structure/bonding.
• Memorize reactions with state symbols, products, and balanced equations for exams.