🔷 Periodicity of Physical Properties of Period 3 Elements (Na to Ar)
1. Atomic Radius
- Trend: Decreases across Period 3.
- Reason: Nuclear charge increases, electrons are added to the same shell → stronger attraction between nucleus and electrons → smaller size.
2. Ionic Radius
- Cations (Na⁺, Mg²⁺, Al³⁺): Much smaller than parent atoms; lose electrons, fewer electron shells, increased proton-to-electron ratio.
- Anions (P³⁻, S²⁻, Cl⁻): Larger than parent atoms; gain electrons → increased repulsion and larger radius.
- Trend: Decreases across the cations (Na⁺ to Al³⁺); then anions (P³⁻ to Cl⁻) are larger but also decrease across the period.
3. Melting Point
- Na, Mg, Al: High melting points due to metallic bonding (strength increases from Na to Al as charge density increases).
- Si: Very high melting point due to giant covalent (macromolecular) structure.
- P₄, S₈, Cl₂, Ar: Lower melting points due to simple molecular structures and weak van der Waals’ forces.
- Trend: Increases from Na to Si → sharp decrease from P onwards.
4. Electrical Conductivity
- Na, Mg, Al: Good conductors due to delocalised electrons.
- Si: Poor conductor; semi-metal/semiconductor.
- P, S, Cl, Ar: Non-conductors; no free-moving charges.
- Trend: High for metals → drops sharply after Al.
🔷 Periodicity of Chemical Properties of Period 3 Elements
1. Reactions with Oxygen
| Element | Product | Equation |
|---|---|---|
| Na | Sodium oxide | 4Na + O₂ → 2Na₂O |
| Mg | Magnesium oxide | 2Mg + O₂ → 2MgO |
| Al | Aluminium oxide | 4Al + 3O₂ → 2Al₂O₃ |
| P | Phosphorus(V) oxide | P₄ + 5O₂ → P₄O₁₀ |
| S | Sulfur dioxide | S + O₂ → SO₂ |
2. Reactions with Chlorine
| Element | Product | Equation |
|---|---|---|
| Na | Sodium chloride | 2Na + Cl₂ → 2NaCl |
| Mg | Magnesium chloride | Mg + Cl₂ → MgCl₂ |
| Al | Aluminium chloride | 2Al + 3Cl₂ → 2AlCl₃ |
| Si | Silicon tetrachloride | Si + 2Cl₂ → SiCl₄ |
| P | Phosphorus pentachloride | PCl₃ + Cl₂ → PCl₅ (or P₄ + 10Cl₂ → 4PCl₅) |
3. Reactions with Water
| Element | Reaction | Equation |
|---|---|---|
| Na | Vigorous, forms alkaline solution | 2Na + 2H₂O → 2NaOH + H₂ |
| Mg | Very slow with cold water, faster with steam | Mg + 2H₂O → Mg(OH)₂ + H₂ (cold) |
4. Oxidation Numbers in Oxides & Chlorides
| Compound | Oxidation No. of Central Atom |
|---|---|
| Na₂O, NaCl | +1 |
| MgO, MgCl₂ | +2 |
| Al₂O₃, AlCl₃ | +3 |
| SiO₂, SiCl₄ | +4 |
| P₄O₁₀, PCl₅ | +5 |
| SO₂ | +4 |
| SO₃ | +6 |
5. Reactions of Oxides with Water
| Oxide | Reaction | pH |
|---|---|---|
| Na₂O | Na₂O + H₂O → 2NaOH | ~14 |
| MgO | Slightly soluble: MgO + H₂O ⇌ Mg(OH)₂ | ~10 |
| Al₂O₃ | Insoluble; amphoteric → no simple reaction | ~7 |
| SiO₂ | Insoluble | ~7 |
| P₄O₁₀ | P₄O₁₀ + 6H₂O → 4H₃PO₄ | ~2 |
| SO₂ | SO₂ + H₂O → H₂SO₃ | ~3 |
| SO₃ | SO₃ + H₂O → H₂SO₄ | ~1 |
6. Acid/Base Behaviour of Oxides and Hydroxides
| Compound | Behaviour | Reaction with Acid/Base |
|---|---|---|
| Na₂O, NaOH | Strongly basic | NaOH + HCl → NaCl + H₂O |
| MgO, Mg(OH)₂ | Basic | Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O |
| Al₂O₃, Al(OH)₃ | Amphoteric | Al(OH)₃ + HCl → AlCl₃ + 3H₂O; Al(OH)₃ + NaOH → Na[Al(OH)₄] |
| P₄O₁₀, SO₂, SO₃ | Acidic | SO₃ + 2NaOH → Na₂SO₄ + H₂O |
7. Reactions of Chlorides with Water
| Chloride | Hydrolysis | pH |
|---|---|---|
| NaCl, MgCl₂ | No hydrolysis | ~7 |
| AlCl₃ | Hydrolysed: Al³⁺ + 6H₂O ⇌ [Al(H₂O)₆]³⁺ → Al(OH)₃ + 3H⁺ | ~3 |
| SiCl₄ | SiCl₄ + 4H₂O → Si(OH)₄ + 4HCl | ~2 |
| PCl₅ | PCl₅ + 4H₂O → H₃PO₄ + 5HCl | ~2 |
8. Trends and Explanations (Bonding and Electronegativity)
- Left to right: More covalent character due to smaller size and higher electronegativity (AlCl₃ covalent, NaCl ionic).
- Acid–base behaviour of oxides linked to:
- Ionic oxides (basic)
- Covalent oxides (acidic)
- Hydrolysis occurs in covalent chlorides (e.g., AlCl₃, SiCl₄, PCl₅) → acidic solutions.
9. Types of Bonding in Oxides and Chlorides
| Compound | Bonding Type |
|---|---|
| Na₂O, MgO | Ionic |
| Al₂O₃ | Ionic with covalent character |
| SiO₂ | Giant covalent |
| P₄O₁₀, SO₂, SO₃ | Simple covalent |
| NaCl, MgCl₂ | Ionic |
| AlCl₃, SiCl₄, PCl₅ | Covalent |
🔷 Chemical Periodicity of Other Elements
1. Predicting Properties of Elements
- Based on trends in groups and periods:
- Atomic/ionic size
- Oxidation states
- Reactivity
- Acid–base nature of oxides
- Electrical conductivity
- E.g., Group 1: all react with water to give alkali and hydrogen.
2. Deducing Position of Unknown Elements
Given properties like:
- Atomic radius
- Melting/boiling point
- Reactivity
- Nature of oxides/chlorides
- You can infer the group and period of an unknown element.
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