Period Elements 3 (Na to Ar)
13.2 Period 3 (Na to Ar)
Elements in Period 3
Period 3 of the periodic table contains eight elements, spanning from sodium (Na) to argon (Ar): In this period, the 3s and 3p orbitals are filling with electrons. The shortened electronic configurations for the eight elements are as follows:
Period 3 Elements:
| Element | Electron Configuration |
|---|---|
| Sodium (Na) | [Ne] 3s¹ |
| Magnesium (Mg) | [Ne] 3s² |
| Aluminium (Al) | [Ne] 3s² 3p¹ |
| Silicon (Si) | [Ne] 3s² 3p² |
| Phosphorus (P) | [Ne] 3s² 3p³ |
| Sulfur (S) | [Ne] 3s² 3p⁴ |
| Chlorine (Cl) | [Ne] 3s² 3p⁵ |
| Argon (Ar) | [Ne] 3s² 3p⁶ |
Reactions of Period 3 Elements
1. Reactions with Water
The reactivity with water decreases across the period:
| Element | Reaction with Water | Equation |
|---|---|---|
| Sodium (Na) | Violent reaction, produces H₂ gas | 2Na + 2H₂O → 2NaOH + H₂ |
| Magnesium (Mg) | Slow with cold water, burns in steam | Mg + H₂O → MgO + H₂ (steam) |
| Aluminium (Al) | Reacts slowly with steam | 2Al + 3H₂O → Al₂O₃ + 3H₂ |
| Chlorine (Cl) | Dissolves to form an acidic solution | Cl₂ + H₂O ⇌ HCl + HOCl |
2. Reactions with Oxygen
All Period 3 elements except argon react with oxygen, forming oxides with varying properties:
Key Reactions:
- Sodium: Forms oxide and peroxide
- 4Na + O₂ → 2Na₂O (oxide)
- 2Na + O₂ → Na₂O₂ (peroxide)
- Magnesium: Burns with a bright flame
- 2Mg + O₂ → 2MgO
- Phosphorus: Forms two oxides
- P₄ + 3O₂ → P₄O₆ (phosphorus(III) oxide)
- P₄ + 5O₂ → P₄O₁₀ (phosphorus(V) oxide)
- Sulfur: Burns with a blue flame
- S + O₂ → SO₂
3. Reactions with Chlorine
Most Period 3 elements form chlorides with varying properties:
- Sodium: Forms ionic NaCl with a bright orange flame
- Magnesium: Forms ionic MgCl₂ with an intense white flame
- Aluminium: Forms AlCl₃ that sublimes easily
- Silicon: Forms covalent SiCl₄ (colorless liquid)
- Phosphorus: Forms PCl₃ (liquid) and PCl₅ (solid)
- Sulfur: Forms S₂Cl₂ (orange liquid)
Reactions of Oxides and Chlorides with Water
1. Oxides of Period 3 Elements
The behavior of oxides changes from basic to acidic across the period:
| Oxide | Reaction with Water | pH | Nature |
|---|---|---|---|
| Na₂O | Forms NaOH (strong base) | 14 | Strongly basic |
| MgO | Forms Mg(OH)₂ (weak base) | 9 | Weakly basic |
| Al₂O₃ | No reaction (amphoteric) | – | Amphoteric |
| SiO₂ | No reaction | – | Acidic |
| P₄O₁₀ | Forms H₃PO₄ (phosphoric acid) | 1-2 | Strongly acidic |
| SO₃ | Forms H₂SO₄ (sulfuric acid) | 0-1 | Very strongly acidic |
2. Chlorides of Period 3 Elements
Chlorides show a transition from ionic to covalent bonding across the period:
Key Reactions:
- NaCl: Dissolves to form a neutral solution
- MgCl₂: Forms a slightly acidic solution
- MgCl₂ + 6H₂O → [Mg(H₂O)₆]²⁺ + 2Cl⁻
- AlCl₃: Violent reaction producing HCl gas
- AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl
- SiCl₄: Hydrolyzes completely
- SiCl₄ + 2H₂O → SiO₂ + 4HCl
- PCl₅: Forms phosphoric acid and HCl
- PCl₅ + 4H₂O → H₃PO₄ + 5HCl
Physical Properties and Characteristics
1. Oxides of Period 3 Elements
The physical properties of oxides change dramatically across the period:
| Oxide | Bonding | Melting Point | Electrical Conductivity | Solubility in Water |
|---|---|---|---|---|
| Na₂O | Ionic | High (1275°C) | Conducts when molten | Soluble |
| MgO | Ionic | Very high (2852°C) | Conducts when molten | Slightly soluble |
| Al₂O₃ | Ionic with covalent character | Very high (2072°C) | Poor conductor | Insoluble |
| SiO₂ | Giant covalent | Very high (1713°C) | Non-conductor | Insoluble |
| P₄O₁₀ | Molecular covalent | Low (340°C sublimes) | Non-conductor | Reacts with water |
| SO₃ | Molecular covalent | Low (16.8°C) | Non-conductor | Reacts with water |
2. Chlorides of Period 3 Elements
Chlorides show a clear trend from ionic to covalent bonding:
| Chloride | Bonding | State at RT | Electrical Conductivity | Reaction with Water |
|---|---|---|---|---|
| NaCl | Ionic | Solid | Conducts when molten | Dissolves (neutral) |
| MgCl₂ | Ionic | Solid | Conducts when molten | Dissolves (slightly acidic) |
| AlCl₃ | Ionic (solid) Covalent (liquid) |
Solid (sublimes) | Doesn’t conduct | Violent reaction (HCl gas) |
| SiCl₄ | Covalent | Liquid | Non-conductor | Hydrolyzes completely |
| PCl₅ | Covalent | Solid (sublimes) | Non-conductor | Forms H₃PO₄ + HCl |
Key Trends Summary
- Left to Right Transition:
- Metallic → Covalent bonding
- Basic → Amphoteric → Acidic oxides
- Ionic → Covalent chlorides
- Physical Properties:
- High MP/BP (ionic/giant covalent) → Low MP/BP (molecular)
- Good conductors (metals) → Poor conductors (non-metals)
- Water Reactions:
- Basic solutions (left) → Acidic solutions (right)
- Simple dissolution → Violent hydrolysis
Summary of Trends Across Period 3 Elements
1. Atomic Properties
- Atomic Radius: Decreases (Na → Cl) due to increasing nuclear charge pulling electrons closer.
- Ionization Energy: Increases (Na → Ar) with exceptions at Al and S due to sub-shell stability.
- Electronegativity: Increases (Na: 0.9 → Cl: 3.0) as atoms attract bonding electrons more strongly.
2. Physical Properties
- Bonding:
- Metallic (Na, Mg, Al) → Giant covalent (Si) → Simple molecular (P, S, Cl, Ar)
- Melting/Boiling Points:
- Peaks at Si (giant covalent), then drops sharply for molecular elements
- Metals: High MP/BP due to metallic bonding
- Electrical Conductivity:
- Good conductors (Na, Mg, Al) → Semiconductor (Si) → Insulators (P to Ar)
3. Chemical Reactivity
- With Water:
- Vigorous (Na) → Slow (Mg/Al) → No reaction (Si to Ar, except Cl dissolves)
- With Oxygen:
- Forms basic oxides (Na₂O, MgO) → Amphoteric (Al₂O₃) → Acidic oxides (P₄O₁₀, SO₃, Cl₂O₇)
- With Chlorine:
- Forms ionic chlorides (NaCl, MgCl₂) → Covalent chlorides (AlCl₃ to S₂Cl₂)
4. Oxide Characteristics
- Acid-Base Nature:
- Basic (Na₂O, MgO) → Amphoteric (Al₂O₃) → Acidic (SiO₂ to Cl₂O₇)
- Reactions with Water:
- Form alkaline solutions (NaOH) → No reaction (Al₂O₃, SiO₂) → Form acidic solutions (H₃PO₄, H₂SO₄)
5. Chloride Characteristics
- Bonding:
- Ionic (NaCl, MgCl₂) → Covalent with ionic character (AlCl₃) → Covalent (SiCl₄ to S₂Cl₂)
- Reactions with Water:
- Dissolve (NaCl) → Hydrolyze violently (AlCl₃, SiCl₄, PCl₅)
- Conductivity:
- Conduct when molten (NaCl, MgCl₂) → Non-conductive (covalent chlorides)
Key Trend: Properties change from metallic to non-metallic across the period, with silicon as the semi-metal transition point.
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