The Nitrogen Family (Pnictogens)
1. Introduction and General Characteristics
Group 15, also known as the Nitrogen Group or Pnictogens, consists of:
- **Nitrogen (N)**
- **Phosphorus (P)**
- **Arsenic (As)**
- **Antimony (Sb)**
- **Bismuth (Bi)**
**Key Properties:**
- **Electronic Configuration:** \( ns^2 np^3 \)
- **Oxidation States:** **-3, +3, +5** (Nitrogen exhibits **-3 to +5**, while Bi favors **+3** due to the inert pair effect).
- **Nature:** Non-metallic (N, P), metalloid (As, Sb), metallic (Bi).
**2. Trends in Physical Properties**
| Property | Nitrogen (N) | Phosphorus (P) | Arsenic (As) | Antimony (Sb) | Bismuth (Bi) |
|----------|--------------|----------------|--------------|---------------|--------------|
| **State (RT)** | Gas | Solid (allotropes) | Solid | Solid | Solid |
| **Melting Point (°C)** | -210 | 44 (white P) | 817 (sublimes) | 631 | 271 |
| **Electronegativity** | 3.04 | 2.19 | 2.18 | 2.05 | 2.02 |
| **Atomic Radius (pm)** | 56 | 98 | 114 | 133 | 143 |
**Trends Observed:**
- **Atomic size increases** down the group.
- **Ionization energy decreases** (N has very high IE due to small size).
- **Metallic character increases** (N & P are non-metals, Bi is a metal).
**3. Chemical Properties and Reactivity**
**(a) Nitrogen (N₂)**
- **Diatomic gas (N≡N)** with a **strong triple bond** (945 kJ/mol).
- **Inert at room temperature** but reacts at high temperatures.
- **Key Compounds:**
- **Ammonia (NH₃)** – Weak base, produced via the **Haber process**.
- **Nitric Acid (HNO₃)** – Strong oxidizer (Ostwald process).
- **Oxides (NO, NO₂, N₂O, N₂O₅)** – Acidic in nature.
**(b) Phosphorus (P)**
- **Allotropes:**
- **White P** – Highly reactive, glows in dark (chemiluminescence).
- **Red P** – Less reactive, used in matches.
- **Black P** – Most stable, semiconductor.
- **Key Compounds:**
- **Phosphine (PH₃)** – Toxic, self-ignites in air.
- **Phosphoric Acid (H₃PO₄)** – Used in fertilizers.
- **PCl₃ & PCl₅** – Important in organic synthesis.
**(c) Arsenic (As), Antimony (Sb), Bismuth (Bi)**
- **Arsenic & Antimony:** Metalloids, form **toxic hydrides (AsH₃, SbH₃)**.
- **Bismuth:**
- **Diamagnetic**, used in low-melting alloys (e.g., Wood’s metal).
- Forms **Bi₂O₃ (basic oxide)** unlike N₂O₅ (acidic).
**4. Important Compounds and Reactions**
**(a) Ammonia (NH₃)**
- **Preparation:** Haber process (\( N₂ + 3H₂ ⇌ 2NH₃ \)).
- **Properties:**
- **Lone pair on N** makes it a Lewis base.
- Forms **ammonium salts (NH₄⁺)** with acids.
**(b) Nitric Acid (HNO₃)**
- **Preparation:** Ostwald process (oxidation of NH₃).
- **Reactions:**
- With metals → **NO/NO₂** (depends on concentration).
- **Aqua regia (HNO₃ + HCl)** dissolves gold & platinum.
**(c) Phosphorus Compounds**
- **Phosphine (PH₃):**
- **Preparation:** \( P₄ + 3NaOH + 3H₂O → PH₃ + 3NaH₂PO₂ \).
- **Self-ignites** in air due to **P₂H₄** impurity.
- **Phosphoric Acid (H₃PO₄):**
- Used in **fertilizers, detergents, and food additives**.
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The Oxygen Family (Chalcogens)
**1. Introduction and General Characteristics**
Group 16, known as the **Oxygen Family** or **Chalcogens**, includes:
- **Oxygen (O)**
- **Sulfur (S)**
- **Selenium (Se)**
- **Tellurium (Te)**
- **Polonium (Po)**
Key Properties:
- Electronic Configuration:\( ns^2 np^4 \)
- Oxidation States:-2, +2, +4, +6 (O mostly -2, Po shows +2, +4).
- Nature: Non-metallic (O, S), metalloid (Se, Te), radioactive metal (Po).
2. Trends in Physical Properties
| Property | Oxygen (O) | Sulfur (S) | Selenium (Se) | Tellurium (Te) | Polonium (Po) |
|----------|------------|------------|---------------|----------------|---------------|
| State (RT) | Gas | Solid (S₈ rings) | Solid | Solid | Solid |
| Melting Point (°C)| -219 | 115 (rhombic) | 221 | 450 | 254 |
| Electronegativity| 3.44 | 2.58 | 2.55 | 2.10 | 2.00 |
| Atomic Radius (pm) | 48 | 88 | 103 | 123 | 135 |
**Trends Observed:**
- Electronegativity decreases down the group (O is the **2nd most electronegative** element).
- Metallic character increases (O & S are non-metals, Po is a metal).
3. Chemical Properties and Reactivity
(a) Oxygen (O₂ & O₃)
- Dioxygen (O₂):
- Supports combustion, paramagnetic.
- Forms oxides (ionic & covalent).
- Ozone (O₃):
- Allotrope, absorbs UV radiation (ozone layer).
- Strong oxidizing agent (\( 2KI + O₃ + H₂O → 2KOH + I₂ + O₂ \)).
(b) Sulfur (S)
- Allotropes:
- Rhombic S (α-Sulfur) – Stable at RT.
- Monoclinic S (β-Sulfur)– Forms above 95.5°C.
- Key Compounds:
- H₂S (Hydrogen sulfide)– Rotten egg smell, toxic.
- SO₂ & SO₃ – Acid rain contributors.
- H₂SO₄ (Sulfuric acid) – "King of chemicals" (industrial use).
(c) Selenium (Se) & Tellurium (Te)
- Semiconductors (used in photocopiers & solar cells).
- H₂Se & H₂Te – Foul-smelling, unstable.
(d) Polonium (Po)
- Radioactive (α-emitter), decays to lead.
- Highly toxic (used in nuclear batteries).
4. Important Compounds and Reactions**
(a) Water (H₂O)
- Universal solvent, high heat capacity.
- Amphoteric nature (can act as acid or base).
(b) Sulfuric Acid (H₂SO₄)
- Preparation: Contact process (\( S → SO₂ → SO₃ → H₂SO₄ \)).
- Uses:Fertilizers, petroleum refining, batteries.
(c) Hydrogen Peroxide (H₂O₂)
- Oxidizing & reducing agent.
- Used as bleach, antiseptic, and rocket fuel.
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Conclusion
- Group 15 (Pnictogens): Nitrogen and phosphorus are crucial for life (DNA, proteins) and industry (fertilizers, explosives).
- Group 16 (Chalcogens): Oxygen and sulfur are vital for respiration, acid production, and semiconductors.
