🔹 What are Allotropes?
Allotropes are different structural forms of the same element in the same physical state.
- Example: Carbon → Diamond, Graphite, Fullerene
🔹 Oxygen – Basics
- Symbol: O
- Atomic Number: 8
- Properties: Colorless, odorless gas, essential for life, combustion, and chemical reactions.
🔹 Main Allotropes of Oxygen
1️⃣ Dioxygen (O₂) – The Air We Breathe
- Molecule: 2 oxygen atoms
- Makes up 21% of the atmosphere
✅ Key Points:
- Essential for respiration (life process)
- Supports combustion (burning)
- Released by plants in photosynthesis
- Stored in oxygen cylinders for medical use
2️⃣ Ozone (O₃) – Earth’s Bodyguard
- Molecule: 3 oxygen atoms
- Found mainly in the stratosphere (ozone layer)
- Appearance: Light blue, sharp smell
✅ Key Points:
- Protects Earth from harmful UV rays
- Used for water & air purification
- Harmful if inhaled at ground level
- Formed by: lightning or sunlight + pollutants
🔹 Comparison Table
| Feature | Dioxygen (O₂) | Ozone (O₃) |
|---|---|---|
| Atoms per molecule | 2 | 3 |
| Common name | Oxygen gas | Ozone |
| Color | Colorless | Light blue |
| Smell | Odorless | Sharp, chlorine-like |
| Found in | Atmosphere (21%) | Ozone layer (stratos.) |
| Main Use | Breathing, burning | UV protection, disinfection |
👉 Quick Tip for Exams:
- O₂ → Life supporter
- O₃ → UV protector
📝 MCQs on Allotropes of Oxygen
1. Which of the following is an allotrope of oxygen?
a) O₂ and O₃
b) O₂ only
c) O₃ only
d) CO₂
2. Dioxygen (O₂) makes up about how much of Earth’s atmosphere?
a) 10%
b) 21%
c) 50%
d) 78%
3. Ozone (O₃) is mostly found in which layer of the atmosphere?
a) Troposphere
b) Stratosphere
c) Mesosphere
d) Exosphere
4. What is the molecular formula of dioxygen?
a) O₃
b) O₂
c) O
d) O₄
5. The smell of ozone is often described as:
a) Odorless
b) Sweet
c) Sharp, chlorine-like
d) Rotten egg-like
6. Which process releases dioxygen (O₂) into the atmosphere?
a) Photosynthesis
b) Respiration
c) Combustion
d) Fermentation
7. Which allotrope of oxygen is essential for respiration in living organisms?
a) O₃
b) O₂
c) CO₂
d) O
8. Ozone protects Earth by absorbing:
a) Infrared radiation
b) Ultraviolet radiation
c) Visible light
d) Gamma rays
9. Which of the following statements is true about O₂?
a) It is blue in color
b) It has a sharp smell
c) It is odorless and colorless
d) It is harmful to humans
10. Which of the following is NOT a use of ozone?
a) Purifying water
b) Air disinfection
c) Supporting breathing
d) Blocking UV rays
11. Which gas cylinder is given to patients for breathing problems?
a) Nitrogen cylinder
b) Oxygen cylinder (O₂)
c) Ozone cylinder
d) Carbon dioxide cylinder
12. Ozone at ground level is considered:
a) Useful for breathing
b) Harmful to breathe
c) Essential for combustion
d) Odorless
13. Which natural phenomenon can produce ozone?
a) Volcanic eruptions
b) Lightning strikes
c) Earthquakes
d) Snowfall
14. Which statement is correct about allotropes?
a) Allotropes are different compounds of an element
b) Allotropes exist in different physical states
c) Allotropes are different structural forms of the same element in the same state
d) Allotropes must always be gases
15. Which of the following colors is associated with ozone?
a) Colorless
b) Blue
c) Red
d) Green
