CAPF AC 2023 Q13: Science & Technology – Chemistry
- (a) Silicon
- (b) Aluminium
- (c) Carbon
- (d) Oxygen
Answer: (d) Oxygen
Explanation
Most abundant element on Earth
How to think: the question is about the Earth's crust, where we find rocks, water and air.
Oxygen is everywhere: in silicate rocks, in water and in the air.
It makes up nearly half of the crust by mass.
| Rank in the crust | Element | Approximate share by mass |
|---|---|---|
| 1 | Oxygen | 46% |
| 2 | Silicon | 28% |
| 3 | Aluminium | 8% |
| 4 | Iron | 5 to 6% |
| 5 | Calcium | 3.6% |
Related facts: aluminium is the most abundant metal in the crust.
Iron is the most abundant element in the whole Earth by mass, because of the core.
Hydrogen is the most abundant element in the universe.
Answer: (d) Oxygen.
Why each option is right or wrong
- (a) Incorrect. Silicon is second, at about 28% of the crust.
- (b) Incorrect. Aluminium is third, though it is the most abundant metal.
- (c) Incorrect. Carbon forms only a tiny fraction of the crust.
- (d) CORRECT. Oxygen is the most abundant element, nearly half the crust by mass.
Key point
Earth's crust by mass: oxygen (about 46%), silicon (28%), aluminium (8%), iron, calcium, sodium, potassium, magnesium. Most abundant metal in the crust: aluminium. Universe: hydrogen. Atmosphere: nitrogen. Human body: oxygen.
Remember
Crust: oxygen, silicon, aluminium, iron.
Common mistakes
- Marking silicon because rocks and sand are silicates.
- Choosing carbon because all life is carbon-based.
- Mixing up the most abundant element (oxygen) with the most abundant metal (aluminium).
Exam tip
Educated guess: sand is silicon dioxide and most rocks are oxides and silicates. Every atom of silicon in them comes with two or more atoms of oxygen.
Sources
- NCERT Class XI Geography — Fundamentals of Physical Geography, Chapter 5: Minerals and Rocks (major elements of the Earth's crust)
- NCERT Class XI Chemistry — The p-Block Elements (Group 16: occurrence of oxygen)