CAPF AC 2022 Q40: Science & Technology – Chemistry
The atomic radius of hydrogen atom is
- (a) 37 nanometer
- (b) 37 picometer
- (c) 17 picometer
- (d) 57 picometer
Answer: (b) 37 picometer
Explanation
Size of a hydrogen atom
How to think: atoms are measured in picometres (pm).
1 pm is 10⁻¹² metre.
Hydrogen is the smallest atom, and NCERT gives its radius as 37 pm.
| Unit | Size | Used for |
|---|---|---|
| Picometre (pm) | 10⁻¹² m | Atomic radii |
| Nanometre (nm) | 10⁻⁹ m | Molecules, viruses, wavelength of light |
| Angstrom | 10⁻¹⁰ m | Older unit for atoms; 1 angstrom = 100 pm |
Why 37 nm is absurd: 37 nanometres is 37,000 picometres.
That is the size of a small virus, a thousand times bigger than an atom.
Some atomic radii from NCERT: lithium 152 pm, sodium 186 pm, carbon 77 pm, oxygen 66 pm.
Answer: (b) 37 picometer.
Why each option is right or wrong
- (a) Incorrect. 37 nanometre is a thousand times too large for an atom.
- (b) CORRECT. The atomic radius of hydrogen is 37 picometre.
- (c) Incorrect. 17 picometre is too small; the accepted value is 37 pm.
- (d) Incorrect. 57 picometre is too large for hydrogen.
Key point
NCERT Class X: atomic radius is measured in picometres (1 pm = 10⁻¹² m); hydrogen = 37 pm. Radius decreases across a period and increases down a group.
Remember
Hydrogen atom radius: 37 pm (picometre).
Common mistakes
- Picking 37 nanometre by matching the number and ignoring the unit.
- Mixing up picometre (10⁻¹²) with nanometre (10⁻⁹).
- Thinking hydrogen's radius is 53 pm; that is the Bohr radius, a different quantity.
Exam tip
Educated guess: the number 37 appears twice, so the examiner is testing the unit. Atoms are sized in picometres.
Sources
- NCERT Class X Science — Periodic Classification of Elements (atomic size)
- NCERT Class XI Chemistry — Classification of Elements and Periodicity in Properties (atomic radius)