Statement I: The molecularity of an elementary reaction is practically not more than three.
Statement II: The probability of more than three molecules colliding simultaneously is very small.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I
The molecularity of an elementary reaction is the number of reacting species (atoms, ions or molecules) that must collide at the same time for the reaction to occur.
Statement I: The molecularity of an elementary reaction is practically not more than three. — True. Elementary reactions are unimolecular, bimolecular or, rarely, termolecular (three species); reactions of molecularity three are already very rare and slow.
Statement II: The probability of more than three molecules colliding simultaneously is very small. — True. The more particles that must meet at the same instant, the less likely such a collision becomes.
Why Statement II explains Statement I?
A step of molecularity four or more would need four or more particles to collide at the same moment.
Such collisions almost never happen, so no practical elementary step has a molecularity above three.
NCERT (2026-27 edition):
3.2.4 Molecularity of a Reaction – “The probability that more than three molecules can collide and react simultaneously is very small. Hence, reactions with the molecularity three are very rare and slow to proceed.”