For a chemical reaction, which can never be a fractional number?

The correct answer is Option (3) → Molecularity

Molecularity is the number of reacting species (atoms, ions or molecules) that must collide at the same time in an elementary reaction. A count of particles is a whole number (1, 2 or 3), so molecularity can never be zero or a fraction.

Why the other options are incorrect

Option (1) Order — Incorrect. Order is found by experiment from the rate law and can be a fraction; for example, a rate law \( \text{Rate}=k[\mathrm{A}][\mathrm{B}]^{1/2} \) gives an order of 3/2.

Option (2) Half-life — Incorrect. Half-life is a time (for a first order reaction \( t_{1/2}=\frac{0.693}{k} \)), and a time can have any value, such as 2.5 min.

Option (4) Rate constant — Incorrect. The rate constant is a measured quantity that can have any value, such as 1.15 × 10⁻³ s⁻¹.

NCERT (2026-27 edition):
3.2.4 Molecularity of a Reaction – “… The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.”

“(i) Order of a reaction is an experimental quantity. It can be zero and even a fraction but molecularity cannot be zero or a non integer.”