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

Molecularity

The correct answer is option 3. Molecularity.

Let us delve into each term related to chemical reactions and why molecularity is the one that cannot be a fractional number.

1. Order of a Reaction:

The order of a reaction with respect to a particular reactant is determined experimentally and represents the exponent of its concentration term in the rate law. If a reaction is first-order with respect to reactant A, the rate law would be \( \text{Rate} = k [\text{A}]^1 \). If it is second-order, the rate law would be \( \text{Rate} = k [\text{A}]^2 \).

Fractional Order: Yes, the order of a reaction can be fractional (e.g., 0.5, 1.5) if the rate depends on a fraction of the concentration of a reactant.

2. Half-life of a Reaction:

The half-life of a reaction is the time required for half of the reactants to be consumed or for half of a substance to undergo a change. For a first-order reaction, the half-life (\( t_{1/2} \)) is constant and can be calculated using the equation \( t_{1/2} = \frac{\ln 2}{k} \), where \( k \) is the rate constant.

Fractional Half-life: Yes, half-life can be fractional (e.g., 0.5 seconds) depending on the units of time used.

3. Molecularity:

Molecularity refers to the number of molecules or ions participating as reactants in an elementary reaction (a reaction that occurs in a single step). In an elementary reaction like \( A + B \rightarrow C \), the molecularity is 2 because two molecules (A and B) collide to form the product C.

Fractionality: Molecularity is always an integer because it represents a discrete count of reacting species involved in an elementary reaction. There cannot be a fraction of a molecule participating in a reaction step.

4. Rate Constant:

The rate constant (k) is a proportionality constant that relates the reaction rate to the concentrations of reactants. For a first-order reaction \( \text{Rate} = k [\text{A}] \), where \( k \) is the rate constant.

Fractional Rate Constant: Yes, rate constants can be fractional depending on the specific reaction kinetics and units used (e.g., mol/L/s).

Conclusion:
While terms like order, half-life, and rate constant can all be expressed as fractional numbers under certain conditions, molecularity specifically refers to the exact number of molecules or ions colliding in an elementary reaction. Since molecules and ions are discrete entities, molecularity is always a whole number and cannot be fractional. Therefore, molecularity is the term related to chemical reactions that cannot be a fractional number.