During the study of kinetics of chemical or nuclear reaction, `t_(1//3)` can be defined as

Options

A. A. One-third of the reaction molecules are left.
B. B. Two-third of the reaction molecules are left.
C. C. One-third of half of the reaction molecules are left.
D. D. One-third of two-thirds of the reaction molecules are left.

Explanation

To define \( t_{1/3} \) in the context of chemical or nuclear reaction kinetics, we can follow these steps:

### Step 1: Understanding \( t_{1/3} \)
- \( t_{1/3} \) refers to the time at which one-third of the reactant has been consumed during a reaction. This implies that two-thirds of the reactant remains.

### Step 2: Defining the Concept
- In a chemical reaction, if we denote the initial amount of reactant as \( R_0 \), after time \( t_{1/3} \), the amount of reactant consumed will be \( \frac{1}{3} R_0 \).

### Step 3: Calculating Remaining Reactant
- If one-third of the reactant is consumed, the remaining amount of reactant will be:
\[ R_{\text{remaining}} = R_0 - \frac{1}{3} R_0 = \frac{2}{3} R_0 \]

### Step 4: Conclusion
- Therefore, \( t_{1/3} \) can be defined as the time taken for one-third of the reactant to be consumed, which also means that two-thirds of the reactant molecules are left.

### Final Answer
- The correct definition of \( t_{1/3} \) is that it represents the time at which two-thirds of the reactant molecules are left. Thus, the answer is option B.

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