The number of atoms or molecules whose concentration determines the rate expression is

order of a reaction

The correct answer is option 2. order of a reaction.

The order of a reaction refers to the sum of the powers of the concentrations of the reactants in the rate law equation. It determines how the rate of a reaction depends on the concentration of each reactant.

Rate Law Expression:

The rate law expresses how the rate of a chemical reaction relates to the concentrations of the reactants. It generally takes the form:

\(\text{Rate} = k [A]^m [B]^n\)

where:

\( k \) is the rate constant.

\( [A] \) and \( [B] \) are the concentrations of reactants A and B, respectively.

\( m \) and \( n \) are the orders of the reaction with respect to reactants A and B, respectively.

The order of a reaction is experimentally determined and may or may not be the same as the stoichiometric coefficients in the balanced chemical equation.

It is determined by examining how changes in the concentration of a reactant affect the rate of reaction, typically through experiments where one reactant's concentration is varied while keeping others constant.

Significance:

The order of a reaction dictates how changes in reactant concentrations affect the rate of reaction. For example: A reaction with an order of 1 with respect to a reactant will show a proportional change in rate when that reactant's concentration is changed.

A reaction with an order of 2 with respect to a reactant will show a quadratic change in rate when that reactant's concentration is changed.

Conclusion: In the context of chemical kinetics, the order of a reaction is indeed the term used to describe the number (or sum of powers) of the concentrations of reactants in the rate law expression that determines how the rate of reaction depends on those concentrations. Therefore, for the question about which term describes the number of atoms or molecules whose concentration determines the rate expression, the correct answer is: order of a reaction.