The reaction \( \text{A}_2 + \text{B}_2 \to 2\text{AB} \) follows the mechanism:
\[
\text{A}_2 \xrightarrow{k_1} \text{A} + \text{A} \, (\text{fast}), \quad \text{A} + \text{B}_2 \xrightarrow{k_2} \text{AB} + \text{B} \, (\text{slow}), \quad \text{A} + \text{B} \to \text{AB} \, (\text{fast}).
\]
The overall order of the reaction is: