Rewrite into a different sentence .

“If the atomic number of the nuclei undergoing the reaction is smaller than the atomic number of the nuclei produced, it indicates a fusion reaction.”

A fusion reaction is indicated when the atomic number of the nuclei undergoing the reaction is lower than the atomic number of the nuclei produced.

A fusion reaction is indicated when the atomic number of the produced nuclei is greater than the atomic number of the nuclei undergoing the reaction.

A fusion reaction is indicated when the atomic number of the nuclei produced is greater than the atomic number of the nuclei undergoing the reaction.