This describes a fission chain reaction:
Initially, the reaction starts with the fission of an atom with an unstable nucleus. In fission, neutrons are produced, and they travel through the surrounding medium. Eventually, they meet other atoms and they are captured by their nuclei: the nuclei become unstable and they undergo fission as well.
The order will be: answer
1) One atom undergoes fission.
2) A number of neutrons are produced.
3) Each neutron finds another atom.
4) A number of atoms undergo fission.