The correct order is the following:
1) An atom fission
2) Neutrons are produced
3) Neutrons find other atoms
4) Many atoms fission
This is the description of a fission chain reaction: initially, the reaction starts with the fission of an atom with unstable nucleus. In the 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.
