because photons dont have mass
1st fission event
n + fission reactant -> fission product + n + n
so the 2 n created can initiate 2 new reactions
2nd fission event, for each n
n + fission reactant -> fission product + n
so the 1 n created can initiated 1 new reaction
in 1st event 2n are created.
in 2nd event, the 2n from 1st created 1n each for a total of 2n.
combining both events, 2n + 2n = 4n are produced in total.
scientists can reflect on the elements that are already there and predict the next ones