Suppose three neutrons are released when an atom in a sample of fissionable nuclei undergoes fission. Each of these neutrons has enough energy to cause another fission reaction in another nucleus of the material. If the reaction is not controlled, how many atoms will have undergone fission after 2 more stages of the chain reaction?

13 nuclei

Explanation:

From the question, the fission of one nucleus produces three neutrons which causes  more nuclei to undergo fission.

This implies that, after the first fission, three neutrons cause three nuclei to undergo fission. The three nuclei that underwent fission produces nine neutrons which causes the fission of nine nuclei.

All together we the number of nuclei that underwent fission as;

1 + 3 + 9 = 13 nuclei.

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.