The splitting of an atom’s nucleus into two smaller nuclei is called
a. nuclear fusion.
b. nuclear fission.
c. a chain reaction.
d. a meltdown.

Option (b) is the correct answer.

Explanation:

When a large nuclei of an atom splits into two smaller nuclei then this type of reaction is known as nuclear fission.

For example,

Fission is a nuclear reaction and there is also release of energy during this reaction.

Thus, we can conclude that the splitting of an atom’s nucleus into two smaller nuclei is called nuclear fission.

why do the isotopes of a particular element behave differently in nuclear reactions but the same in chemical reactions? chemical reactions involve changes to electrons in an atom; isotopes do not differ in electrons, and so they react the same way during chemical changes.