### Video Transcript

Consider the following example of
nuclear fission: 235 92 U plus 1 0 n forms 142 56 Ba plus 91 36 Kr plus three 1 0
X. Which subatomic particle is
represented by X? (A) An electron, (B) a quark, (C) a
neutron, (D) a proton, or (E) a positron.

Nuclear fission is a process where
large nuclei split into smaller ones, releasing large amounts of energy. Nuclear fission reactions are
commonly used in nuclear reactors and power plants to produce electricity. In nuclear reactors, uranium-235
atoms frequently split into two smaller parts: barium-144 and krypton-89. In the process, a lot of energy is
released. And two or three neutrons are
usually released as well.

In a nuclear reactor, the fission
process is a chain reaction. Each fission, each splitting of
uranium-235 to release energy, requires one neutron but produces around three
neutrons. So let’s say we start with one
neutron that splits the uranium-235 atom, releasing energy and three neutrons. Then those three neutrons can split
three more uranium-235 atoms, releasing three more neutrons each, for a total of
nine neutrons. Then those nine neutrons can split
nine uranium-235 atoms, releasing a total of 27 neutrons, and so on.

If it’s not controlled, the chain
reaction grows exponentially, very quickly generating more and more neutrons and
splitting more and more uranium-235 atoms. Each fission releases a lot of
energy, so this can be very dangerous. Instead, the reaction is controlled
using control rods, which absorb neutrons and slow down the reaction so that it
continues at a fairly constant rate.

In each fission of uranium-235,
three neutrons are usually produced. So the subatomic particle
represented by X is (C), a neutron.