Neutrons Emitted in Fission Reaction of Uranium 235

To find the number of neutrons emitted during the given nuclear fission reaction, we first write the balanced nuclear equation.

A standard nuclear fission reaction is initiated by a thermal neutron ($^{1}_{0}\text{n}$). Let $x$ be the number of neutrons emitted in the reaction:

$^{1}_{0}\text{n} + {^{235}_{92}\text{U}} \longrightarrow {^{142}_{54}\text{Xe}} + {^{90}_{38}\text{Sr}} + x \, {^{1}_{0}\text{n}}$

Now, we apply the law of conservation of mass number ($A$) and atomic number ($Z$):

1. Conservation of Atomic Number ($Z$):
   $\text{Left side: } 0 + 92 = 92$
   $\text{Right side: } 54 + 38 + x(0) = 92$
   The atomic number is conserved.

2. Conservation of Mass Number ($A$):
   $\text{Left side: } 1 + 235 = 236$
   $\text{Right side: } 142 + 90 + x(1) = 232 + x$

   Equating the total mass numbers on both sides:
   $236 = 232 + x$
   $x = 236 - 232 = 4$

Thus, the number of neutrons emitted in the reaction is $4$.