"_92^238"U" is chemical symbol of one of the isotopes of element uranium. Some times it is also written as "U"-238.
By convention the figure 92 is the atomic number of the element represented.
Atomic number is number equal to the number of protons in its nucleus. And the other number 238 represents the Atomic mass of the element in amu (atomic mass units).
We know that Atomic Mass
=Number of Protons in the nucleus + Number of neutrons in it.
Hence,
238=92+Number of neutrons.
Solving it for the Numbers of neutrons,m, we obtain
Number of neutrons=238-92=146.
Similarly, a nucleus of U-235 isotope of uranium will have 92 protons and 143 neutrons. Difference between the mass number of isotopes being difference between the number of neutrons in the respective nuclei.
Its symbol is "_92^235"U".
