#"_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"#.
