Gary Dougherty, Age 11, Of Mesa, Ariz., for His question:

What is U 235?

The u stands for uranium, most vital of the radioactive minerals in this atomic age. The atomic number of the element uranium is 92, meaning that the uranium atom has 92 protons in its nucleus. Its atomic weight or mass number is 238.

Elements, however, have isotopes or brother atoms with extra, neutrons or more neutrons. This changes the atomic mass number. Uranium 235 is like uranium 238 in every respect but one. It has fewer particles than uranium 238. The element uranium is composed of 99.3% u 238. The isotope u 235 composes less than one percent, and a mere fraction of the uranium is u 234. Both these isotopes are radioactive.