Uranium-238's decay equation is 238U92 -> 234Th90 + 4He2 + Gamma rays. Uranium-238 is an isotope that undergoes alpha decay to form thorium, helium and gamma rays. While the formation of gamma rays does not affect the atomic mass of the resulting elements, it is key in nuclear science.
Any given isotope is represented as a chemical symbol of the element in question along with an atomic number and mass number. The mass numbers of the elements on one side of a correctly balanced equation can be added together to equal the sum of the mass numbers on the other side. The same rule holds true for the atomic numbers. Gamma rays are high-energy photons; any photons with an energy rating higher than 10keV are considered to be gamma rays.