Final answer:
In the nuclear reaction of Uranium-238 decaying into Thorium-234, an alpha particle (He) with a mass number of 4 and atomic number of 2 is emitted. Uranium-235 contains 92 protons and 143 neutrons.
Step-by-step explanation:
The nuclear reaction presented is 238 92U → 234 90Th + 4 2He. Here, we see Uranium-238 decaying into Thorium-234 and an alpha particle, which is identical to a helium nucleus with a mass number of 4 and an atomic number of 2. The original equation given in the question lacks the alpha particle (He), but by understanding the law of conservation of mass and charge, we can deduce the identity of the missing particle.
An isotope of uranium with an atomic number of 92 and a mass number of 235 will thus contain 92 protons, as the atomic number defines the number of protons in an atom's nucleus. To find the number of neutrons, we subtract the atomic number from the mass number: 235 - 92 = 143 neutrons.