When uranium-238 undergoes alpha decay, it emits an alpha particle (which is a helium nucleus, consisting of 2 protons and 2 neutrons). This results in the following:

* The atomic number decreases by 2: Uranium has an atomic number of 92. Subtracting 2 gives us 90.

* The mass number decreases by 4: Uranium-238 has a mass number of 238. Subtracting 4 gives us 234.

The element with atomic number 90 is Thorium, so the resulting isotope is Thorium-234 (²³⁴Th).