The radioactive decay of uranium into thorium is represented by the equation: ${}_{92}{}^{238}\mathrm{U}\to {}_{90}{}^{234}\mathrm{Th}+\mathrm{x}$.

What is x?

The mass number of x must be 238 –  234 = 4 and atomic number of x must be 92 - 90 = 2. Hence, it is an alpha particle which has atomic number as 2 and mass number as 4.