Final answer:
Thorium-234 undergoes alpha decay to become the element radium (Ra) with an atomic number of 88 and a mass number of 230.
Step-by-step explanation:
When thorium-234 undergoes alpha decay, it emits an alpha particle, which is composed of 2 protons and 2 neutrons. This process decreases the atomic number by 2 and the mass number by 4. Thorium-234 (Th-234), which has an atomic number of 90 and a mass number of 234, will transform into an element X with an atomic number of 88 and a mass number of 230.
By examining the periodic table, we see that the element with atomic number 88 is radium (Ra). Therefore, the complete nuclear equation for the alpha decay of Thorium-234 is:
^234_90Th → ^4_2He + ^230_88Ra
This equation respects the law of conservation of matter, ensuring that the total number of protons and neutrons is the same on both sides of the equation.