In alpha decay, 238U decays to 234Th. What is the emitted particle and the Z and A of the daughter nucleus?

• A. Emitted beta particle; daughter Z=91, A=234
• B. Emitted alpha particle; daughter Z=92, A=238
• C. Emitted gamma ray; daughter Z=90, A=234
• D. Emitted alpha particle; daughter Z=90, A=234

Answer: (D)

In alpha decay, the nucleus releases an alpha particle, which is a helium nucleus containing 2 protons and 2 neutrons. This reduces the mass number by 4 and the atomic number by 2. For uranium-238 (Z = 92, A = 238), the daughter nucleus thus has Z = 92 − 2 = 90 and A = 238 − 4 = 234, which corresponds to thorium-234. The emitted particle is an alpha particle.

Other options don’t fit because beta decay would change the atomic number without changing the mass number, gamma emission changes neither Z nor A, and the mass-number and atomic-number changes here match alpha decay exactly.