WAEC Physics 2011 Paper

Concept:Alpha particle emission reduces both mass number and atomic number by fixed amounts.Explanation:Uranium-234 has mass number $234$ and atomic number $92$.Thorium-230 has mass number $230$ and atomic number $90$.Decrease in mass number: $234 - 230 = 4$.Decrease in atomic number: $92 - 90 = 2$.An alpha particle is $\mathrm{^{4}_{2}He}$, which lowers mass number by $4$ and atomic number by $2$.Thus, the emitted particle is an alpha particle.Answer:A. alpha particles