Uranium-234 disintegrated to form Thorium-230 by emitting

Step 1: Analyze the question Uranium-234 (atomic number 92) changes into Thorium-230 (atomic number 90). We need to find which radiation was emitted.

Step 2: Compare the nuclei

• Uranium-234: mass number 234, atomic number 92
• Thorium-230: mass number 230, atomic number 90

Change in mass number = 234 − 230 = 4 Change in atomic number = 92 − 90 = 2

Step 3: Identify the emitted particle The nucleus lost 4 mass units and 2 proton units. That is exactly a helium nucleus, which is an alpha particle (⁴₂He).

Equation: ²³⁴₉₂U → ²³⁰₉₀Th + ⁴₂He

Check:

• Mass: 234 = 230 + 4 ✔
• Charge: 92 = 90 + 2 ✔

Why not the others?

• Gamma rays: no change in mass or atomic number, only energy is released.
• Beta negative: the atomic number would increase by 1, and the mass number would stay the same.
• Beta positive: the atomic number would decrease by 1, and the mass number would stay the same.

Only alpha emission reduces the mass number by 4 and the atomic number by 2.

Answer: A. alpha particles

Exam tip: Alpha decay: mass number drops by 4, atomic number drops by 2. Beta decay: mass number is unchanged. Gamma: nothing changes in the nucleus's identity. Always check the changes in A and Z first.