what dose the following equation illustrate? $^{238}U_{92} → ^{234}Th_{90} + ^4 He_2$

Analyze the question

The equation shows uranium-238 changing into thorium-234 and releasing a helium nucleus (an alpha particle). We need to name the type of process this is.

Concept

• Alpha decay: an unstable nucleus emits an alpha particle (⁴He₂), so its mass number drops by 4 and its atomic number drops by 2.
• Spontaneous disintegration (natural radioactivity): an unstable nucleus breaks down on its own, with no outside bombardment, giving off alpha, beta or gamma radiation.
• Nuclear fission: a heavy nucleus is split into two comparable medium-sized fragments, usually after being hit by a neutron, and releases a lot of energy.
• Nuclear fusion: light nuclei join to form a heavier one.
• Artificial radioactivity: radioactivity induced by bombarding stable nuclei with particles.

Solution

Check the balance:

• Mass numbers: 238 = 234 + 4 ✔
• Atomic numbers: 92 = 90 + 2 ✔

So the equation is balanced and shows U-238 emitting an alpha particle to become Th-234.

• Nothing bombards the uranium, so it is not artificial radioactivity or induced fission.
• Only a small particle (He nucleus) is emitted, so it is not fission, which would produce two large fragments.
• Two nuclei are not combining, so it is not fusion.
• "Radio-decomposition" is not the standard term for this process.
• The nucleus decays by itself, emitting an alpha particle. This is spontaneous disintegration (natural radioactive decay, alpha decay).

Answer: E. Spontaneous disintegration

Tip: If you see one nucleus on the left, a small particle (α, β or γ) on the right, and no bombarding particle, think natural or spontaneous decay. For fission, look for a neutron going in and two large fragments plus extra neutrons coming out.