Which noble gas is radioactive and is produced as a decay product of uranium and thorium?

1. Analyze the Question

The question asks for a specific noble gas that possesses two distinct characteristics:

• It is radioactive.
• It is formed as a result of the radioactive decay of heavier elements like uranium and thorium.

2. Explain the Concept

Noble gases (Group 18 elements) are generally known for being chemically inert (unreactive). However, they have different origins and properties:

• Argon, Krypton, and Neon: These are stable, non-radioactive gases found in the atmosphere. They are obtained primarily through the fractional distillation of liquid air.
• Radon: This is the heaviest noble gas. Unlike its group members, it is unstable and radioactive. It is a naturally occurring gas produced in the Earth's crust during the radioactive decay series of uranium-238 and thorium-232. Because it is a gas, it can seep out of soil and rocks into buildings, which is a significant environmental health concern.

3. Show the Solution

• Step 1: Evaluate the options based on radioactivity. Argon, Krypton, and Neon are stable isotopes and are not radioactive.
• Step 2: Identify the decay process. The decay of Uranium ($^{238}\text{U}$) and Thorium ($^{232}\text{Th}$) involves the emission of alpha and beta particles, eventually leading to the formation of Radon ($^{222}\text{Rn}$ or $^{220}\text{Rn}$).
• Step 3: Confirm the properties. Radon is the only noble gas that is radioactive and is a known byproduct of the decay chains of these heavy radioactive elements.

4. Identify the Answer

The correct option is C.

5. Exam Tip

To remember this for your JAMB exam, associate the word "Radon" with "Radiation." Since it is the only radioactive noble gas, the name itself serves as a hint. Also, remember that Radon is a health hazard in basements because it is a dense, radioactive gas that accumulates in poorly ventilated areas!