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The correct answer is Radon. Unlike other noble gases such as argon, helium, neon, krypton, and xenon, radon is not normally present in the Earth’s atmosphere in measurable quantities. The main reason is that radon is a radioactive noble gas with no stable isotopes. It is produced in trace amounts during the radioactive decay of uranium and thorium found in rocks, soil, and minerals.

Because radon atoms are unstable, they decay quickly into other radioactive elements, releasing alpha particles. This short half-life prevents radon from accumulating in the atmosphere to any significant level. Instead, it is usually found only in localized areas, especially in underground spaces, mines, and basements where uranium-rich rocks exist.

In contrast, other noble gases are stable and present naturally in the air. For example, argon is the most abundant noble gas, making up about 0.93% of the atmosphere. Helium and neon occur in trace amounts, while xenon and krypton are also present but at very low concentrations.

Radon is also a health hazard due to its radioactivity. Prolonged exposure to radon gas can increase the risk of lung cancer, which is why monitoring radon levels in homes and workplaces is important in certain regions.

Radon was discovered in 1900 by Friedrich Ernst Dorn. Unlike its stable counterparts, it is not extracted for large-scale commercial uses because of its limited presence and hazardous nature.

Thus, while all noble gases are chemically inert, radon is unique because it is unstable, radioactive, and not normally present in the general composition of the Earth’s atmosphere.