Question

The noble gas which is not found in atmosphere:
(A) Ne
(B) Ar
(C) Rn
(D) Kr

Answer 1

Hint: Before answering this question we must have an idea about the six inert gases. We should also know about their chemical properties and that some of them are radioactive.

Complete step by step answer:
Helium, Neon, Argon, Krypton and Xenon present in group 18 of the modern periodic table are called Noble gases.
They are the most stable elements known as they have the complete octet configuration. That means the maximum number of valence electrons their outer shell can hold.
All gases conduct electricity, fluoresce, are odorless and colorless, and are used in many conditions when a stable element is needed to maintain a safe and constant environment.
Neon is used in advertising signs. Argon is used in light bulbs. Helium is used in balloons and to cool things. Xenon is used in headlights for cars.

Radon (Atomic number 86) is a radioactive substance produced due to decay of Radium-226 found in Uranium ore. This is not present in the atmosphere.

Hence, the correct answer is Option (C) Radon.

Additional information:
Radon has application in radiotherapy due to its radioactive nature. It has been used in implantable seeds, made of glass or gold, primarily used to treat cancers.

Note: We should remember that the term ‘inert gases’ for noble gases is no longer valid. As some of them have been known to react and form compounds. All noble gases including Xenon, Radon and Argon form compounds with fluorine under extremes—like reactive reagents, low temperatures, or high pressures.