ANS:D - argon

The noble gas which occurs most abundantly in the atmosphere is argon. Argon is the noble gas that occurs most abundantly in the atmosphere, making up about 0.93% of the Earth's atmosphere. Here’s an explanation of its significance and properties:

Abundance:

1. Atmospheric Composition: Argon is the third most abundant gas in the atmosphere after nitrogen and oxygen. Its relative abundance is due to its inert nature and stability, which prevent it from reacting with other elements or compounds.

Characteristics:

1. Noble Gas: Argon belongs to the group of noble gases, which are known for their low reactivity due to their complete valence electron shell. This makes argon non-toxic and safe for various applications.
2. Colorless and Odorless: Argon is a colorless, odorless gas at room temperature, making it unnoticeable in the atmosphere.

Sources and Formation:

1. Noble Gas Formation: Argon is primarily produced through the radioactive decay of potassium-40 in the Earth's crust and is continuously replenished from geological sources.
2. Extraction: It can also be extracted from the air through fractional distillation of liquefied air, separating it from nitrogen and oxygen.

Applications:

1. Industrial Uses: Argon is widely used in industrial applications, such as in welding and metal fabrication, to provide an inert atmosphere that prevents oxidation.
2. Lighting: It is commonly used in incandescent and fluorescent lighting, as well as in certain types of lasers.
3. Scientific Research: Argon is used in laboratories for various experiments where a non-reactive environment is needed.

Summary: