Problem 97 - A Complete Resource Book in Chemistry for JEE Main

Question

What is the correct order of occurrence (\% by weight) in air of \(\mathrm{Ne}, \mathrm{Ar}, \mathrm{Kr} ?\) (a) \(\mathrm{Ne}>\mathrm{Ar}>\mathrm{Kr}\) (b) \(\mathrm{Ar}>\mathrm{Ne}>\mathrm{Kr}\) (c) \(\mathrm{Ar}>\mathrm{Kr}>\mathrm{Ne}\) (d) \(\mathrm{Ne}>\mathrm{Kr}<\mathrm{Ar}\)

Step-by-Step Solution

Answer

Option (b) \( \mathrm{Ar} > \mathrm{Ne} > \mathrm{Kr} \) is correct.

Step 1: Recognize Elemental Abundances

Understand that in the Earth's atmosphere, Argon (Ar), Neon (Ne), and Krypton (Kr) occur at different abundances. Typically, Argon is more abundant than Neon, and Neon is more abundant than Krypton. This is due to their different sources and formation processes on Earth.

Step 2: Verify Abundances

Recall or research the approximate percentage concentrations of these gases by weight: Argon is about 0.93%, Neon is about 0.0018%, and Krypton is about 0.00011% of the Earth's atmosphere. This is based on their natural occurrence as trace gases.

Step 3: Compare Elements

Using the verified abundances, we can compare: \( \mathrm{Ar} (0.93\%) > \mathrm{Ne} (0.0018\%) > \mathrm{Kr} (0.00011\%) \). This confirms the relative order of abundance by weight.

Step 4: Choose the Correct Option

With the order established as \( \mathrm{Ar} > \mathrm{Ne} > \mathrm{Kr} \), check which option matches this sequence. Option (b) \( \mathrm{Ar} > \mathrm{Ne} > \mathrm{Kr} \) is consistent with our findings.

Key Concepts

Argon

Argon is the third most prevalent gas in the Earth's atmosphere, following nitrogen and oxygen. It makes up approximately 0.93% by weight of the atmosphere. This might not sound like much, but compared to other noble gases, it is relatively abundant.

Argon's abundance can be attributed to its stability and inertness. As a noble gas, it doesn't easily react with other elements, allowing it to persist in the atmosphere for long periods. This stability is a result of having a complete outer electron shell, which makes it energetically favorable to remain unreacted.

It's interesting to note that most of the Argon in our atmosphere comes from the radioactive decay of potassium-40 to argon-40. This decay occurs over millions of years, replenishing the Argon in our air. Argon's gaseous form makes it prevalent despite being heavier than other gases like helium or hydrogen, which escape into space due to Earth's gravitational pull.

Neon

Neon, although not as abundant as argon, holds an essential place in the family of noble gases. In Earth's atmosphere, Neon accounts for about 0.0018% by weight. While it's much less common than argon, you've likely seen Neon’s vibrant effects in neon signs.

Neon's name originates from the Greek word 'neos', meaning new. It was discovered relatively recently in 1898 by Sir William Ramsay and Morris Travers while they were studying liquified air. When a high voltage is applied to Neon in a vacuum tube, it emits a bright reddish-orange glow, which has made it popular in signage and lighting.

The rarity of Neon in the atmosphere is a result of it being a lighter noble gas formed by stellar nucleosynthesis far away and then trapped in the Earth's atmosphere. Its low abundance is also due to its non-reactive nature, meaning it is not replenished by chemical reactions like some other gases.

Krypton

Krypton is one of the rarest gases in the Earth's atmosphere, comprising just 0.00011% by weight. Despite its low abundance, Krypton has unique properties that make it valuable.

This noble gas is best known for its use in lighting and photography. Krypton is especially useful in high-speed photography due to its brightness when it is used in flash lamps. Additionally, it's used in fluorescent lamps and certain types of gas discharge tubes.

Discovered in 1898, along with Neon and Xenon, Krypton's name originates from the Greek word 'kryptos', meaning hidden. Its low concentration in the atmosphere can partly be explained by its atomic mass and inertness; it doesn't readily react with other elements, much like its noble gas siblings. The rarity and uses of Krypton make it a fascinating subject for study despite its small presence in the air.