Among the noble (rare) gases present in Earth's atmosphere, which one is the most abundant by volume?

Introduction / Context:
The noble gases, sometimes called rare gases, include helium, neon, argon, krypton and xenon. Although they are chemically inert, some of them are present in significant amounts in the atmosphere. Knowing which noble gas is most abundant in the air is a common general knowledge and chemistry question that connects the periodic table to the composition of the atmosphere we breathe.

Given Data / Assumptions:

• The noble gases considered are xenon, helium, neon, argon and krypton.
• We are talking about Earth's atmosphere at ground level.
• Abundance refers to volume percentage in air, not abundance in the universe.
• We assume standard atmospheric composition.

Concept / Approach:
In Earth's atmosphere, nitrogen and oxygen are the major components, followed by argon at about 0.93 percent by volume. The concentrations of other noble gases are much smaller: neon, helium, krypton and xenon exist only in trace amounts. Although helium is more abundant in the universe as a whole, in Earth's atmosphere argon dominates among noble gases. Therefore, argon is the most abundant rare gas in the air around us.

Step-by-Step Solution:
Step 1: Recall the approximate composition of dry air: about 78 percent nitrogen, 21 percent oxygen and around 1 percent other gases.Step 2: Among these other gases, argon makes up nearly 0.93 percent by volume.Step 3: Helium and neon are present only at parts per million levels, which is far less than argon.Step 4: Krypton and xenon are even rarer, present only in tiny trace amounts.Step 5: Therefore, when comparing noble gases in the atmosphere, argon is clearly the most abundant.Step 6: Match this conclusion with the options and select argon (Ar).

Verification / Alternative check:
Atmospheric composition tables from reference books list argon as the third most abundant gas in dry air, after nitrogen and oxygen. Helium, neon, krypton and xenon are listed below with much smaller volume percentages. While helium is very abundant in stars, that is a different context from the air we breathe. This confirms that argon is the correct answer for Earth's atmosphere.

Why Other Options Are Wrong:
Option a, xenon, is extremely rare in the atmosphere and is present only in trace amounts. Option b, helium, escapes Earth's gravity over time and is present at much lower levels than argon. Option c, neon, is also a trace gas. Option e, krypton, is even less abundant than neon. None of these noble gases approach the nearly 1 percent volume concentration that argon has in air.

Common Pitfalls:
Students may confuse cosmic or stellar abundance with atmospheric abundance and choose helium because it is abundant in the universe. Another pitfall is to remember the word rare gas and assume that all noble gases must be equally rare. In reality, argon is not rare in our atmosphere, even though it is chemically inert. Keeping the distinction between different contexts of abundance helps avoid these errors.

Final Answer:
The most abundant noble gas in Earth's atmosphere by volume is argon (Ar).