Arrange the following permanent gases present in the atmosphere in ascending order: 1. Helium 2. Hydrogen 3. Neon 4. Xenon Select the answer using the code given below:

1. Primary Composition of the Atmosphere (basic)

To understand the chemistry of our planet, we must first look at the atmosphere—the thin, protective envelope of gases that makes life possible. While we often think of 'air' as a single substance, it is actually a complex mixture of gases, water vapor, and tiny solid particles. These gases are generally classified into two categories: permanent gases, which remain in fixed proportions regardless of where you are, and variable gases (like water vapor and CO₂), which change depending on the location and time. As noted in Fundamentals of Physical Geography, Composition and Structure of Atmosphere, p.64, the bulk of this mixture is concentrated near the surface, with oxygen becoming nearly negligible by the time you reach an altitude of 120 km.

The vast majority of our atmosphere—about 99%—is composed of just two gases: Nitrogen (N₂) at roughly 78% and Oxygen (O₂) at about 21%. However, the remaining 1% contains crucial 'trace' gases. Argon (Ar) is the most significant of these at 0.93%, followed by Carbon Dioxide (CO₂). Below these are the noble gases and hydrogen, present in tiny, microscopic amounts. According to Physical Geography by PMF IAS, Earths Atmosphere, p.271, when we look at these trace elements in terms of volume, a distinct hierarchy emerges. For instance, Neon (0.002%) is more abundant than Helium (0.0005%), which in turn is more abundant than Xenon or Hydrogen.

It is fascinating to note that our atmosphere hasn't always looked this way. Our primordial atmosphere was mostly Hydrogen and Helium, but these light gases were largely stripped away by intense solar winds during the Earth's early formation Fundamentals of Physical Geography, The Origin and Evolution of the Earth, p.15. The current oxygen-rich composition is actually a 'biological byproduct' created over billions of years by photosynthetic life. Today, gravity keeps the heavier gases like Nitrogen and Oxygen closer to the Earth's surface, while the lighter gases are found in much smaller concentrations.

Sources: Fundamentals of Physical Geography, Composition and Structure of Atmosphere, p.64; Fundamentals of Physical Geography, The Origin and Evolution of the Earth, p.15; Physical Geography by PMF IAS, Earths Atmosphere, p.271

2. Permanent vs. Variable Gases (basic)

To understand our atmosphere, we first categorize its gases based on how much their concentration changes over time and space. The atmosphere is essentially a mixture of Permanent Gases and Variable Gases. Permanent gases (also called constant gases) are those whose proportion remains nearly fixed in the lower atmosphere, regardless of where you are on Earth. For example, whether you are in a desert or over an ocean, Nitrogen (N₂) always makes up about 78% of dry air, and Oxygen (O₂) remains at roughly 21% Physical Geography by PMF IAS, Earths Atmosphere, p.271. Even the trace gases like Argon (0.93%), Neon (0.002%), and Helium (0.0005%) maintain these specific ratios relative to the total volume of the atmosphere.

In contrast, Variable Gases are those whose concentrations fluctuate significantly due to natural or human-induced processes. The most famous example is Water Vapour, which can vary from nearly 0% in freezing polar regions to 4% in humid tropical areas. Other variable gases include Carbon Dioxide (CO₂), which is increasing globally due to fossil fuel combustion, and Ozone (O₃), which varies by altitude and location Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), World Climate and Climate Change, p.96. These variable gases, despite being present in small amounts, are critical because they act as greenhouse gases, regulating Earth's temperature.

When studying the permanent trace gases, it is helpful to know their relative abundance, as they are often found in very tiny percentages. For instance, the sequence from most abundant to least abundant among common trace gases is: Neon (0.002%) > Helium (0.0005%) > Xenon (0.00009%) > Hydrogen (0.00005%) Physical Geography by PMF IAS, Earths Atmosphere, p.271. While these gases are "permanent" in their proportion, their total density decreases as we move higher into space. Interestingly, some gases disappear faster than others; for example, Oxygen becomes almost negligible at a height of 120 km, while CO₂ and water vapour are found only up to 90 km from the Earth's surface Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.66.

Category	Characteristics	Examples
Permanent Gases	Fixed proportions; constant ratio to total volume.	Nitrogen, Oxygen, Argon, Neon, Helium, Hydrogen.
Variable Gases	Concentration varies by time, location, and altitude.	Water Vapour, CO₂, Methane, Ozone, Nitrous Oxide.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.271; Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.66; Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), World Climate and Climate Change, p.96

3. Atmospheric Structure: Homosphere and Heterosphere (intermediate)

When we look at the atmosphere from a chemical perspective, we categorize it into two primary regions based on how well its constituent gases are mixed: the Homosphere and the Heterosphere. This classification is vital because it explains why the air we breathe at sea level is so different from the thin, layered gases found at the edge of space.

The Homosphere extends from the Earth's surface up to an altitude of approximately 80 km. In this region, the atmosphere is a "homogeneous" or uniform blend of gases. Even though the air becomes much thinner (less dense) as you go up, the proportion of major gases remains remarkably constant—roughly 78.08% Nitrogen (N₂), 20.95% Oxygen (O₂), and 0.93% Argon (Ar) Physical Geography by PMF IAS, Earths Atmosphere, p.270. This uniformity is maintained by turbulent mixing and convection currents that act like a giant blender, preventing the gases from settling. However, there are minor exceptions: water vapor and pollutants vary near the surface, and the Ozone layer creates a concentrated belt of O₃ between 19–50 km Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.7.

Above 80 km, we enter the Heterosphere, where the rules of the game change. At these extreme altitudes, the air is so thin that gas molecules rarely collide, meaning there is no longer enough turbulence to keep them mixed. Instead, gravity takes over, sorting the gases into distinct layers based on their atomic weight—a process known as diffusive separation. Heavier gases settle at the bottom of this region, while lighter ones float to the top Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6. The layering typically follows this ascending order of lightness:

• Molecular Nitrogen (N₂): The heaviest, at the base of the heterosphere.
• Atomic Oxygen (O): Dominant up to about 120 km, though it becomes negligible soon after Physical Geography by PMF IAS, Earths Atmosphere, p.272.
• Helium (He): Found in the higher reaches.
• Hydrogen (H): The lightest element, forming the outermost fringe of the atmosphere in the exosphere.

Sources: Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6-7; Physical Geography by PMF IAS, Earths Atmosphere, p.270-272

4. Noble Gases in the Atmosphere (intermediate)

In our atmosphere, most gases are busy reacting with one another—oxygen supports combustion, and carbon dioxide is absorbed by plants. However, there is a special group of elements known as Noble Gases (or inert gases) that rarely join the party. Chemically, these elements belong to Group 18 of the periodic table. Their defining feature is a stable electronic configuration, typically having a complete octet (8 electrons) in their outermost shell, which makes them highly unreactive Science, Class X (NCERT 2025), Metals and Non-metals, p.47. Because they do not easily form compounds, they remain in the atmosphere as individual atoms.

While we often focus on Nitrogen and Oxygen, Argon (Ar) is actually the third most abundant gas in the atmosphere, making up about 0.93% of dry air Physical Geography by PMF IAS, Earth's Atmosphere, p.270. The other noble gases—Neon, Helium, Krypton, and Xenon—exist in much smaller "trace" amounts. These are classified as permanent atmospheric gases because their proportions remain fixed relative to the total volume of the atmosphere over long periods, unlike variable gases like water vapor or CO₂ which fluctuate based on location and time Physical Geography by PMF IAS, Earth's Atmosphere, p.271.

Interestingly, the presence of these gases is a result of Earth's long history of degassing, where gases were released from the interior during volcanic eruptions and cooling Fundamentals of Physical Geography, Class XI (NCERT 2025), The Origin and Evolution of the Earth, p.15. For instance, most of the Argon in our air comes from the radioactive decay of Potassium-40 in the Earth's crust. Understanding their relative concentrations is vital for scientific precision in atmospheric studies.

Noble Gas	Approx. % by Volume	Status
Argon (Ar)	0.93%	Major Component
Neon (Ne)	0.002%	Trace Gas
Helium (He)	0.0005%	Trace Gas
Xenon (Xe)	0.00009%	Trace Gas

Sources: Science, Class X (NCERT 2025), Metals and Non-metals, p.47; Physical Geography by PMF IAS, Earth's Atmosphere, p.270-271; Fundamentals of Physical Geography, Class XI (NCERT 2025), The Origin and Evolution of the Earth, p.15

5. Light Gases and Atmospheric Escape (intermediate)

Pillars of our atmosphere like Nitrogen and Oxygen are heavy enough to stay tethered to Earth, but the lightest gases—specifically Hydrogen (H₂) and Helium (He)—exist in a constant state of transition. Think of our atmosphere as a layered cake sorted by gravity. In the heterosphere (beginning around 80 km altitude), gases are no longer mixed uniformly but are layered by weight. The lightest elements float to the top, reaching the exosphere, where they are only weakly bonded by gravity Environment and Ecology, Majid Hussain, p.6. At these heights, the atmosphere is so rarefied it is nearly a vacuum, and individual atoms can easily be lost forever. This loss is known as atmospheric escape or atmospheric stripping. It occurs when gas molecules achieve escape velocity—the speed needed to break free from a planet's gravitational pull. For light gases, this energy is often provided by the solar wind (a stream of charged particles from the sun) or simply by the heat energy the atoms absorb. Historically, Earth’s primordial atmosphere was dominated by Hydrogen and Helium, but this early layer was almost entirely stripped away by solar winds during the Earth's early evolution Fundamentals of Physical Geography, NCERT Class XI, p.15. Today, these gases remain only in trace amounts, though they are still being lost to space even now. Fortunately, Earth has a critical defense mechanism: the magnetic field. This field acts as a shield, deflecting the high-energy solar wind and significantly reducing the rate at which Hydrogen is stripped away Physical Geography, PMF IAS, p.280. Despite this protection, the concentrations of these light and noble gases remain extremely low in our current atmosphere. As shown in the table below, even among the trace gases, there is a clear hierarchy of abundance:

Gas	Approximate Concentration (%)
Neon (Ne)	0.002%
Helium (He)	0.0005%
Xenon (Xe)	0.00009%
Hydrogen (H₂)	0.00005%

Sources: Environment and Ecology, Majid Hussain, Basic Concepts of Environment and Ecology, p.6; Physical Geography, PMF IAS, Earth's Atmosphere, p.280; Fundamentals of Physical Geography, NCERT Class XI, The Origin and Evolution of the Earth, p.15

6. Ranking Trace Permanent Gases (exam-level)

When we look at the composition of Earth's atmosphere, we usually focus on the giants: Nitrogen (78.08%) and Oxygen (20.95%). However, a small but critical portion of our air consists of permanent gases whose proportions remain remarkably constant over time and space in the lower atmosphere. Beyond the well-known Argon (0.93%), there are several "trace" gases—mostly noble gases—that exist in microscopic quantities. Understanding their relative abundance is a frequent challenge in competitive exams like the UPSC.

These trace gases are measured in tiny percentages by volume. Among the most common ones encountered in geography and science are Neon, Helium, Krypton, Xenon, and Hydrogen. While they are all "rare," their concentrations vary significantly from one another. For instance, Neon is roughly 40 times more abundant than Hydrogen in our atmosphere. These gases are chemically inert (with the exception of Hydrogen, though it behaves as a permanent gas in its atmospheric distribution) and are distributed uniformly through the homosphere due to atmospheric mixing and convection Environment and Ecology by Majid Hussain, Basic Concepts of Environment and Ecology, p.6.

Gas Name	Symbol	Percentage by Volume	Relative Ranking (among trace gases)
Neon	Ne	0.002%	Highest (of this group)
Helium	He	0.0005%	High
Xenon	Xe	0.00009%	Low
Hydrogen	H₂	0.00005%	Lowest

To rank these gases correctly, especially in an ascending order (from least to most abundant), we start with the rarest. Hydrogen sits at the bottom of the list, followed by Xenon. We then move up to Helium, and finally Neon, which is the most abundant among these specific four trace constituents Physical Geography by PMF IAS, Earth's Atmosphere, p.271. Note that while Helium is the second lightest element in the universe, its light mass allows it to escape into space more easily than heavier noble gases, which partly explains its low concentration compared to its cosmic abundance.

Sources: Physical Geography by PMF IAS, Earth's Atmosphere, p.271; Environment and Ecology by Majid Hussain, Basic Concepts of Environment and Ecology, p.6; Fundamentals of Physical Geography, Geography Class XI (NCERT), Composition and Structure of Atmosphere, p.66

7. Solving the Original PYQ (exam-level)

Now that you have mastered the Composition and Structure of the Atmosphere, this question acts as the ultimate test of your precision. While you know that Nitrogen and Oxygen make up 99% of the air, UPSC often probes the remaining 1%—the permanent trace gases. As detailed in NCERT Class 11 Geography (Fundamentals of Physical Geography), these gases remain in fixed proportions. To solve this, you must apply your knowledge of volumetric concentration to the four noble gases listed, transitioning from general concepts to specific data points.

To arrive at the correct answer, we must arrange the gases in ascending order (from the least concentrated to the most concentrated). Think through the decimal placements: Hydrogen (2) is the most scarce at 0.00005%, followed by Xenon (4) at 0.00009%. As we move up the scale, Helium (1) occupies a larger share at 0.0005%, and Neon (3) is the most abundant of this specific group at approximately 0.0018% (often rounded to 0.002%). This logical progression (2 → 4 → 1 → 3) confirms that Option (C) is the correct choice.

The primary trap in this question lies in Option (D), which switches the positions of Neon and Helium. UPSC frequently uses numerical proximity to confuse candidates; because both gases are often discussed together in chemistry, it is easy to forget that Neon is actually about four times more abundant than Helium in our atmosphere. Always be wary of the sequence—ascending means smallest to largest—and remember that for permanent gases, the specific volumetric hierarchy is a non-negotiable fact you must memorize to avoid these common distractor traps.