Argon and the Other Noble Gases in Air

Introduction

After nitrogen and oxygen, the third most common gas in air is not carbon dioxide but argon. Together with tiny amounts of neon, helium, krypton and xenon, it makes up the unreactive "last one per cent" of dry air. For over a century after air was first studied, nobody noticed these gases at all. On this page you will see why they went unnoticed, why they are so unreactive, and how we now put them to work.

Core explanation

How much is there? By volume, dry air contains approximately:

Noble gas Symbol Percentage in dry air --- --- --- Argon Ar 0.93% Neon Ne 0.0018% Helium He 0.0005% Krypton Kr 0.0001% Xenon Xe less than 0.00001%

So argon is by far the most important noble gas in air: about 1 litre in every 107 litres of air is argon. The others are present only as traces.

Why they are unreactive. The noble gases are the elements of Group 0 (also called Group 18) of the Periodic Table. Their atoms have a full outer shell of electrons — two for helium, eight for the others. A full outer shell is a very stable arrangement, so these atoms have no tendency to gain, lose or share electrons. That is why they do not form bonds with each other and exist as separate single atoms: they are monatomic (Ar, not Ar₂). It is also why they hardly react with anything else.

Discovery. Because the noble gases do not react, the usual chemical tests of the eighteenth and nineteenth centuries missed them. Argon was discovered in 1894 when Lord Rayleigh noticed that nitrogen extracted from air was slightly denser than nitrogen made from chemical compounds. William Ramsay showed that the "air nitrogen" contained a new, heavier gas. The name argon comes from the Greek for "lazy".

Physical properties. All the noble gases are colourless, odourless gases at room temperature, with very low boiling points. Their densities increase down the group: helium is much less dense than air, while argon, krypton and xenon are denser than air.

Uses. Their lack of reactivity is exactly what makes them useful:

- Argon provides an inert atmosphere . It fills ordinary filament light bulbs so that the hot tungsten filament does not react; it shields hot metal during welding so it does not oxidise; and it fills the gap in some double-glazed windows because it conducts heat poorly. - Neon glows red-orange when electricity passes through it, which gives the classic "neon sign". - Helium has a very low density and cannot burn, so it is safe for filling balloons and airships. Liquid helium cools the superconducting magnets in MRI scanners. - Krypton and xenon are used in some high-intensity lamps, such as photographic flash tubes and some vehicle headlamps.

Where they come from. Argon, neon, krypton and xenon are obtained commercially by the fractional distillation of liquid air. Helium is present in air in such small amounts that it is extracted instead from some natural gas deposits.

Step-by-step reasoning

To explain a use of a noble gas in an exam answer:

1. Name the property the use depends on (unreactive, low density, glows in electric discharge, poor conductor of heat). 2. Link the property to the structure: a full outer shell makes the atoms unreactive. 3. Explain what would go wrong if a reactive gas such as oxygen were used instead.

Visual explanation

Draw an argon atom as a nucleus with three circles of electrons: 2, 8 and 8. The outer ring is completely filled, like a jigsaw with no missing pieces. Next to it draw an oxygen atom with only six outer electrons and two gaps. Oxygen's gaps make it grab electrons from other atoms; argon has no gaps, so it has nothing to gain.

Real-world analogy

A noble gas atom is like a person who already owns everything they want: offers to trade or swap leave them uninterested. Oxygen is the keen collector with gaps in the collection, always looking for a deal. That difference in "wanting" explains the difference in reactivity.

Real-world example

In a welding workshop, a stream of argon gas flows around the electric arc and the molten metal. Without it, the white-hot metal would react with oxygen and nitrogen in the air, making the weld weak and brittle. The argon simply keeps the air away and takes no part in the process.

Why?

Why was argon discovered so late when there is more of it in air than carbon dioxide? Carbon dioxide reacts with limewater and so was easy to detect. Argon reacts with nothing used in ordinary tests, so it was hidden inside the "nitrogen" left after oxygen had been removed. Only a careful density measurement revealed it.

Common misconception

"Noble gases are rare, so they must be expensive and unimportant." Argon is not rare at all — it is almost 1% of the atmosphere, and it is produced cheaply in huge amounts as a by-product of making oxygen and nitrogen from air.

Worked example

Question: A room has a volume of 50 m³. Estimate the volume of argon in the air in the room.

Reasoning: Argon is about 0.93% of dry air. Volume of argon = 0.93 ÷ 100 × 50 = 0.465 m³.

Answer: About 0.47 m³ of argon, nearly half a cubic metre.

Quick check

1. Why are the noble gases so unreactive? Answer: Their atoms have full outer shells of electrons, a very stable arrangement, so they do not need to gain, lose or share electrons.

Exam focus

Learn that argon is about 0.9% (roughly 1%) of dry air. For each use, give the matching property — "unreactive" for light bulbs and welding, "low density and non-flammable" for helium balloons, "glows when electricity passes through" for neon signs. Say that noble gases are monatomic.

Advanced insight

"Unreactive" is not quite the same as "completely inert". In 1962 Neil Bartlett made the first noble gas compound, containing xenon, and several xenon and krypton fluorides and oxides are now known. The larger atoms hold their outer electrons less tightly, which is why only the heavier noble gases form compounds.

Summary

Argon makes up about 0.93% of dry air; neon, helium, krypton and xenon are present only in traces. These Group 0 gases have full outer electron shells, so they are monatomic and very unreactive. That unreactivity makes them ideal for providing inert atmospheres, while other properties give uses in lighting, balloons and cooling. Most are obtained from liquid air.

Practice questions

1. Which noble gas is the most abundant in air, and roughly what percentage of dry air is it? Answer: Argon, about 0.9% (almost 1%). 2. Why is argon used to fill filament light bulbs? Answer: It is unreactive, so the very hot tungsten filament does not react with it, whereas it would burn away in oxygen. 3. Give two reasons why helium, rather than hydrogen, is used in party balloons. Answer: Helium has a low density so the balloon floats, and it is non-flammable, whereas hydrogen can burn explosively. 4. Explain what "monatomic" means and why noble gases are monatomic. Answer: It means existing as single atoms; noble gas atoms have full outer shells, so they do not form bonds with each other.