Argon makes up 1% of the air.
Why did it take over 100 years to find it? ― Nitrogen's weight was off by just 0.5%
Argon is a gas that bonds with nothing. So it slipped straight past every chemical test. What finally gave it away was a tiny mismatch — nitrogen that should have been identical came out about one part in 200 too heavy.
At school you learn that air is roughly 80% nitrogen and 20% oxygen. Add them up and you get nearly 100%. It feels like that must be everything.
Yet carbon dioxide, the gas we hear about constantly in the news, makes up only about 0.04% of the air. So what's the remaining 1%? The answer is a gas called argon — more than 20 times as much as the carbon dioxide in every breath you take.
Despite being so abundant, argon wasn't discovered until 1894 — even though serious research into air had been going on for over a century before that. Why did nobody notice it?
Only two reasons it went unnoticed
Back then, the way to identify a gas was to react it with something else and see what formed. Argon doesn't burn and won't bond with anything. It simply never showed up in chemistry's net.
Once oxygen and moisture were stripped from air, whatever remained was labeled "nitrogen." Argon stayed mixed in at about 1%, and the only clue to tell it apart was a barely-there difference in weight.
The person who broke through both obstacles was Lord Rayleigh, a British physicist, through relentlessly careful weight measurements. Let's go through it step by step.
Why did chemical tests pass right over argon?
Chemists of the 18th and 19th centuries told gases apart by what they reacted with. Oxygen makes things burn. Carbon dioxide clouds limewater. The way a gas reacted was its name tag.
Argon has no such tag. Its outermost shell of electrons is already completely full. It has no need to trade electrons with other atoms, so it forms almost no compounds. As far as reaction-based tests were concerned, argon might as well not have existed.
In fact, there had been a clue much earlier. In 1785, the British scientist Cavendish sparked electricity through nitrogen in air to react it with oxygen and remove it. No matter what he did, a small bubble of gas always remained — recorded as less than 1/120th of the original volume. But nobody chased down what that bubble actually was, and more than a century slipped by.
How did weighing it finally reveal argon?
Around 1890, Lord Rayleigh was working on weighing gases as precisely as possible. He prepared nitrogen two different ways: one by stripping oxygen and moisture out of air, the other by breaking down compounds such as ammonia.
Both should have been "pure nitrogen." But when he compared equal volumes, the nitrogen taken from air was always about 0.5% heavier (Figure 1, left). Changing the measuring method, changing which compound he started from — the gap never went away.
A one-in-200 discrepancy could easily have been written off as measurement error. Rayleigh refused to do that. In 1892 he wrote to a science journal asking whether anyone could explain the gap. The chemist Ramsay took up the challenge.
Together they reacted nitrogen taken from air as thoroughly as they could, using heated magnesium and electric sparks to strip everything reactive away. What remained — about 1% — simply would not react. When they split its light into a spectrum, it showed lines that matched no known element. In 1894 the pair announced a new element, naming it after the Greek word for "lazy."
Argon's discovery revealed that an entire family of unreactive gases was missing from the periodic table. Ramsay went on to extract helium from a mineral on Earth, and in 1898, by slowly evaporating liquefied air, found neon, krypton, and xenon. In 1904, Rayleigh received the Nobel Prize in Physics and Ramsay the Nobel Prize in Chemistry.
Most of the argon in the air is thought to have formed as radioactive potassium in rocks slowly transformed over billions of years. It has been seeping out from deep inside the Earth, through volcanoes and other routes, and accumulating ever since. Because it reacts with nothing, once it reaches the air it simply stays there.
Summary
Because argon reacts with nothing, it slipped past chemical testing for over a hundred years. It was found because someone refused to ignore the fact that nitrogen, which should have been identical, was 0.5% off in weight. Not writing off a tiny discrepancy as "error" led to the discovery of an entire column of the periodic table.
1% of the air in your next breath is a gas with no name tag.
It wasn't a chemical reaction that found it — it was a scale.
For another case where a trace component of air has an outsized effect, see Carbon dioxide is only 0.04% of the air. Why does it change the temperature? For another story of accumulating evidence overturning conventional wisdom, see Why didn't scholars used to believe that meteorites fell from the sky?
- Fill a 500ml plastic bottle with water — think of this as "one breath."
- Using a measuring spoon, take out 5ml (one teaspoon) of water into a separate container — 1% of the total. That's roughly how much argon is in one breath.
- Now use a dropper or eyedropper to measure out about 0.2ml (a few drops) — that's roughly how much carbon dioxide is in one breath. Compare the two side by side.
A teaspoon versus a few drops. Both look like "barely anything," yet one is more than 20 times the other. That's the kind of quantity that stayed hidden for so long simply because it wouldn't react. One drop of water is roughly 0.05ml.
Want to go deeper? Terms, formulas, and how this fits the curriculumLabels show whether each part is middle-school level or university-level, all the way up
- MSCovered in middle-school science
- HSCovered in high-school "Chemistry Basics/Chemistry"
- HS+High-school advanced content, or textbook sidebar material
- UnivNot taught in high school — university-level inorganic chemistry/geochemistry
- ResearchNot yet settled even at university level — an active research question
MSTerms: this phenomenon has a name
- Argon: element 18. A colorless, odorless gas making up about 0.93% of air.
- Noble gases: helium, neon, argon, krypton, xenon, and others — the rightmost column of the periodic table. They form almost no compounds.
- Density: weight per fixed volume. At the same temperature and pressure, a gas made of heavier particles has a higher density.
MSHSCheck the numbers: does 1% argon really add up to 0.5% extra weight?
The "nitrogen" taken from air has argon mixed into it. From that mixture, we can calculate how much heavier the nitrogen should come out, and compare it to Rayleigh's measurements. The molecular weights in the table are relative weights of a single particle and have no unit.
| Molecular weight of nitrogen | ~28.0 |
| Atomic weight of argon | ~39.9 |
| Share of nitrogen in dry air | ~78.1% |
| Share of argon in dry air | ~0.93% |
| Density of nitrogen from air (Rayleigh's measurement) | said to be about 1.2572g per liter |
| Density of nitrogen from compounds (same) | said to be about 1.2505g per liter |
| Share of gas left as "nitrogen" (%) | 78.1 + 0.93 = 79.03 |
| Argon's share within that | 0.93 ÷ 79.03 ≒ 0.0118 |
| How much heavier argon is than nitrogen | 39.9 − 28.0 = 11.9 |
| Average weight added by the mixing | 11.9 × 0.0118 ≒ 0.14 |
| That as a share of nitrogen's weight | 0.14 ÷ 28.0 = 0.005 |
| Measured weight ratio | 1.2572 ÷ 1.2505 ≒ 1.0054 |
| Argon vs. carbon dioxide | 0.93 ÷ 0.04 ≒ 23 |
| Argon in one breath (500ml) | 500 × 0.0093 ≒ 4.7 |
The calculation gives 0.5%; the measurement gives about 0.54% — a close match. A 1% impurity only shows up as a 0.5% weight difference. That smallness is exactly why the discovery took so long.
| Symbol/term | Meaning and units |
| Molecular/atomic weight | Relative weight of one particle, with carbon set to 12 (no unit) |
| % | Share by volume (for gases, essentially the same as share by particle count) |
| Density | Grams per liter (compared at 0°C, 1 atmosphere) |
HSHS+Why don't noble gases react?
HSAn atom becomes stable once its outermost electron shell reaches a set number (often 8). Sodium and chlorine react readily because they give or take electrons to get closer to that state. Argon already starts with a full outer shell of 8 electrons, so it has no reason to exchange electrons with anything else.
HS+A gas's density being proportional to the weight of its particles follows from Avogadro's law: equal volumes of gas at the same temperature and pressure contain the same number of particles, regardless of the gas. Rayleigh's method used this law to weigh the average mass of particles too small to see.
UnivWhere argon comes from, and the limits of "unreactive"
Most atmospheric argon is the isotope with mass number 40, thought to form from the radioactive decay of potassium-40 in rocks. Across the universe as a whole, argon-36 is said to be more common, which shows that the composition of Earth's air records the planet's internal history. This decay is even used to date volcanic rock. That said, "completely unreactive" overstates things: since a xenon compound was made in 1962, compounds of the heavier noble gases have been known. For argon, only unstable compounds that survive at extremely low temperatures have been reported.
ResearchWhat's still unsettled
- How much argon has actually escaped from Earth's interior? Estimates of the split between an early burst right after Earth formed and slow release afterward vary depending on the mantle-mixing model used.
- Why do other planets' noble gases differ from Earth's? The noble-gas proportions in the atmospheres of Venus and Mars differ from Earth's, and are studied as clues to how planets gained or lost their atmospheres over time.
- Can a stable argon compound be made? Calculations suggest it might bond with other elements under high pressure, but experimental confirmation remains limited.
In other words, everything in this article reflects "the current state of understanding." In particular, when and how Earth's air formed is still being revised, with noble gases as a key clue.
How this maps to the curriculum (by level)
| Level | Subject/unit | Where in this article |
|---|---|---|
| MS | Science — properties of gases, composition of air | Proportions and densities of nitrogen, oxygen, argon |
| HS | Chemistry Basics — periodic table and electron configuration | Why noble gases don't react |
| HS+ | Chemistry — gas laws | Avogadro's law and comparing densities |
| Univ | Inorganic chemistry / geochemistry | Potassium-40 decay, noble-gas compounds |
| Research | Planetary science / outgassing of Earth's interior | Origin of atmospheric argon, differences between planets |
| ― | Everyday connection | What's in the air you breathe; not dismissing small discrepancies |
- The Nobel Foundation, 1904 Nobel Prize in Physics (Lord Rayleigh)
- The Nobel Foundation, 1904 Nobel Prize in Chemistry (Ramsay)
- Rayleigh, Lord and Ramsay, W. (1895) Argon, a New Constituent of the Atmosphere. Philosophical Transactions of the Royal Society of London A 186
- National Astronomical Observatory of Japan (国立天文台), ed., Chronological Scientific Tables (理科年表), composition of the atmosphere
- High-school "Chemistry Basics" (化学基礎) textbook, periodic table and electron configuration, properties of gases
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding of the underlying mechanisms. Historical measurements vary slightly between sources.