Is Kryptonite Real? Krypton, the Noble Gases and One Very Famous Rock

No, kryptonite is fiction, but krypton is a real noble gas, element 36, found in air at about 1 part per million. Here is what it really does.

No, kryptonite is not real. It was invented for Superman stories, and it is not an element, a mineral or anything else on the periodic table. Krypton, though, is completely real: a colourless, odourless gas with atomic number 36, drifting around in the air you are breathing right now. It is just far less dramatic than a glowing green rock, and it has never once weakened a superhero.

That is the short answer. The long answer involves a hunt for something hidden and a ruler made of light. Settle in.

What is krypton?

Krypton has the symbol Kr and sits at atomic number 36. The atomic number is simply the number of protons, the positively charged particles in the centre of every atom, and it is what decides which element you have. Our guide to atoms has the full tour. Krypton sits in group 18, the far right-hand column of the table, which is home to the noble gases.

At room temperature it is a gas with no colour, no smell and no taste, so you would never know it was there. Pack it into a bottle and it is about three times heavier than the same bottle of air. Here are the numbers from the site's data, with the kelvin figures converted to degrees Celsius:

• Atomic mass: about 83.8.
• Melting point: 115.78 K, which is minus 157.37 °C.
• Boiling point: 119.93 K, which is minus 153.22 °C.
• Density as a gas: about 3.75 grams per litre, compared with about 1.2 for ordinary air.

Those two points are only about four degrees apart, so krypton is a liquid for the chemical equivalent of a very short lunch break.

The element sits between argon above it and xenon below it. If you ever want a side-by-side look, there is an argon versus krypton comparison page.

Where did the name krypton come from?

The name comes from the Greek word kryptos, meaning hidden. Fair enough: it was hiding in plain sight.

By the 1890s chemists had found that air was not just oxygen and nitrogen. In 1894 Lord Rayleigh and William Ramsay turned up argon, an odd leftover gas that refused to react with anything. The history of the periodic table shows why a whole new column was needed.

In 1898 Ramsay and his assistant Morris Travers set out to see whether more gases were lurking in air. They cooled air until it became a liquid, then let it warm up slowly, so that the different gases boiled off one at a time. This trick is called fractional distillation: each gas turns back to vapour at its own boiling point, so you can collect them in turn, like people leaving a party one by one as their taxis arrive.

Once the oxygen, nitrogen and argon had gone, a small residue was left. When they examined it, it gave out light with bright lines of colours that no known element made. A new gas, then, and it had been hiding the whole time. They called it krypton. The same method soon turned up neon and xenon too.

How much krypton is there in the air?

Very little. Krypton makes up roughly 1 part per million of the air by volume. That means that if you could line up a million air molecules, about one of them would be krypton. Imagine a stadium of a million fans where only one is wearing a krypton scarf: you would struggle to spot them.

Here is how the noble gases compare in dry air. These percentages and parts per million are rounded, and the exact values depend on the source, so treat them as approximate.

Boiling points are converted from kelvin by subtracting 273.15. Going down the column, each gas is heavier and boils at a higher temperature.

Because air has so little krypton in it, the gas is collected as a by-product when factories separate air into oxygen and nitrogen for industry. Nobody mines it.

Why is there so much more argon than krypton?

This is a lovely puzzle. Argon is several thousand times more common in air than krypton, even though they are next-door neighbours in the same column with very similar habits. What happened?

The big reason is that argon had a second source. Most of the argon in our atmosphere was made inside the Earth.

Potassium has a radioactive version, potassium-40, hiding in rocks. (Atoms of the same element with different numbers of neutrons are called isotopes, as our article on relative atomic mass and isotopes explains.) Over billions of years some of it has slowly decayed into argon-40, which then seeped out of the rocks and into the sky.

Krypton has no comparable big source, so what we have is mostly a trace left over from when the planet formed.

So if you ever wonder why the air is nearly one per cent argon, the answer is that the planet is a very slow, very patient argon factory. Krypton just watches.

What is krypton used for?

Quite a lot, for such a shy gas. It is not a household name, but you may have met it without noticing.

Lighting and photography flashes

Krypton is used in some lamps, where a gas fill can slow the wearing away of a hot filament or glow when electricity passes through it. It is also used in some high-speed photography flashes, where a burst of light has to be very short and very bright.

Lasers

Krypton takes part in some lasers. A laser is a device that makes a narrow beam of light of a single colour. Certain lasers use krypton, and a few of the ultraviolet ones used in precise manufacturing combine it with fluorine.

Measuring the metre

This is the best party trick. For 23 years, from 1960 to 1983, the metre was officially defined using krypton. The definition said a metre was 1,650,763.73 wavelengths of a particular orange-red light given out by krypton-86 atoms. The wavelength is the distance from one crest of a light wave to the next, so in effect scientists were using light waves as a ruler, and krypton-86 gave the ruler its notches.

The definition changed in 1983. A metre is now defined as the distance light travels in a vacuum in 1/299,792,458 of a second, a figure that is easier to reproduce around the world. Krypton was retired with, presumably, a small cake.

So is kryptonite real?

No. Kryptonite is a made-up substance from Superman stories, where it is the remains of Superman's destroyed home planet, which was called Krypton. It first appeared in the 1940s, as a handy weakness for an almost invincible hero. The planet and the chemical element share a name, but they are not related in any way that chemistry cares about.

Kryptonite is not an element, because there is no kryptonite on the periodic table, and it is not a real compound of krypton either. Krypton is very reluctant to join anything, as we are about to see.

Why are the noble gases called noble?

The word comes from old ideas about metals. Gold was called a "noble metal" because it hardly reacts with other things, staying shiny and unchanged while other metals rusted and tarnished. The aloof behaviour reminded people of nobility too grand to mingle with the crowd. When chemists found a whole column of gases that refused to react, the name was borrowed. They are also called the inert gases, though "inert" turned out to be slightly too strong.

The reason is the electrons. Electrons are the tiny charged particles that whizz around the centre of an atom, and chemistry is almost entirely about swapping, sharing and rearranging them. They sit in layers called shells. The outermost layer is the one that matters, because those outer electrons do the joining, as the valence electrons article explains.

A noble gas has a full outer shell. That is a very stable arrangement, a bit like a jigsaw with every piece in place. Helium has two electrons, which fills its only shell. Neon, argon, krypton, xenon and radon each have eight in their outermost shell. Atoms of other elements react because they are trying to reach that tidy state by losing, gaining or sharing electrons. A noble gas is already there, so it has no reason to join in. Our electron configuration article shows how shells are filled, and the noble gases article covers the whole family in more detail.

So krypton exists as lone atoms, not pairs like oxygen or nitrogen: a very polite person at a bus stop with no intention of making conversation.

Can krypton make compounds?

Yes, but only if you push it. For decades the noble gases were treated as completely unreactive, and then in 1962 a chemist made a compound of xenon, and the textbooks had to be changed. Krypton joined the list in the 1960s too.

The compound is krypton difluoride, written KrF₂. It joins krypton to fluorine, which is the most reactive element of all and so is the one most likely to persuade a noble gas to cooperate. You do not find it in nature or on a shelf. It has to be made in a laboratory under extreme conditions, including very low temperatures, and it breaks down easily as soon as things warm up.

Why krypton, and not helium or neon? The answer is size. Krypton atoms are bigger, so their outer electrons are further from the centre and held less tightly, and a very hungry atom such as fluorine can just about pull on them. In helium and neon the outer electrons sit so close and are held so firmly that no ordinary chemical method has managed it. That is the pattern: the heavier the noble gas, the more willing it is to bond, and xenon is the most cooperative of the common ones.

So "inert" is not quite right for krypton, but "stubborn" is.

Try it on Periodic Bubbles. Open the bubbles and find krypton. The whole noble gas family shares one colour on the bubbles view, so it is easy to spot in the far right-hand column. Then pop argon and xenon and compare. Where is the "kryptonite" bubble? There is none, which is rather the point.

Frequently asked questions

Is kryptonite a real element?

No. It was invented for Superman stories and does not appear on the periodic table. Krypton is the real element with a similar name.

Is krypton dangerous?

Krypton is not poisonous and does not burn. Like any gas that is not oxygen, though, it can push the air out of a closed space, so it should be handled sensibly and never in a sealed room. In practice you are far more likely to meet it sealed inside a lamp.

Does krypton glow?

When electricity passes through krypton gas in a sealed tube, it gives out light, like the other noble gases. The exact colours and why they differ are a story of their own, so we will leave that to the neighbouring articles.

Why is krypton so rare?

It is a heavy atom, and the universe generally makes less of the heavier elements, and Earth has no big ongoing source of it. Argon, by comparison, keeps being made by the slow decay of potassium-40 in rocks.

Fancy testing yourself? Take the Pop Quiz, or see which element turns up in today's Daily Bubble. No capes required.