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Krypton and xenon: what’s the difference?

What’s the deal with krypton and xenon? And why are millions of liters of these colorless, odorless, (mostly) inert rare gases bought and used each year? Let’s dig in.

Thruster-Xenon-Gas
This photo is a Hall thruster using xenon. It was provided by Mitchell Walker of Georgia Institute of Technology.

Krypton and xenon are both noble gases (also known as rare gases). If you remember chemistry from high school, this puts them in the group of elements to the right of the periodic table. Just above them are helium, neon, and argon. And just below are radon and oganesson.

All of these gases are colorless and odorless. For the most part, these elements don’t take part in chemical reactions or form compounds. Krypton and xenon will form some compounds like KrCl₂ or XeF₂, but these compounds tend to be reactive and not particularly stable. This reactive property makes XeF₂ useful as a vehicle for providing fluorine to desired sites in semiconductor manufacturing.

Some compounds are only stable in the excited state (KrCl, XeF, etc.) and are known as exciplexes. These exciplexes are formed and break apart repeatedly in excimer lasers. Xenon will also bind to the protein metmyoglobin, and it acts as an anesthetic when it does. The (mostly) non-reactive property of krypton and xenon is useful for space propulsion, as it reduces the concern that the expelled propellant will react with the spacecraft.

Both krypton and xenon are found in the atmosphere. In fact, you are breathing some right now. Both are produced by extraction from the air using cryogenic distillation through the same collection process in a roughly 10 to 1 ratio, krypton to xenon. They are found in the atmosphere at that ratio.

Krypton is found in air at about 1 ppm and xenon is in the air at less than 100 ppb. With these low concentrations, it is easy to understand the classification as “rare gases.” The more abundant krypton is less expensive than xenon. Would you believe that krypton has historically been about 1/10 the price of xenon? Ignoring the frequent volatility of the krypton and xenon markets and, of course, the laws of supply and demand apply.

Compared to other noble gases, krypton and xenon are heavy atoms. Krypton has a mass of 84 amu, and xenon has a mass of 131 amu, which makes xenon the heaviest of all the stable noble gases (radon undergoes radioactive decay, with a half-life of less than 4 days). This heavier mass comes into play with sputter deposition. Argon is most frequently used for sputtering, transferring target atoms to a substrate to create a coating. It is cheap and works best for coating with light target atoms, but a higher yield is obtained when the inert gas has a similar atomic mass to the target atom.

Krypton and xenon are used for heavier coatings like titanium. The mass is also important when etching semiconductor materials. The higher mass of xenon also contributes to making it more efficient to use for electric propulsion of spacecraft. In this application, the xenon is ionized and ejected from a spacecraft at hundreds of kilometers per second. Following Newton’s third law of motion, with every action having an equal and opposite reaction, at these high velocities, ejecting a very small amount of mass can generate useful thrust. We sell xenon to NASA for this application.

Krypton can also be used for space propulsion, but due to the lower mass, almost twice as many atoms would be needed as compared to xenon. If atoms lighter than krypton are used, still more atoms need to be ionized and ejected to generate the same thrust. The large masses also lead to slower diffusion of both gases. This slower diffusion makes for better insulation, slower transfer of heat when used in double (or triple) pane windows or in lightbulbs. Both insulate better than air or argon.

Ionization energy is defined as the energy required to remove an electron from a neutral atom. Noble gases tend to be very stable and don’t give up electrons easily. The group of elements is sometimes referred to as inert gases due to this stability. We know this is not completely true, as we noted above that some compounds can be formed with them. The reactivity of the noble gases increases and the ionization energy decreases as one moves down the periodic table. The ionization energy of krypton is less than that of the lighter noble gases (He, Ne, Ar), and xenon is less than that of krypton.

The ionization energy affects the efficiency of electric propulsion, as before being ejected from the thruster, the krypton or xenon needs to be ionized. The lower ionization energy contributes to xenon being the more efficient propellant compared to krypton and other higher ionization energy atoms, as a larger portion of the energy used goes into ejecting the ions (besides needing to ionize fewer atoms).

Ionization energy also affects the reactivity of the ions. With higher ionization energy, ions produced from krypton atoms are more aggressive—taking electrons and degrade other substances, like the wall of a thruster. The properties of these ions are also important in plasmas employed in semiconductor manufacturing. Due to their lower ionization energies, compared to halocarbons, xenon and krypton are used with halocarbons to influence the etching rates of silicon nitrides, silicon oxides, and polysilicon films. These are critical for manufacturing the memory chips in solid-state drives.

Also, xenon and krypton act both as surface disruption agents and plasma modulators, influencing the composition of plasmas through secondary ionization. Historically, secondary ionization and surface disruption was achieved using argon due to cost and availability, but xenon and krypton provide the ability to select for ions present in plasma in additional ways.

As you move down the periodic table through the noble gases – starting with helium – each gas is less like a (theoretical) ideal gas. Helium is most like an ideal gas, and krypton and xenon are quite different from an ideal gas. Both gases compress significantly at higher pressures and xenon is more compressible than krypton.

At 1000 psi and room temperature, xenon is a supercritical fluid. Drop the temperature a bit, and the xenon liquifies. This compressibility makes xenon the better gas for use in spacecraft (beyond the effects of atomic mass and ionization energy mentioned above). By compressing the gas into a small space, the propulsion system can be smaller and lighter, making larger payloads and smaller spacecraft possible. Using xenon electric propulsion made it possible for NASA’s Dawn spacecraft to visit not one, but two asteroids! This longer mission would not have been possible using conventional chemical propellants.

So, you can see there is a lot to value from these colorless, odorless, and useful gases. If you have any questions, please let us know.