The noble gases krypton and xenon are similar (also known as rare gases). This places them in the group of elements to the right of the periodic table, if you remember chemistry from high school. Helium, neon, and argon are just above them. Below that are radon and oganesson.
These gases are all odourless and colourless. These elements, for the most part, do not participate in chemical reactions or form compounds. Krypton and xenon can combine to form compounds such as KrCl2 or XeF2, but these are highly reactive and unstable. Because of its reactive nature, XeF2 can be used to deliver fluorine to specific locations in semiconductor manufacturing.
Exciplexes are compounds that are only stable in the excited state (KrCl, XeF, etc.). In excimer lasers, these exciplexes are formed and broken apart repeatedly. When xenon binds to the protein metmyoglobin, it produces an anaesthetic effect. Krypton and xenon are useful for space propulsion because their (mostly) non-reactive properties reduce the risk of the expelled propellant reacting with the spacecraft.
In the atmosphere, both krypton and xenon can be found. You are, in fact, breathing some right now. In a roughly 10 to 1 ratio, krypton and xenon are extracted from the air using cryogenic distillation through the same collection process. At that ratio, they can be found in the atmosphere.
Krypton is found in the air at a concentration of about 1 ppm, while xenon is found at a concentration of less than 100 ppb. With such low concentrations, it's easy to see why they're classified as "rare gases." Krypton is less expensive than xenon because it is more abundant. Would you believe that historically, krypton was about a tenth of the price of xenon? Ignoring the krypton and xenon markets' frequent volatility, and the laws of supply and demand, of course.
Krypton and xenon are heavy atoms when compared to other noble gases. Krypton has an atomic mass of 84 amu, while xenon has an atomic mass of 131 amu, making it the heaviest of the stable noble gases (radon undergoes radioactive decay, with a half-life of less than 4 days). With sputter deposition, this heavier mass comes into play. The most common application of argon is sputtering, which involves transferring target atoms to a substrate to create a coating. It's inexpensive and best for coating light target atoms, but it produces a higher yield when the inert gas has a similar atomic mass to the target atom.
For heavier coatings, such as titanium, krypton and xenon are used. When etching semiconductor materials, mass is also important. The higher mass of xenon contributes to its efficiency as an electric propulsion material for spacecraft. The xenon is ionised and ejected from a spacecraft at hundreds of kilometres per second in this application. Following Newton's third law of motion, which states that every action has an equal and opposite reaction, ejecting a small amount of mass at these high velocities can generate useful thrust. For this application, we sell xenon to NASA.
Krypton can also be used for space propulsion, but it requires nearly twice as many atoms as xenon due to its lower mass. To generate the same thrust with atoms lighter than krypton, more atoms must be ionised and ejected. Both gases diffuse more slowly due to the large masses. When used in double (or triple) pane windows or lightbulbs, this slower diffusion results in better insulation and a slower transfer of heat. Both are more insulating than air or argon.
The energy required to remove an electron from a neutral atom is known as ionisation energy. Noble gases are extremely stable and do not readily give up electrons. Because of this stability, the group of elements is sometimes referred to as inert gases. We know this isn't entirely true because, as we mentioned earlier, some compounds can be made with them. As one moves down the periodic table, the reactivity of noble gases increases while the ionisation energy decreases. Krypton has a lower ionisation energy than the lighter noble gases (He, Ne, Ar), and xenon has a lower ionisation energy than krypton.
Because krypton or xenon must be ionised before being ejected from the thruster, the ionisation energy has an impact on the efficiency of electric propulsion. Because xenon has a lower ionisation energy than krypton and other atoms with higher ionisation energies, a larger portion of the energy used goes into ejecting the ions, making it a more efficient propellant (besides needing to ionise fewer atoms).
The reactivity of the ions is also affected by the ionisation energy. Ions produced by krypton atoms have a higher ionisation energy and are more aggressive in taking electrons and degrading other substances, such as the wall of a thruster. The properties of these ions are also crucial in semiconductor manufacturing plasmas. Xenon and krypton are used with halocarbons to influence the etching rates of silicon nitrides, silicon oxides, and polysilicon films because they have lower ionisation energies than halocarbons. These are necessary for the production of solid-state drive memory chips.
Furthermore, xenon and krypton act as surface disruption agents as well as plasma modulators, influencing plasma composition through secondary ionisation. Due to cost and availability, argon has traditionally been used for secondary ionisation and surface disruption, but xenon and krypton provide additional ways to select for ions present in plasma.
Each noble gas, starting with helium and moving down the periodic table, becomes less like a (theoretical) ideal gas. Helium is the closest gas to an ideal gas, while krypton and xenon are far from it. At higher pressures, both gases compress significantly, but xenon is more compressible than krypton.
Xenon is a supercritical fluid at 1000 psi and room temperature. The xenon liquefies when the temperature is reduced. Because of its compressibility, xenon is a better gas for use in spacecraft (beyond the effects of atomic mass and ionisation energy mentioned above). The propulsion system can be smaller and lighter by compressing the gas into a small space, allowing for larger payloads and smaller spacecraft. NASA's Dawn spacecraft was able to visit not one, but two asteroids thanks to xenon electric propulsion! Using conventional chemical propellants, this longer mission would not have been possible.