The Chemical Reactivity of Krypton
Krypton (Kr), a chemical element with atomic number 36, belongs to Group 18 of the periodic table, known as the noble gases. This classification provides a significant clue to its chemical behaviour. Elements in this group are characterized by having a complete outer electron shell, which is an exceptionally stable electronic configuration. This stability means krypton has very little tendency to gain, lose, or share electrons with other atoms.
Interaction with Water and Air
Krypton exhibits extremely low chemical reactivity under normal conditions.
- With Water: Krypton does not react chemically with water. It is sparingly soluble in water, meaning a small amount can dissolve physically, but no chemical bonds are formed. The gas remains as discrete krypton atoms within the water.
- With Air: Krypton is a trace component of Earth’s atmosphere, constituting approximately one part per million (ppm) by volume. It does not react with nitrogen, oxygen, argon, or any other gases present in the air under typical atmospheric conditions. Its presence in the atmosphere over India, like anywhere else, is as an unreactive constituent.
Safety Profile: Toxicity, Radioactivity, and Flammability
The safety aspects of krypton are generally favourable under most circumstances.
- Toxicity: Krypton is considered a non-toxic gas. It does not participate in biological processes and is chemically inert within the body. However, like any inert gas (e.g., nitrogen, helium), krypton can act as a simple asphyxiant in high concentrations by displacing oxygen in the air, leading to oxygen deprivation. This is a physical, not chemical, hazard.
- Radioactivity: Naturally occurring krypton consists of several stable isotopes (e.g., Kr-80, Kr-82, Kr-83, Kr-84, Kr-86). These isotopes are not radioactive. However, some artificial isotopes of krypton, such as Krypton-85 (Kr-85), are produced as by-products of nuclear fission in reactors. Kr-85 is radioactive and undergoes beta decay, emitting radiation. These artificial isotopes require careful handling and disposal due to their radioactivity.
- Flammability: Krypton is a non-flammable gas. It does not burn and does not support combustion. It can be used in some lighting applications, such as high-performance incandescent bulbs or certain types of discharge lamps (like some airport runway lights), where its inertness and ability to reduce filament evaporation are beneficial.
Chemical Reactions of Krypton: Krypton Difluoride
Despite its general inertness, krypton can be forced to react under extreme conditions, particularly with highly electronegative elements like fluorine. The discovery of noble gas compounds in the 1960s was a significant scientific breakthrough, challenging the long-held belief of their complete inertness.
The most famous and well-studied chemical compound of krypton is Krypton Difluoride (KrF2).
- Formation: Krypton difluoride is synthesized by subjecting a mixture of krypton gas and fluorine gas to conditions that provide significant energy input. This can be achieved through methods such as:
- Electrical discharge at low temperatures (e.g., -196 °C).
- Irradiation with ultraviolet light.
- Proton bombardment.
- Cooling a mixture of Kr and F2 below -150 °C.
- Reaction Equation: Under these specific, energy-intensive conditions, the reaction proceeds as: Kr (g) + F2 (g) → KrF2 (s)
- Properties: Krypton difluoride is a white crystalline solid that is stable only at low temperatures (below -30 °C). It is a powerful fluorinating agent, meaning it readily donates fluorine atoms to other substances. Its extreme reactivity makes it useful in specific chemical syntheses where highly potent fluorinating agents are required, but it is not a compound encountered in everyday life. Its existence demonstrates that even noble gases, under the right conditions, can form chemical bonds.