Krypton (Kr): Properties, Uses & Reactions for JEE/NEET & CBSE

Introduction

Krypton (Kr) is a chemical element, a noble gas, and a member of Group 18 of the periodic table. Despite its general inertness, Krypton finds crucial applications in specialized lighting, high-speed photography, and laser technology due to its unique spectroscopic properties. Understanding its chemical and physical characteristics is vital for competitive examinations like JEE and NEET.

CBSE/JEE Quick Revision Notes

Here are the key properties of Krypton frequently tested in examinations:

• Symbol: Kr
• Atomic Number (Z): 36
• Atomic Mass (A): 83.798 u (approximately 84 u)
• Group: 18 (Noble Gases)
• Period: 4
• Block: p-block
• Nature: Colourless, odourless, tasteless, non-flammable gas at standard temperature and pressure. Monatomic.
• Electronic Configuration: [Ar] 3d¹⁰ 4s² 4p⁶
• Valency: Generally 0 (forms compounds like KrF₂ under specific, extreme conditions due to its high ionization energy).
• Ionization Enthalpy: High (1st IE: 1350.8 kJ/mol)
• Electron Gain Enthalpy: Negligible/positive
• Electronegativity (Pauling scale): Not generally assigned due to its inertness, but very low for compound formation.
• Density (at STP): 3.749 g/L
• Boiling Point: -153.22 °C (119.93 K)
• Melting Point: -157.36 °C (115.79 K)
• Occurrence: Trace amounts in Earth’s atmosphere (approximately 1 ppm by volume).

Electron Configuration & Bonding Behavior

Electron Configuration

Krypton’s atomic number is 36, meaning it has 36 electrons. Its ground-state electron configuration is: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶

The shorthand (condensed) electron configuration uses the preceding noble gas, Argon (Ar), which has 18 electrons: [Ar] 3d¹⁰ 4s² 4p⁶

This configuration signifies a completely filled valence shell (n=4 shell with 8 electrons, 4s² 4p⁶, and a completely filled d-subshell).

Bonding Behavior

As a noble gas, Krypton exhibits exceptional stability due to its complete octet in the outermost shell. This leads to:

• High Ionization Enthalpy: A significant amount of energy is required to remove an electron.
• Nearly Zero Electron Gain Enthalpy: It has little tendency to accept an electron.
• Chemical Inertness: It generally does not form chemical bonds under normal conditions.

However, under extreme conditions (e.g., high pressure, high temperature, electrical discharge, or UV radiation) and in the presence of highly electronegative elements (primarily fluorine), Krypton can be forced to form compounds. This is primarily observed with Krypton difluoride (KrF₂). Its reactivity is less extensive than that of Xenon due to Krypton’s higher ionization energy and smaller atomic size, making its valence electrons held more tightly.

Crucial Chemical Reactions

Krypton’s chemistry is limited compared to other elements. The most significant and well-studied compound is Krypton difluoride (KrF₂).

1. Synthesis of Krypton Difluoride (KrF₂)

Krypton difluoride is synthesized by reacting Krypton gas with Fluorine gas under specific, energetic conditions:

Kr(g) + F₂(g) \xrightarrow\{\text\{Electrical Discharge / UV Radiation, -196°C\}\} KrF₂(s)

• Conditions: Requires low temperatures (e.g., liquid nitrogen temperature, -196°C), intense UV radiation, or electrical discharge to overcome the high activation energy for breaking the Kr-Kr and F-F bonds and forming Kr-F bonds.
• Properties: KrF₂ is a white crystalline solid, highly unstable, and a potent fluorinating and oxidizing agent.

2. Reactions of Krypton Difluoride (KrF₂)

KrF₂ is a strong Lewis acid and an exceptionally strong oxidizing agent. It reacts readily with strong Lewis acids to form KrF⁺ and Kr₂F₃⁺ ions.

Example Reaction (Formation of a KrF⁺ salt): KrF₂(s) + AsF₅(g) → [KrF⁺][AsF₆⁻](s)

• In this reaction, KrF₂ acts as a fluoride ion donor (Lewis base) to AsF₅, forming the complex anion AsF₆⁻ and the KrF⁺ cation. This highlights its ability to form stable species with high oxidation states, although the overall compound remains highly reactive.

Due to its high oxidizing power, KrF₂ can oxidize gold to AuF₅, making it one of the few reagents capable of reacting directly with gold.

Industrial and Biological Importance

Industrial Importance

1. Lighting:
   • Incandescent Bulbs: Krypton is used in some incandescent light bulbs (often mixed with Argon) to reduce the evaporation rate of the tungsten filament, allowing it to burn at a higher temperature for increased brightness and longer life.
   • Fluorescent Lamps: Used in certain high-efficiency fluorescent lamps.
   • Discharge Lamps: Creates a bright, white light when electrically excited.
2. Lasers:
   • Krypton Fluoride (KrF) Excimer Lasers: These lasers produce ultraviolet light (248 nm) and are used in photolithography for manufacturing microelectronic devices (e.g., microprocessors), in nuclear fusion research, and in eye surgery.
3. Photography:
   • High-Speed Photography: Krypton-filled flash lamps produce very fast, intense flashes of light, useful for high-speed photography.
4. Insulation: Krypton’s low thermal conductivity makes it a good insulating gas, sometimes used in double-paned windows.

Biological Importance

• Inertness: Krypton is biologically inert. It is not known to have any biological role and is non-toxic.
• Medical Research: Radioactive isotope Krypton-81m (⁸¹mKr) is used in nuclear medicine for lung ventilation/perfusion scans (V/Q scans) to diagnose pulmonary embolism.