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Element Fact
The liquid helium-4 possesses the most unusual physical properties that defy common sense. Below 2.2 K it becomes a superfluid that can flow, against gravity, up the wall of a container.

Krypton, Kr

krypton sample

Atomic number: 36
Atomic mass: 83.80(1)
Natural abundance: -
78Kr (77.920386) - 0.35%
80Kr (79.916378) - 2.52%
82Kr (81.913485) - 11.6%
83Kr (82.914136) - 11.5%
84Kr (83.911507) - 57.0%
86Kr (85.910610) - 17.3%

History and Production

Derived from the Greek word kryptos, meaning hidden. It was discovered by W. Ramsey and M.W. Travers, in 1898, in the residual almost all liquid air had boiled away. The uses of the element are limited, partly due to its rarity and hence its high costs. It is used in discharged tubes (advertisement signs) and high-speed photohgraphy. The isotope 85Kr has found its use in chemical analysis, such as estimation of reactant concentrations. Krypton was once used (1960) to define the fundamental unit of length, the meter, in terms of the orange-red spectral line of 86Kr.

Physical Data

It is colorless, odorless and generally inert. The concentration of krypton in air is about 1.14 ppm.

Interatomic distance: 380 pm (van der Waal diameter)
Melting point: -157.36C
Boiling point: -153.22C
Thermal conductivity/Wm-1K-1: 0.00949 (27C)
Density/kgm-3: 2823 (m.p.), 2413 (b.p.), 3.7493 (0C)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 0 kJ/mol
Gibbs free energy of formation: -
Entropy: 164.1 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Ar] 3d10 4s2 4p6 = [Kr]
Term symbol: 1S0
Electron affinity: (not stable)   Electronegativity (Pauline): -
Ionization energy (first, second, third): 1350.76, 2350.37, 3565.14 kJ/mol

Chemical property

Krypton's outer-most electron shell has already achieved the most stable configuration and hence the atom is particularly resistant to chemical reaction. However, along with other heavier noble gasses, water molecules can be arranged to form cavity to encapsulate krypton under high pressure. These species are known as clathrates. Krypton does not form chemical bonds, rather it is weakly held via van der Waals forces with the water molecules. Despite its inert chemical behavior, some krypton compounds have been identified, particularly those of fluorides. For instance, krypton difluoride is formed when mixtures of krypton and fluorine are cooled to temperatures near -196C and then follow by irradiation of X-rays or high-energy electrons.

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