Noble gases

Why are they called the 'Noble Gases'?

▪They are all very unreactive.
▪ Noble gases were originally called 'inert gases', as they were thought not to react with anything. Then a British chemist, Neil Bartlett, made a compound with xenon.
▪ The name was changed to 'noble gases' as they were considered like the very unreactive precious metals gold and platinum, which are sometimes called 'noble' metals.

What contains helium?

▪Crude natural gas often contains helium. Commercially available helium is obtained via its separation from these gaseous mixtures by fractional distillation.
▪The other noble gases, apart from radon, are present in small amounts in air and are commonly obtained by fractional distillation of liquid air.

Uses of the noble gases

▪He-inert atmosphere
▪Helium-deep-sea diving.
▪Liquid helium (with a boiling point of 4.2 K)-coolant for superconducting magnets in NMR and MRI instruments.
▪heliox
▪Argon-inert atmosphere.
▪The noble gases emit light of vivid colors when an electrical discharge is passed through them; the color depends on the gases used. Neon's emission spectrum is responsible for the bright orange-red neon signs.
▪Kr- energy-saving fluorescent
▪Kr-flash lamps
▪Xe- specialized light sources
▪Xe- ruby lasers
▪Radon- cancer therapy

Physical properties of noble gases

-monoatomic
-weak interatomic forces
-solidify
-enthalpy

Chemical properties of noble gases

-extremely stable
-behaves similarly under standard conditions
-largest ionization energy
-weak Van der Waals forces

Reactivity of noble gases: Xenon fluorides

XeF2 (670 K, 2:1 ratio)
XeF4 (670 K, 6 bar, 1:5 ratio)
XeF6 (570 K, 50 bar, 1:20 ratio)
•Xenon forms the neutral fluorides XeF2, XeF4 and XeF6 which are prepared from Xe and F2 under different conditions.
•All the fluorides are crystalline solids at room temperature.

Structure of XeF6

•XeF6 is stereochemically non-rigid with the lone pair of electrons occupying different sites and causing the overall structure to be distorted octahedral.

Reactivity of noble gases: Xenon oxides

▪XeO2 and other forms of xenon may occur in deeply buried silicate minerals.
▪Xenon(VI) oxide can be prepared by the hydrolysis of XeF4 or XeF6 but is highly explosive.
▪In alkaline solution, Xenon(VI) oxide gives rise to the formation of the xenate anion, [HXeO4]- ,which slowly disproportionate giving the perxenate ion and xenon.

Reactivity of Radon

In theory, radon is more reactive than xenon, and therefore should form chemical bonds more easily than xenon does. However, due to the high radioactivity and short half-life of radon isotopes, only a few fluorides and oxides of radon have been formed in practice.

Significance of the reactivity of noble gases

-Reagent
-Fluorinating agents
-XeO3 is a powerful oxidizing agent
-Perxenate ion

Radon

▪All isotopes of radon are radioactive; the longest-lived isotope, has a half-life of only 3.825 days. There is concern regarding the level of radon in many homes.
▪A potential cause of lung cancer, radon is formed from the decay of trace amounts of uranium in certain rock formations and itself undergoes alpha decay, generating radioactive daughter isotopes.

Helium, a superfluid

Helium has several unique qualities when compared with other elements: it has a very low boiling point and is monoatomic; at temperatures close to absolute zero helium condenses to a liquid with the properties of a superfluid, flowing with zero friction

Impurities with Crude Helium Gas and Removal of These Impurities

=> Crude oil (mixture of water, carbon dioxide, hydrogen sulfide, heavy hydrocarbons)
=> carbon dioxide and hydrogen sulfide - removed using amine systems
=> Heavy hydrocarbons -removed by a refrigeration system, an oil absorption cycle or a combination refrigeration-oil absorption process
=> water removal - glycols and solid desiccants

Extraction of helium from natural gas by cryogenic process: Refrigeration

=> -320-degree Fahrenheit
=> auto-refrigeration - achieved by pressure drop on the gas system
=> auto-refrigeration is accomplished by
=> Refrigerants - methane, ethylene, propane, propylene, nitrogen

Extraction of helium from natural gas by cryogenic process: Purification

=> crude helium is cooled
=> gaseous helium is passed through charcoal
=> charcoal removes the remaining contaminants

What is the second most abundant element in the universe?

Helium

Reactivity of noble gases

•Xenon (and to a lesser extent, krypton), combines with fluorine and oxygen to form a wide range of compounds and this is because:
•F2 and O2 are strong oxidizing agents.
•F and O are highly electronegative elements.

Structure of XeF2

•XeF2 is linear, with three lone pairs on Xe.

noble gases

The elements in group 18, on the right of the modern periodic table, are called the noble gases.

Structure of XeF4

•XeF4 is planar, with two lone pairs.