Group 18 (Inert gases) elements

Group 18 (Inert gases) elements:

Occurrence:

All the noble gases occur in the atmosphere.

Physical properties:

As we move along the noble gas elements, their atomic radius and boiling point increases from helium to radon. The first ionization energy decreases from helium to radon. Noble gases have the largest ionisation energy compared to any other elements in a given row as they have completely filled orbital in their outer most shell. They are extremely stable and have a small tendency to gain or lose electrons. The common physical properties of the group 18 elements are listed in the Table.

Table 3.10 Physical properties of group 18 elements

Properties of inert gases:

Physical properties:

Noble gases are monoatomic, odourless, colourless, tasteless, and non-inflammable. They are highly unreactive. They are non-metallic in nature.

Chemical Properties:

Only the xenon and krypton show some chemical reactivity. Xenon fluorides are prepare by direct reaction of xenon and fluorine under different conditions as shown below.

When XeF₆ is heated at 50 °C in a sealed quartz vessel it forms XeOF₄.

2XeF₆ + SiO₂ →^{50 0 C} →2XeOF₄ + SiF₄

When the reaction is continued the following reaction takes place.

2XeOF₄ + SiO₂ → 2XeO₂F₂ + SiF₄

2XeO₂F₂ + SiO₂ → 2XeO₃ + SiF₄

On hydrolysis with water vapour XeF₆ gives XeO₃

XeF₆ + 3H₂O → XeO₃ + 6HF

When XeF₆ reacts with 2.5 M NaOH, sodium per xenate is obtained.

2XeF₆ + 16NaOH → Na₄XeO₆ + Xe + O₂ + 12NaF + 8H₂O

Sodium per xenate is very much known for its strong oxidizing property. For example, it oxidises manganese (II) ion into permanganate ion even in the absence of the catalyst.

5XeO₆⁴⁻ + 2Mn²⁺ + 14H⁺ → 2MnO₄⁻ + 5XeO₃ + 7H₂O

Xenon reacts with PtF₆ and gave an orange yellow solid [XePtF₆] and this is insoluble in CCl₄.

Xenon difluoride forms addition compounds XeF₂.2SbF₅ and XeF₂.2TaF₅. Xenon hexa fluorides forms compound with boron and alkali metals. Eg : XeF₆.BF₃, XeF₆MF, M-alkali metals.

There is some evidence for existence of xenon dichloride XeCl₂.

Krypton form krypton difluoride when an electric discharge is passes through Kr and fluorine at 183° C or when gases are irradiated with SbF₅ it forms KrF₂.2SbF₃.

Table 3.11 Structures of compounds of Xenon:

Uses of noble gases:

The inertness of noble gases is an important feature of their practical uses.

Helium:

·           Helium and oxygen mixture is used by divers in place of air oxygen mixture. This prevents the painful dangerous condition called bends.

·           Helium is used to provide inert atmosphere in electric arc welding of metals

·           Helium has lowest boiling point hence used in cryogenics (low temperature science).

·           It is much less denser than air and hence used for filling air balloons

Neon:

Neon is used in advertisement as neon sign and the brilliant red glow is caused by passing electric current through neon gas under low pressure.

Argon:

Argon prevents the oxidation of hot filament and prolongs the life in filament bulbs

Krypton:

Krypton is used in fluorescent bulbs, flash bulbs etc...

Lamps filed with krypton are used in airports as approaching lights as they can penetrate through dense fog.

Xenon:

Xenon is used in fluorescent bulbs, flash bulbs and lasers.

Xenon emits an intense light in discharge tubes instantly. Due to this it is used in high speed electronic flash bulbs used by photographers

Radon:

Radon is radioactive and used as a source of gamma rays

Radon gas is sealed as small capsules and implanted in the body to destroy malignant i.e. cancer growth