Chemistry Group 18 Elements
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Class 12 chapter7 p-block elements

Group 18 Elements

Topics Covered :

● Group 18 Elements
● Occurrence
● Electronic Configuration
● Ionisation Enthalpy
● Atomic Radii
● Electron Gain enthalpy
● Physical Properties

Group 18 Elements :

`=>` Group 18 consists of six elements : helium, neon, argon, krypton, xenon and radon.

`=>` All these are gases and chemically unreactive.

`=>` They form very few compounds. Because of this, they are termed noble gases.

Occurrence :

`=>` All the noble gases except radon occur in the atmosphere.

`=>` Their atmospheric abundance in dry air is `~ 1%` by volume of which argon is the major constituent.

`=>` Helium and sometimes neon are found in minerals of radioactive origin e.g., pitchblende, monazite, cleveite.

`=>` The main commercial source of helium is natural gas.

`=>` Xenon and radon are the rarest elements of the group.

`=>` Radon is obtained as a decay product of `color{red}(text()^(226)Ra)`

`color{red}(text()_(88)^(226)Ra → text()_(86)^(222) Rn + text()_(2)^(4)He)`

`=>` The important atomic and physical properties of the Group 18 elements along with their electronic configurations are given in Table 7.12..

`=>` The trends in some of the atomic and physical properties of the group are discussed here.
Q 3030891712

Why are the elements of Group 18 known as noble gases ?

Solution:

The elements present in Group 18 have their valence shell orbitals completely filled and, therefore, react with a few elements only under certain conditions. Therefore, they are now known as noble gases.

Electronic Configuration :

`=>` All noble gases have general electronic configuration `color{red}(ns^2 np^6)` except helium which has `1s^2` (Table 7.12).

`=>` Many of the properties of noble gases including their inactive nature are ascribed to their closed shell structures.

Ionisation Enthalpy :

`=>` Due to stable electronic configuration these gases exhibit very high ionisation enthalpy.

`=>` But, it decreases down the group with increase in atomic size.

Atomic Radii :

Atomic radii increase down the group with increase in atomic number.

Electron Gain Enthalpy :

Because of stable electronic configurations, they have no tendency to accept the electron and therefore, have large positive values of electron gain enthalpy.

Physical Properties :

`=>` All the noble gases are monoatomic.

`=>` They are colourless, odourless and tasteless.

`=>` They are sparingly soluble in water.

`=>` They have very low melting and boiling points because the only type of interatomic interaction in these elements is weak dispersion forces.

● Helium has the lowest boiling point (`4.2 K`) of any known substance.

● It has an unusual property of diffusing through most commonly used laboratory materials such as rubber, glass or plastics.
Q 3050891714

Noble gases have very low boiling points. Why?

Solution:

Noble gases being monoatomic have no interatomic forces except weak dispersion forces and therefore, they are liquefied at very low temperatures. Hence, they have low boiling points.
 
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