Chemistry 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 ?


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?


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.