Characteristics and Physical Properties of Group 18 Elements - Chemistry, Class 12, The p-Block Elements

Contents

1 Group 18
2 Occurrence of Noble Gases
3 General Characteristics of Group 18 Elements
- 3.1 (1) Electronic Configurations
- 3.2 (2) Atomic and Physical Properties
4 Properties of Noble Gases
- 4.1 Physical Properties
- 4.2 Chemical Properties

Group 18

The group 18 consists of elements helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). These gases at ordinary temperature do not have chemical reactivity and therefore, these were called inert gases. Because of the low abundance of these gases on earth, they have also been called rare gases.

Occurrence of Noble Gases

Due to the inert nature of noble gases, they always occur in the free state. Except radon, all these gases are present in atmosphere in the atomic state. Their total percentage in dry air is about 1% by volume, of which argon (0.93%) in the major component. It originates in the air mostly from electron capture of potassium.

Helium is also present in natural gas to the extent of 2 to 7%. Helium and sometimes neon are found in small quantities in minerals of radioactive element such as monazite, clevite, pitch blende, etc. Helium, neon and argon are found in the water of certain springs.

Radon is radioactive and does not occur in the free state because it decays very rapidly.

General Characteristics of Group 18 Elements

(1) Electronic Configurations

Except helium, the atoms of all noble gases have eight electrons in the valence shell. The general electronic configuration of noble gases (except He) may be expressed as ns²np⁶. On the other hand, helium has 1s² electronic configuration.

These configurations being stable, the noble gases neither have any tendency to gain nor lose electrons, and, therefore, they do not enter into chemical combinations. It is, therefore, reasonable to assume that inert nature of the noble gases is due to their stable electronic configurations.

(2) Atomic and Physical Properties

(a) Existence

All the noble gases are monoatomic, colourless and odourless gases. The monoatomic state of these gases is due to stable electronic configuration (ns²np⁶) of their atoms. As a result, they are not capable of combining even amongst themselves.

(b) Atomic radii

As we go down the group, the van der Waal’s radius increases due to the addition of new electronic shells.

(c) Ionisation Enthalpies

The ionisation enthalpies of noble gases are very high. This is attributed to the stable completely filled configurations of noble gases. However, the ionisation enthalpies decrease with increase in atomic number from He to Rn due to increasing atomic size.

(d) Electron Gain Enthalpies

Due to the stable ns²np⁶ electronic configurations, noble gas atoms have no tendency to accept additional electron. Therefore, their electron gain enthalpies are zero or have large positive values.

(e) Melting and Boiling Points

The melting and boiling points of noble gases are very low in comparison to those of other substances of comparable atomic and molecular masses. This indicates that only weak van der Waal’s forces are present between the atoms of the noble gases in the liquid or the solid state.

These van der Waal’s forces increase with the increase in atomic size of the atom, and therefore, the boiling points and melting points increase from He to Rn. Helium has the lowest boiling point (4.2 K) of any known substance.

(f) Ease of liquefaction

The noble gases are not easily liquefied. This is due to the fact that there are only weak van der Waal’s forces which hold atoms together.

Due to increase in atomic size and, therefore, increase in van der Waal forces, the ease of liquefaction increases down the group from He to Rn.

(g) Solubility in water

The noble gases are only slightly soluble in water. The solubility, in general, increases from He to Rn.

Properties of Noble Gases

Physical Properties

(i) All the noble gases are colourless, odourless and tasteless.

(ii) All the noble gases are monoatomic.(iii) They are sparingly soluble in water.

Chemical Properties

Noble gases are not very reactive. Their inertness to chemical reactivity is attributed to the following reasons:

(i) The noble gases have completely filled ns²np⁶ electronic configurations in their valence shells.

(ii) The noble gases have very high ionisation energies.

(iii) The electron affinities of noble gases are almost zero or large and positive.

Therefore, they have neither tendency to gain nor to lose any electron and do not enter into chemical combinations.

In March 1962, Neil Bartlett noticed that platinum hexafluoride, PtF₆ is a powerful oxidising agent which combines with molecular oxygen to form red ionic compound, dioxygenyl hexafluoroplatinate (V), O₂⁺ [PtF₆]^–

O₂ (g) + PtF₆ (g) → O₂[PtF₆]-

This indicates that PtF₆ has oxidised O₂ to O₂⁺.

Oxygen and xenon have some similarities:

(i) The first ionisation energy of xenon gas (1170 kJ mol^-1) is fairly close to that of oxygen (1175 kJ mol^-1).

(ii) The molecular diameter of oxygen and atomic radius of xenon are similar (4A°).

On this assumption, Bartlett reacted xenon and platinum hexafluoride in gas phase and an orange yellow solid of the composition XePtF₆ was obtained.

Xe(g) + PtF₆ (g) → Xe⁺[PtF₆]‾ (s)

The formation of this compound has shown that xenon is not totally inert and after this a number of compounds of xenon were prepared.