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Group-18 Elements

The elements helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) belong to group 18 of the periodic table. They are also known as rare gases, because they are found in very small amounts in nature. They are highly non-reactive and, barring few exceptions, they do not take part in chemical reactions and are, therefore, called inert gases or noble gases. They always occur in free state because of their inert nature.
The valence shell electronic configuration of helium is 1s2 and for other members of this group is ns2np6. Thus, except helium, other noble gases have a closed octet of electrons in their outermost shell. This electronic configuration is very stable and is related to their chemical inertness.
Ionization energy (kJ/mol)
Density at STP (g/cm3)
Melting point (K)
Boiling point (K)
1.8 × 10â4
9.0 × 10â4
1.8 × 10–3
3.7 × 10–3
5.9 × 10–3
9.7 × 10–3

General trends in physical properties

  1. State: All are monoatomic, colorless, odorless, ad tasteless gases.
  2. Solubility: They are sparingly soluble in water. The solubility generally increases with increase in the atomic number.
  3. Boiling point and melting point: Due to weak intermolecular van der Waal’s forces between them, they possess very low boiling point and melting point in comparison to those of other substances of comparable atomic and molecular masses. However, the boiling point and melting point increase with increase in the atomic number because van der Waal’s forces become stronger with increase in size of the atoms or molecules.
  4. Liquefaction: It is extremely difficult to liquefy these gases as there are only weak van der Waal’s forces which hold atoms together. Since these forces increase with the increase in atomic size and population of electrons, ease of liquefaction increases down the group from He to Rn.
  5. Atomic radii: In the case of noble gases, the atomic radii correspond to van der Waal’s radii. Therefore, these are quite large as compared with atomic radii of the other atoms belonging to the same period. As we go down the group, the van der Waal’s radius increases due to the addition of new electronic shells and increase in screening effect.
  6. Ionization energies: The ionization energies of noble gases are very high. This is due to the stable configurations of noble gases. However, the ionization energies decrease with increase in atomic number from He to Rn due to increasing atomic size and decrease in effective nuclear charge.
  7. Electron affinities: Due to the stable ns2np6 electronic configurations, noble gas atoms have no tendency to accept additional electron. Therefore, their electron affinities are almost zero.
  8. Enthalpy of fusion and enthalpy of vaporization: In general, the enthalpies of fusion and the enthalpies of vaporization are low and increase down the group.

Chemical properties

The noble gases are generally inert and do not participate in chemical reactions due to
  1. Very low electron affinities,
  2. Exceptionally high ionization energies, and
  3. Stable electronic configuration.

Compounds of xenon: fluorides, oxides, and oxofluorides

Xenon reacts directly only with F2 forming three fluorides, namely, XeF2, XeF4, and XeF6. These can be obtained by the direct combination of xenon and fluorine at 400°C in a sealed nickel vessel and the products depend on the Xe/F2 molar ratio.
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Compounds XeF2, XeF4, and XeF6 are all white solids, which sublime at room temperature. The lower fluorides form higher fluorides when heated with F2 under pressure. They are extremely strong oxidizing and fluorinating agents.
They react quantitatively with hydrogen as follows:
XeF2 + H2 2HF + Xe
XeF4 + 2H2 4HF + Xe
They oxidize Cl to Cl2, I to I2, and cerium(III) to cerium(IV).
XeF2 + 2HCl 2HF + Xe + Cl2
XeF4 + 4KI 4KF + Xe + 2I2
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They fluorinate compounds:
XeF4 + 2SF4Xe + 2SF6
XeF4 + PtXe + PtF4
The fluorides differ in their reactivity with water. XeF2 in soluble in water, but undergoes slow hydrolysis. Hydrolysis is more rapid with alkali.
2XeF2 + 2H2O 2Xe + 4HF + O2
XeF4 reacts violently with water, giving xenon trioxide XeO3.
3XeF2 + 6H2O 2Xe + XeO3 + 12HF + 1½ O2
XeF6 also reacts violently with water, but slow hydrolysis by atmospheric moisture gives the highly explosive solid XeO3.
XeF6 + 6H2O XeO3 + 6HF
With small quantities of water, partial hydrolysis occurs, giving a colorless liquid xenon oxofluoride XeOF4. The same product is formed when XeF6 reacts with silica or glass.
XeF6 + H2O XeOF4 + 2HF
2XeF6 + SiO2 2XeOF4 + SiF4
XeO3 is an explosive, white hygroscopic solid. It reacts with XeF6 and XeOF4.
XeO3 is soluble in water, but does not ionize. However, in alkaline solution above pH 10.5, it forms the xenate ion [HXeO4]:
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