Notes On Group 18: Occurrence And Atomic Properties - CBSE Class 12 Chemistry
The elements in group eighteen are helium, neon, argon, krypton, xenon and radon. These elements are collectively referred to as noble gases. The term "noble gases" implies that these elements tend to be chemically inert or un-reactive. Compounds of xenon, such as fluorides, oxides and oxyfluorides, and fluorides of krypton are known. These compounds can be prepared only under drastic conditions at high pressure and temperature Electronic configuration: The general valence shell electronic configuration of these elements is ns2 np6, except for helium electronic configuration of helium is 1s2. Due to the duplet or octet configuration in their valence shells, the elements of this group are highly stable. In other words, as these elements have closed shell structures, they have less or no tendency to form compounds. As these elements do not have a tendency to lose, gain or share electron with atoms of other elements, their valency is zero. Occurrence: Owing to their inert nature, all the noble gases occur in Free State. All the noble gases, helium, neon, argon, krypton and xenon, except radon, occur in the atmosphere. These gases make up approximately 1% by volume of the atmosphere, in which argon alone constitutes 0.93%. Argon can be obtained from liquid air by fractional distillation. Natural gas deposits are the most important commercial and economical source of helium. Certain water springs contain helium, neon and argon in the form of dissolved gases. Radon is a radioactive substance that is produced by the decay of radium and thorium minerals. Trends in some of the atomic properties: Atomic radii: The atomic radii increase on descending the group with an increase in the atomic number. This is because of the addition of a new shell at each successive element on descending the group. Ionisation potential: These elements have closed shell structures hence these elements have very high ionisation potentials. Ionisation enthalpies decrease gradually on descending the group with an increase in the atomic size. Electron gain enthalpy: Elements of group eighteen have stable electronic configurations. They do not have a tendency to accept electrons. Hence, they have very high positive values of electron gain enthalpy.

#### Summary

The elements in group eighteen are helium, neon, argon, krypton, xenon and radon. These elements are collectively referred to as noble gases. The term "noble gases" implies that these elements tend to be chemically inert or un-reactive. Compounds of xenon, such as fluorides, oxides and oxyfluorides, and fluorides of krypton are known. These compounds can be prepared only under drastic conditions at high pressure and temperature Electronic configuration: The general valence shell electronic configuration of these elements is ns2 np6, except for helium electronic configuration of helium is 1s2. Due to the duplet or octet configuration in their valence shells, the elements of this group are highly stable. In other words, as these elements have closed shell structures, they have less or no tendency to form compounds. As these elements do not have a tendency to lose, gain or share electron with atoms of other elements, their valency is zero. Occurrence: Owing to their inert nature, all the noble gases occur in Free State. All the noble gases, helium, neon, argon, krypton and xenon, except radon, occur in the atmosphere. These gases make up approximately 1% by volume of the atmosphere, in which argon alone constitutes 0.93%. Argon can be obtained from liquid air by fractional distillation. Natural gas deposits are the most important commercial and economical source of helium. Certain water springs contain helium, neon and argon in the form of dissolved gases. Radon is a radioactive substance that is produced by the decay of radium and thorium minerals. Trends in some of the atomic properties: Atomic radii: The atomic radii increase on descending the group with an increase in the atomic number. This is because of the addition of a new shell at each successive element on descending the group. Ionisation potential: These elements have closed shell structures hence these elements have very high ionisation potentials. Ionisation enthalpies decrease gradually on descending the group with an increase in the atomic size. Electron gain enthalpy: Elements of group eighteen have stable electronic configurations. They do not have a tendency to accept electrons. Hence, they have very high positive values of electron gain enthalpy.

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