The modern periodic table consists of seven periods and eighteen groups. The reason behind the dividing the periodic table into groups and period is to make the study of elements and compounds easier. The modern periodic table is the period function of atomic number. The chemical and physical properties repeat itself when moving across the period and group. The noble gas (group 18) elements have the Outermost configuration of ns2, np6. Therefore, they have a little or no tendency to react with other elements to form compounds.
Noble Gas Definition
It defined as the group 18 elements which are chemically inert and have little tendency to undergo any reaction, hence these were referred to as inert gases, zero-valent gases or zero group elements.
- These gases have a highly stable configuration.
- However, researchers have shown that under certain specific conditions these gases react with certain elements to form compounds. Therefore, nowadays, these gases are more correctly called as noble gases rather than inert gases; the word ‘noble’ signifies that they undergo very few chemical reactions.
Noble Gases on Periodic Table
The zero group of the periodic table consists of six gaseous elements namely helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). Except for radon all of them are present in very small amounts in the atmosphere (hence their name rare gases). Radon is produced by the disintegration of radium and is radioactive itself.
Mendeleeff did not provide any blank space for the noble gases in his periodic table although he provided such spaces for several other elements not known at that time. He could not imagine the existence of elements that avoid participating in chemical reactivity under ordinary conditions. However, Ramsay, the discoverer of these gases, introduced a new group in the periodic table. He classed them as a separate group of “no valency’ or ‘zero valency’ and hence called it as zero group.
Characteristics of Noble Gases
- Atomic Radii. Their atomic radii increase with the increase in atomic number because at each successive step new shell is being added.
- Ionization energies. Since their outermost orbits have the stable configuration, they possess very high ionization energies. The ionization energy of the noble gas is highest among the members of the same period. However, their ionization energies decrease on moving down the group because of the increase in atomic radii (size effect) and screening effect.
- Electron affinity. Due to complete octet, gas atoms are unable to take extra electron hence their electron affinities are almost zero.
- Monoatomicity. Due to very high ionization energies and nearly zero electron affinity, gas atoms are not capable of combining even amongst themselves and hence they exist as single atoms.
- Forces of attraction between atoms of noble gases. The noble gases are monoatomic but the fact that these can be liquefied indicates the presence of some forces of attraction between the atoms. But since the noble gases have stable octet in their valence shell, the attractive force between gas atoms must be weak i.e. van der Waals type.
- Liquefaction. Since only the very weak van der Waal’s forces of attraction exist in these gases, these are relatively difficult to be liquefied. Further, as the van der Waals forces increase from He to Xe, the ease of liquefaction of these gases increases with the increase in their atomic weights.
- Solubility in water. These gases are only slightly soluble in water. However, solubility in water.
- Chemical nature. Noble gases are almost chemically inert. However, Kr, Xe, and Rn have been found to react chemically; the reactivity increases from Kr to Rn. However, the reactivity of radon has not been studied much because of the radioactive nature of the gas.
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Noble Gas Uses
Noble gases are widely used to provide an inert atmosphere in welding and cutting (antioxidant), in electric bulbs (prolongs filament life), etc. They are also used as coolants for low-temperature work. Amongst the noble gases only helium and argon are easily available and hence these are most widely used.
- Neon tubes are used in various fields of electrotechnology for rectifiers, voltage, regulators, inductors, etc.
- Neon lamps are used in botanical gardens and in the greenhouses as it stimulates growth and is effective in the formation of chlorophyll.
- The chief use of argon is in filling incandescent lamps where it has following advantages over nitrogen.
- The most important use of argon is in welding and other operations which require a non-oxidising atmosphere and absence of nitrogen.
- Xenon is used in discharge tubes for producing a high-speed flash of bluish light which is used in quick photography. The flash sometimes lasts for 1/500,000th part of a second.
- The xenon lamp is used as a source of light.
- Radon, being extremely radioactive, even more than radium, is used in the treatment of cancer (radiotherapy).
Noble Gas Properties
- The property of noble gases to react chemically involves the loss of electrons, hence the highly electronegative element (F and O) can combine with these gases very easily.
- Xenon, having the least I.E., forms the largest number of compounds than any other gas.
- Krypton forms some compounds but with difficulty as compared to xenon.
- The lighter members (He, Ne and Ar) are not found to form any compound. Thus the important gas compounds include fluorides and oxides of xenon.
- Among the three known fluorides (XeF2, XeF4, and XeF6), xenon tetrafluoride is the most important; while among oxides, xenon trioxide is more important than the tetroxide.
- Xenon tetrafluoride was the first xenon fluoride to be prepared by Classen, Selig, and Malm in 1962.