Noble gas or inert gas referred to six naturally occurring chemical elements like helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) which placed in Group 18 or Group VIIIA in the periodic table. All the noble gases are colorless, orderless, tasteless, monoatomic gas with very low reactivity due to their filled valence shell electron configuration and very high ionization energy. Radon is a radioactive noble gas that occurs in uranium mines. Oganesson (Og), also placed with the noble gas family but its chemistry is under investigation. In solid-state, gas like neon argon, krypton, xenon, and radon formed face-centered cubic crystal lattice but helium alone formed a close-packed hexagonal crystalline solid structure.
The physical properties and uses of all the rare gases are very similar. For example, the melting point and boiling point of noble gas a close together, and all the gases are used in fluorescent lamps for light generation. The lack of chemical reactivity of noble gases was the reason to call them inert. In fact, prior to 1962, no true compound was known. They only formed clathrates or cage compounds with water and pare quinol through the hydrogen bonding network.
History and Discovery
The noble gas in the periodic table was discovered through their physical properties like electromagnetic spectrum and gas density. In 1785, English chemist and physicist Henry Cavendish observed that a sample of air always leaves a small residue of substance (about 1/120th part) after repeated sparking air with excess oxygen. The first discovery of noble gas was found nearly one hundred years after this observation. The fine experiment was done after a century by Sir William Ramsay, a Scottish chemist, and Lord Rayleigh, an English physicist in 1894. Lord Rayleigh observed that nitrogen isolated from atmospheric gases was 0.5 percent heavier than nitrogen prepared chemically. In 1894, chemist Sir William Ramsay identified a new element (argon meaning lazy) in the residue left after heating atmospheric nitrogen with magnesium. A yellow line was observed close to the Sodium D-lines in the spectrum of sun chromophore during the total solar eclipse. Lockyer and Frankland assigned this to a new element which they named helium (Greek helios meaning sun). The same line was observed by L Palmieri in the spectrum of volcanic gases. Ramsay and Travers discovered three new noble gas like neon, krypton, and xenon by low –temperature distillation of liquid air. Rutherford and Soddy isolate radon (Latin word meaning ray) from radioactive decay of minerals.
Properties of Noble Gases
The valence shell of noble gas contains complete s- and p-orbital, which is a very stable configuration. Therefore, the energy required for the promotion of an electron to the next available vacant orbital is quite large. For example, the promotional energy for xenon is very large equal to 963 kJ mol-1. The fact suggests that covalent bonding involving sp3d or sp3d2 valence state is highly unfavorable for xenon. Ionic chemical bonding to form the compounds of the type Xe+F– is similarly difficult due to high positive entropy and free energy. For more details about noble gas, see the individual topics of gases like helium, neon, argon, krypton, xenon, radon, oganesson.