Group 18 consists of elements: helium, neon, argon, krypton, xenon, radon and oganesson. All these are gases and chemically unreactive. They form very few compounds, because of this they are termed as noble gases. All noble gases have general electronic configuration ns²np⁶ except helium which has 1s². Many of the properties of noble gases including their inactive nature are ascribed to their closed shell structures. Due to stable electronic configuration these gases exhibit very high ionization enthalpy. However, it decreases down the group with increase in atomic size. Atomic radii increase down the group with increase in atomic number. Since noble gases have stable electronic configurations, they have no tendency to accept the electron and therefore, have large positive values of electron gain enthalpy.

Which of the following noble gases is not obtained via fractional distillation?

Helium

The correct answer is option 4. Helium.

Here is an explanation of how noble gases are obtained, particularly focusing on helium and why it is not obtained via fractional distillation:

Fractional Distillation:

Fractional distillation is a process used to separate components of a mixture based on differences in their boiling points. It is commonly used to separate the components of air, including noble gases like argon, krypton, and neon. In fractional distillation of air, the air is cooled and liquefied, usually by compression and cooling, until it becomes a liquid. This liquid air is then allowed to boil slowly in a fractionating column. As the temperature gradually increases from the bottom to the top of the column, components with lower boiling points vaporize and rise, while components with higher boiling points remain liquid and flow downward. Noble gases, being present in trace amounts in the Earth's atmosphere, can be separated using fractional distillation based on the differences in their boiling points.

Helium Extraction:

Unlike other noble gases, helium is not obtained via fractional distillation of air. This is because helium is present in extremely small concentrations in the Earth's atmosphere (about 5 parts per million) and is much rarer than other noble gases. The primary source of helium is natural gas deposits, where helium is present as a minor component. During natural gas extraction, helium is obtained as a byproduct from certain natural gas wells that contain helium-rich gas. Once extracted, the helium-rich natural gas undergoes purification processes to remove impurities and increase the helium concentration. These purification steps may include cryogenic distillation, adsorption, or membrane separation techniques. Cryogenic distillation involves cooling the helium-rich gas to very low temperatures to liquefy it, then distilling the liquid helium to separate it from other components of the gas mixture. Adsorption and membrane separation techniques use selective adsorbents or membranes to selectively capture helium molecules while allowing other gases to pass through.

While fractional distillation is an effective method for separating noble gases like argon, krypton, and neon from air, it is not suitable for extracting helium due to its extremely low concentration in the atmosphere. Instead, helium is primarily obtained as a byproduct of natural gas production and purified using cryogenic distillation or other separation techniques.

In summary, while noble gases like argon, krypton, and neon can be obtained via fractional distillation of air, helium is primarily obtained as a byproduct of natural gas production and purified using specialized techniques rather than fractional distillation.