What Is Group 18 On The Periodic Table

Group 18 of the periodic table, also known as the noble gases, represents a fascinating collection of elements that possess unique chemical properties and play significant roles in various scientific and technological applications. These elements, including helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn), share a common characteristic: they are exceptionally stable and chemically inert under normal conditions.

The Discovery and History of Noble Gases

The discovery of noble gases unfolded over several decades, beginning in the late 19th century. In 1868, French astronomer Pierre Janssen observed a new yellow spectral line during a solar eclipse. This line, initially attributed to a new element, was later named helium by Norman Lockyer. However, it wasn't until 1895 that William Ramsay isolated helium from a uranium-bearing mineral, confirming its existence as a distinct element.

Following the discovery of helium, Ramsay and his collaborators turned their attention to the composition of air. In 1894, Lord Rayleigh noticed a discrepancy in the density of nitrogen obtained from air compared to that produced chemically. Ramsay hypothesized that air contained another, heavier gas. Through careful experimentation, they isolated argon, an element that constituted nearly 1% of the Earth's atmosphere.

The subsequent years saw the discovery of neon, krypton, and xenon by Ramsay and his student Morris Travers. These elements were identified through spectroscopic analysis of the remaining gases after nitrogen, oxygen, and argon had been removed from air. Radon, the last naturally occurring noble gas, was discovered in 1900 by Friedrich Ernst Dorn as a radioactive decay product of radium.

Electronic Configuration and Properties

The exceptional stability of noble gases stems from their unique electronic configuration. These elements have a full outer electron shell, meaning they possess the maximum number of electrons that can occupy the outermost energy level. Helium has two electrons in its outer shell (1s²), while the remaining noble gases have eight electrons in their outer shells (ns²np⁶). This configuration makes them exceptionally resistant to gaining, losing, or sharing electrons, hence their chemical inertness.

Physical Properties

Noble gases exhibit several distinctive physical properties:

• Monoatomic Nature: Noble gases exist as individual, non-bonded atoms under normal conditions. This contrasts with most other gases, which exist as diatomic molecules (e.g., oxygen, nitrogen).
• Low Boiling Points: Due to weak interatomic forces (primarily London dispersion forces), noble gases have extremely low boiling points. Helium has the lowest boiling point of any element, at -268.9 °C.
• Low Density: Noble gases are relatively low in density compared to other elements and compounds.
• Colorless and Odorless: In their gaseous state, noble gases are colorless and odorless.
• Low Solubility in Water: Noble gases are only sparingly soluble in water.

Chemical Properties

The chemical inertness of noble gases is their defining characteristic. For many years, they were believed to be completely incapable of forming chemical compounds. However, in 1962, Neil Bartlett synthesized xenon hexafluoroplatinate (XePtF₆), demonstrating that noble gases could, in fact, participate in chemical reactions under certain conditions.

Since Bartlett's groundbreaking discovery, numerous other noble gas compounds have been synthesized, primarily involving fluorine and oxygen. These compounds are generally formed under extreme conditions, such as high pressure, low temperature, or intense radiation. The reactivity of noble gases increases with increasing atomic number, with xenon forming the most stable and diverse range of compounds.

Occurrence and Abundance

Noble gases are found in varying concentrations throughout the universe and on Earth. Helium is the most abundant element in the universe after hydrogen, formed primarily during the Big Bang and in stellar nucleosynthesis. On Earth, helium is relatively rare, found mainly in natural gas deposits.

Argon is the most abundant noble gas in the Earth's atmosphere, comprising about 0.93% by volume. It is produced by the radioactive decay of potassium-40 in rocks. Neon, krypton, and xenon are present in trace amounts in the atmosphere. Radon is a radioactive gas produced by the decay of uranium and thorium in rocks and soil. It is found in low concentrations in air but can accumulate in enclosed spaces, posing a health hazard.

Production and Isolation

The primary source of noble gases is air. Fractional distillation of liquefied air is used to separate nitrogen, oxygen, and argon. Neon, krypton, and xenon are obtained as byproducts of this process. Helium is extracted from natural gas deposits, where it accumulates due to the radioactive decay of uranium and thorium. Radon is collected from the radioactive decay of radium.

Fractional Distillation

Fractional distillation is a process that separates different components of a liquid mixture based on their boiling points. In the case of air separation, the air is first cooled to extremely low temperatures, causing it to liquefy. The liquid air is then slowly heated, and the different components boil off at different temperatures. Nitrogen, with the lowest boiling point, is the first to vaporize, followed by argon and oxygen.

Extraction from Natural Gas

Helium is extracted from natural gas by first removing water, carbon dioxide, and other impurities. The remaining gas is then cooled to very low temperatures, causing all components except helium to liquefy. The gaseous helium is then purified by adsorption or distillation.

Applications of Noble Gases

The unique properties of noble gases have led to a wide range of applications in various fields:

• Helium:
  • Cryogenics: Liquid helium is used as a coolant in cryogenics, enabling research at extremely low temperatures, such as in superconducting magnets and MRI machines.
  • Lighter-than-air Craft: Helium is used to inflate balloons and airships due to its low density and non-flammability.
  • Breathing Mixtures: Helium is mixed with oxygen to create breathing mixtures for deep-sea diving, reducing the risk of nitrogen narcosis.
• Neon:
  • Lighting: Neon is used in neon signs, producing a bright orange-red light.
  • High-Voltage Indicators: Neon is used in high-voltage indicators and surge arrestors.
• Argon:
  • Welding: Argon is used as a shielding gas in welding, preventing oxidation of the metal being welded.
  • Incandescent Light Bulbs: Argon is used to fill incandescent light bulbs, preventing the filament from burning out.
  • Preservation: Argon is used to preserve documents and artifacts by displacing oxygen and preventing degradation.
• Krypton:
  • Lighting: Krypton is used in high-intensity lamps, such as those used in airport runway lighting.
  • Lasers: Krypton is used in some types of lasers.
• Xenon:
  • Lighting: Xenon is used in high-intensity lamps, such as those used in car headlights and movie projectors.
  • Anesthesia: Xenon is used as a general anesthetic.
  • Propulsion: Xenon is used as a propellant in ion thrusters for spacecraft.
• Radon:
  • Radiotherapy: In the past, radon was used in radiotherapy to treat certain types of cancer. However, this practice has largely been discontinued due to the risks associated with radiation exposure.

Compounds of Noble Gases

While noble gases are generally inert, they can form compounds with highly electronegative elements, such as fluorine and oxygen. The most well-studied noble gas compounds are those of xenon.

Xenon Fluorides

Xenon forms several fluorides, including xenon difluoride (XeF₂), xenon tetrafluoride (XeF₄), and xenon hexafluoride (XeF₆). These compounds are synthesized by reacting xenon with fluorine under various conditions. XeF₂ is a linear molecule, XeF₄ is a square planar molecule, and XeF₆ has a complex, distorted octahedral structure.

Xenon Oxides

Xenon trioxide (XeO₃) and xenon tetroxide (XeO₄) are two oxides of xenon. XeO₃ is a highly explosive compound that is formed by the hydrolysis of XeF₆. XeO₄ is even more unstable and decomposes spontaneously into xenon and oxygen.

Other Noble Gas Compounds

In addition to xenon compounds, a few compounds of other noble gases have also been synthesized. Krypton difluoride (KrF₂) is a reactive compound that can be used to synthesize other krypton compounds. Argon fluorohydride (HArF) is a metastable compound that can only exist at extremely low temperatures.

Health and Safety Considerations

Noble gases are generally considered to be non-toxic. However, there are some health and safety considerations associated with their use:

• Asphyxiation: Noble gases can displace oxygen in the air, leading to asphyxiation if inhaled in high concentrations.
• Radon Exposure: Radon is a radioactive gas that can accumulate in enclosed spaces, such as basements. Prolonged exposure to high levels of radon can increase the risk of lung cancer.
• Cryogenic Hazards: Liquid helium and other cryogenic liquids can cause severe burns and frostbite upon contact with skin.

The Future of Noble Gas Research

Research on noble gases continues to advance, with ongoing efforts to synthesize new compounds and explore their potential applications. Some areas of current research include:

• Synthesis of New Noble Gas Compounds: Scientists are attempting to synthesize new noble gas compounds with novel structures and properties.
• Applications in Materials Science: Noble gases are being investigated for use in materials science, such as in the creation of new types of semiconductors and insulators.
• Medical Applications: Noble gases are being explored for potential medical applications, such as in drug delivery and neuroprotection.

Conclusion

Group 18 elements, the noble gases, are a unique and fascinating group of elements that have played a significant role in the development of chemistry and technology. Their exceptional stability, stemming from their full outer electron shells, makes them invaluable in various applications, from cryogenics to lighting. While once considered completely inert, the discovery of noble gas compounds has expanded our understanding of chemical bonding and opened new avenues for research. As research continues, we can expect to see even more innovative applications of these remarkable elements in the future.