The noble gases are elements positioned in Group 18 of the periodic table. This group includes seven members: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), and the synthetic element oganesson (Og). Under standard atmospheric conditions, the first six of these elements exist as colorless, odorless, monatomic gases. Their shared characteristics and placement on the table distinguish them as a family.
Alternative Names and Historical Context
The most common collective name for these elements is derived from the German term Edelgas, which translates to “noble gas.” This nomenclature is an analogy to noble metals like gold and platinum, known for their resistance to chemical reaction. The name suggests that these gases stand apart from more reactive elements.
Historically, they have also been referred to as “inert gases” due to the initial belief that they were entirely chemically passive. This label arose because scientists could not get them to react with other substances when they were first discovered. However, this term is now considered inaccurate and has fallen out of favor as researchers have successfully synthesized compounds involving the heavier elements in the group.
Another former designation was the “rare gases,” describing their relative scarcity in the Earth’s atmosphere compared to nitrogen and oxygen. However, argon constitutes about 0.93% of the atmosphere’s volume. Because of this abundance, the “rare gas” label is misleading and is not used in modern chemistry.
Defining Characteristic: Chemical Inertness
The low reactivity of the noble gases is directly linked to their specific electron configuration. With the exception of helium (two electrons), all noble gases possess eight electrons in their outermost electron shell. This arrangement follows the octet rule and represents a state of maximum stability for an atom.
Other elements readily gain, lose, or share electrons to achieve this stable configuration. Since noble gases already have a complete valence shell, they have virtually no tendency to participate in chemical bonding. This inherent stability means they do not need to react with other elements.
Their unreactive nature explains why they exist as individual atoms, making them monatomic gases, rather than forming molecules like oxygen or nitrogen. The only forces holding the atoms together are the extremely weak London dispersion forces, which results in very low boiling points.
While the lighter noble gases like helium and neon are essentially unreactive, the heavier elements, such as xenon and krypton, can be forced to react under specific, high-energy conditions. The first stable noble gas compound, xenon hexafluoroplatinate, was synthesized in 1962, proving that the term “inert” was inaccurate. The ability of the heavier noble gases to form compounds, particularly with highly electronegative elements like fluorine, is due to their outer electrons being less firmly held, making them easier to remove.
Practical Applications
The unique properties of the noble gases are widely exploited across various industries and technologies. Helium, the second lightest element, is used to provide buoyancy for balloons and airships. Its extremely low boiling point makes liquid helium an indispensable cooling medium in cryogenics, necessary for superconducting magnets used in magnetic resonance imaging (MRI) machines.
Argon is the most cost-effective noble gas due to its high abundance, and its primary use relies on its non-reactivity. It is widely used as a shielding gas in arc welding to create an inert atmosphere that prevents the hot metal from reacting with oxygen and nitrogen. Argon also fills incandescent light bulbs, preventing the oxidation and rapid deterioration of the hot tungsten filament.
Neon is famously used in lighting because it emits a vibrant red-orange glow when an electrical discharge passes through it. Other noble gases, including argon and xenon, can be mixed or used to create the wide spectrum of colors seen in advertising signs. Krypton and xenon are used in high-intensity lamps and flash photography, as they produce very bright light flashes.