The noble gases are the six elements located in Group 18 of the periodic table: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn). Historically, these elements were called “inert gases” because they appeared completely unreactive under normal conditions. The term “inert” suggests a substance has little tendency to participate in chemical reactions or form compounds. However, modern chemistry has revealed this traditional label is misleading, as some of these gases can be coaxed into forming stable chemical compounds.
Why Noble Gases Resist Reaction
The belief that Group 18 elements are inert stems from their unique atomic structure. Except for Helium, which has two electrons, all noble gases possess a complete valence shell containing eight electrons (a complete octet). This arrangement represents an extremely stable, low-energy state for an atom. Chemical reactions occur because atoms seek to achieve this stable configuration by gaining, losing, or sharing electrons. Since noble gases already have this configuration, they have no tendency to engage in these processes. They exhibit the highest ionization energies within their periods, meaning significant energy is required to remove an electron and make them reactive. They also have an electron affinity close to zero, showing no attraction for gaining an extra electron. This combination makes bond formation highly unfavorable under standard laboratory conditions.
The Discovery of Noble Gas Reactivity
The belief in inertness was shattered in 1962 by chemist Neil Bartlett. Bartlett had previously created a compound using platinum hexafluoride (\(\text{PtF}_6\)), a powerful oxidizing agent, and oxygen gas. He realized the energy needed to remove an electron from Xenon (\(\text{Xe}\)) was almost identical to the energy required for the oxygen molecule. This insight prompted him to mix Xenon gas with the deep-red platinum hexafluoride gas. The resulting reaction immediately produced a yellow-orange solid, which was the world’s first genuine noble gas compound. This experiment proved that Xenon was not inert if paired with a sufficiently strong oxidizer. The ability of noble gases to react increases as you move down Group 18 (Krypton, Xenon, and Radon). This occurs because the valence electrons are farther from the nucleus in heavier atoms, reducing the nuclear pull and making them easier to ionize.
Xenon Chemistry: The Primary Exception
Xenon is the most chemically versatile of the noble gases, forming the largest number of stable compounds. Its compounds primarily involve the highly electronegative elements, Fluorine and Oxygen. The most studied compounds are the Xenon fluorides, which include Xenon difluoride (\(\text{XeF}_2\)), Xenon tetrafluoride (\(\text{XeF}_4\)), and Xenon hexafluoride (\(\text{XeF}_6\)). These fluorides are colorless, crystalline solids synthesized by reacting Xenon and Fluorine gases under specific conditions of high heat and pressure. Xenon also forms several explosive oxides, such as Xenon trioxide (\(\text{XeO}_3\)) and Xenon tetroxide (\(\text{XeO}_4\)), which are often created by the hydrolysis of the fluorides. While Krypton has also been shown to form a compound, Krypton difluoride (\(\text{KrF}_2\)), Xenon remains the element that most clearly demonstrates the conditional reactivity of the noble gases.
The Truly Inert Gases and Practical Applications
While the heavier noble gases can react, the lighter elements—Helium, Neon, and Argon—remain unreactive under all practical conditions. Their small atomic size and the strong hold the nucleus has on their electrons make their ionization energy too high for compound formation. Their lack of chemical activity is highly valued in industry and technology. Argon, which makes up almost one percent of the Earth’s atmosphere, is used as a shielding gas in arc welding to prevent hot metals from reacting with oxygen and nitrogen. Neon is used in discharge tubes to create bright red-orange light for advertising signs due to its stability. Helium’s inertness and low density make it the preferred, non-flammable gas for filling airships and scientific balloons, as well as providing an inert environment in semiconductor manufacturing. The overall answer is nuanced: the original classification is an oversimplification, but the lighter gases remain chemically aloof, while the heavier ones are conditionally reactive.