Argon is a colorless, odorless, and chemically inert gas that resists forming compounds with other elements. The story of its discovery in the late 19th century is a classic example of scientific rigor, born from the meticulous investigation of a small, persistent atmospheric anomaly. This pursuit led to the identification of an entirely new class of elements and fundamentally changed the understanding of the periodic table. The gas is now recognized as the third most abundant gas in Earth’s atmosphere, making up about 0.934% of its volume.
The Unexplained Density Discrepancy
The discovery process began with a tiny, yet reproducible, measurement error concerning the density of nitrogen gas. In the 1890s, British physicist Lord Rayleigh was working to determine the precise atomic weight of nitrogen, using two distinct methods to prepare his gas samples. The first method involved isolating nitrogen from the atmosphere by removing all known components like oxygen, carbon dioxide, and water vapor, resulting in what was termed “atmospheric nitrogen.”
The second method involved synthesizing nitrogen chemically, typically by decomposing nitrogen-containing compounds such as ammonia, yielding “chemical nitrogen.” Rayleigh’s careful measurements revealed a systematic and undeniable difference between the two samples. Nitrogen isolated from the air was consistently found to be approximately 0.5% heavier than the nitrogen synthesized in the laboratory.
This discrepancy was too significant to be dismissed as experimental error, prompting Rayleigh to theorize that the atmospheric sample contained an additional, heavier, non-reactive gas. This suspicion echoed the forgotten work of Henry Cavendish, who, in 1785, had performed an experiment leaving a small, unreactive residue. Rayleigh’s precise data provided the modern context for this long-standing puzzle, setting the stage for the search for this unknown component.
Experimental Isolation of the New Gas
With the problem clearly defined, two British scientists, Lord Rayleigh and chemist Sir William Ramsay, embarked on independent efforts to isolate the mystery gas. Rayleigh chose to replicate and modernize Cavendish’s original technique, which focused on forcing known atmospheric components to react. His method involved passing a mixture of air and excess oxygen through a chamber where powerful electric sparks were repeatedly discharged.
The intense heat from the sparks caused the atmospheric nitrogen and oxygen to combine, forming nitrogen oxides, which were then absorbed by an alkali solution, such as lye. After many days, the unreactive residue that remained was confirmed to be a gas with a density about one and a half times that of nitrogen.
Ramsay, working simultaneously, employed a distinct chemical absorption method to separate the gases. He passed “atmospheric nitrogen” over red-hot copper to remove oxygen, and then over heated metallic magnesium. The magnesium combined with the nitrogen to form magnesium nitride (Mg3N2), effectively removing the nitrogen from the gaseous mixture. This process left behind a small, non-combustible, and inert gas residue. Their joint announcement in 1895 confirmed the existence of a new element.
Naming and Placement in the Periodic Table
The final steps involved confirming the new substance was an element and determining its properties. Spectroscopy, which analyzes the light emitted by an energized substance, was used to examine the isolated gas. The resulting spectral lines were unique and did not match those of any known element, providing definitive proof of a new atomic species.
The element’s most striking characteristic was its lack of chemical reactivity, which determined its name. It was named “Argon,” derived from the Greek word argos, meaning “lazy” or “inactive.” The name reflected its chemical nature, as the gas resisted all attempts to force it into a chemical reaction.
This inert element presented an immediate challenge to the structure of the periodic table, which was based on increasing atomic weight and similar chemical properties. Argon’s atomic weight was greater than that of the next element, potassium, yet its lack of reactivity meant it could not be placed alongside elements like chlorine or an alkali metal. The solution was the creation of an entirely new column, Group 18, dedicated to the noble gases, which led to the subsequent discovery of neon, krypton, and xenon.