Argon is a colorless, odorless, and non-reactive gas belonging to the noble gases group on the periodic table. It holds a remarkably high concentration in the atmosphere. Argon is the most abundant noble gas found in Earth’s atmosphere, making up a significant portion of the total gaseous composition. Understanding the presence of this gas is important for grasping the overall chemical makeup of the air we breathe.
Atmospheric Concentration of Argon
Argon accounts for approximately 0.934% of dry air by volume. This places it as the third most abundant gas in the atmosphere, following nitrogen at about 78% and oxygen at nearly 21%. While a figure under one percent may seem minor, this fraction represents a massive global volume. Argon’s concentration is far greater than that of other atmospheric components, such as carbon dioxide (about 0.04%). The consistency of this percentage means argon is a stable component of the Earth’s gaseous envelope.
The Unique Origin of Atmospheric Argon
The reason for argon’s substantial atmospheric presence is unlike the biological or chemical cycles that govern nitrogen and oxygen. The vast majority of atmospheric argon, over 99.6%, exists as the isotope Argon-40 (\(\text{Ar}^{40}\)). This specific isotope is not a remnant of the Earth’s primordial formation but is instead generated from within the planet itself. \(\text{Ar}^{40}\) is continuously produced by the radioactive decay of Potassium-40 (\(\text{K}^{40}\)), which is widespread in the Earth’s rocks and minerals.
The \(\text{K}^{40}\) isotope undergoes a process called electron capture, with about 10.7% of its decay path resulting in the formation of \(\text{Ar}^{40}\). This decay has a half-life of 1.248 billion years, meaning the process has been steadily releasing argon over geological time scales. Once created, the gaseous argon slowly escapes from the crust and mantle rocks through volcanic activity and other geological processes in a mechanism known as outgassing. The lighter, primordial isotopes of argon, such as Argon-36 (\(\text{Ar}^{36}\)), make up only a negligible trace amount of the argon found on Earth.
Properties and Practical Applications
Argon’s defining characteristic, which is also reflected in its name derived from the Greek word argos meaning “lazy” or “inactive,” is its chemical inertness. As a noble gas, it possesses a full outer electron shell, making it extremely reluctant to form chemical bonds with other elements. This non-reactive nature is the foundation for its industrial and scientific uses.
Argon is commonly used as a shielding gas in various welding techniques, like gas tungsten arc welding (TIG) and gas metal arc welding (MIG). By surrounding the weld area, argon displaces oxygen and nitrogen, preventing the hot metals from oxidizing or reacting and ensuring a clean, strong joint. It is also employed in lighting, filling incandescent light bulbs to prevent the tungsten filament from oxidizing prematurely. Furthermore, its ability to remain chemically isolated makes it the basis for Potassium-Argon dating, a geological technique that measures the ratio of \(\text{K}^{40}\) to \(\text{Ar}^{40}\) to determine the age of ancient rocks.