The noble gases are a group of elements defined by their extreme lack of chemical reactivity. These colorless, odorless, and monatomic gases include helium, neon, argon, krypton, xenon, and radon. Their defining characteristic stems from having a full valence electron shell, which gives them a highly stable electronic configuration. This stability means they have little tendency to gain or lose electrons, making them reluctant to participate in chemical reactions that form compounds.
Placement on the Periodic Table
The noble gases occupy Group 18, the far-right column of the periodic table. This placement is significant because Group 18 elements possess a complete outer shell of valence electrons, known as a full octet (except helium, which has a full duplet). This configuration is the underlying chemical reason for their stability and non-reactive behavior.
This stable structure translates to the highest ionization energies within their respective periods, meaning a large amount of energy is required to remove an electron. Their placement reflects a completed energy level, which provides the chemical foundation for understanding their natural distribution. Because they do not readily bond, they exist primarily as single, uncombined atoms, and their presence on Earth is dictated by physical processes.
Noble Gases in Earth’s Atmosphere
The Earth’s atmosphere serves as the primary reservoir for four of the six stable noble gases: neon, argon, krypton, and xenon. Argon is the most abundant noble gas in the air, comprising approximately 0.93% of the atmosphere by volume, making it the third most common gas overall after nitrogen and oxygen. Neon, krypton, and xenon are present in much smaller, trace amounts.
This atmospheric concentration provides the main source for their industrial collection, accomplished through cryogenic distillation. The method begins by cooling and compressing air until it liquefies, achieving a temperature near absolute zero. Since each gas has a distinct, extremely low boiling point, the liquefied air is then carefully warmed, allowing the components to boil off and be separated at different temperatures. Neon is separated first, followed by argon, krypton, and xenon. The abundance of these gases is relatively stable because they are heavy enough to be retained by Earth’s gravity.
Sources Beyond the Atmosphere
While most noble gases are harvested from the air, the two exceptions, helium and radon, have their origins deep within the Earth’s crust. Terrestrial helium is not a remnant of the planet’s formation but is continually created by a process called radiogenic decay. Specifically, it forms when heavy radioactive elements like uranium and thorium, found in granitoid rocks, decay and emit alpha particles.
These alpha particles are essentially the nuclei of helium atoms, which then collect electrons to become neutral helium gas. Due to its tiny atomic size and buoyancy, this helium gas slowly migrates upward through faults and porous rock layers. The gas often becomes trapped alongside natural gas deposits under non-porous cap rocks, such as salt or anhydrite, where it is extracted commercially, sometimes reaching concentrations of up to 7% by volume in the raw gas.
Radon, the heaviest noble gas, is also a radiogenic product, specifically an intermediate step in the decay chain of uranium-238. Radon-222 is formed from the decay of radium-226, an element present in virtually all rocks and soil. Because it is a gas, radon escapes the ground and can accumulate in confined spaces like basements, presenting a unique environmental concern.
Cosmic Abundance
On a cosmic scale, helium and neon are overwhelmingly abundant. Helium is the second most plentiful element in the observable universe, having been formed primarily during the Big Bang and continuously produced in stars through nuclear fusion.