Argon
Atomic Data
| Atomic Number | 18 |
| Symbol | Ar |
| Atomic Weight | 39.948 u |
| Density (STP) | 1.784 g/L |
| Melting Point | −189.35 °C (83.80 K) |
| Boiling Point | −185.85 °C (87.30 K) |
| Electronegativity | — |
| Electron Config. | 1s² 2s² 2p⁶ 3s² 3p⁶ |
| Oxidation States | 0 |
| Phase at STP | Gas |
| Category | Noble gas |
| Period / Group | 3 / 18 |
| CAS Number | 7440-37-1 |
Electron Configuration
1s2 2s2 2p6 3s2 3p6
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| 2p | 6 | 10 | ||
| M | 3 | 3s | 2 | 12 |
| 3p | 6 | 18 | ||
| Total | 18 | 18 | ||
Isotopes of Argon
Argon has three naturally occurring isotopes, all stable. 40Ar dominates at over 99.6%, largely because it accumulates in the atmosphere through the radioactive decay of 40K (potassium-40) in Earth’s crust over billions of years.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Argon-36 | ₃₆Ar | 18 | 18 | 0.3365% | Stable |
| Argon-38 | ₃₈Ar | 18 | 20 | 0.0632% | Stable |
| Argon-40 | ₄₀Ar | 18 | 22 | 99.6003% | Stable |
Abundance & Occurrence
Argon is the third most abundant gas in Earth’s dry atmosphere at approximately 0.93% by volume, surpassing carbon dioxide. It is produced commercially by the fractional distillation of liquid air, which makes it one of the most cost-effective inert gases available for industrial use. In the broader universe argon is relatively scarce, contributing around 100 parts per million by mass.
COMPOSITION OF EARTH'S ATMOSPHERE (BY VOLUME)
NOBLE GASES IN THE UNIVERSE (PPM BY MASS)
Discovery & History
Safety & Handling
- Argon is non-toxic and chemically inert — it causes no chemical injury to tissue and is not classified as a chemical hazard under normal exposure conditions.
- Asphyxiation hazard: being denser than air, argon accumulates in low-lying and confined spaces such as pits, tanks, and poorly ventilated rooms. Oxygen-deficiency monitors should be used wherever argon is stored or used in quantity.
- No sensory warning: argon is colourless, odourless, and tasteless, providing no indication of dangerous concentrations. Exposure can cause rapid unconsciousness without prior warning.
- Cryogenic liquid: liquid argon (boiling point −185.85 °C) causes severe cryogenic burns on contact with skin and eyes. Insulated gloves, a face shield, and closed-toe footwear are required when handling liquid argon.
- Cylinder storage: pressurised cylinders must be secured upright, kept away from heat sources, and stored away from flammable or oxidising gases.
Argon in the Real World
Real-World Uses
- Welding shielding gas — Argon is the dominant shielding gas in TIG (tungsten inert gas) and MIG (metal inert gas) welding. It surrounds the hot weld pool and prevents oxidation and nitridation, producing clean, strong welds in stainless steel, aluminium, and titanium.
- Incandescent and fluorescent lighting — Argon fills incandescent light bulbs to slow tungsten filament evaporation and extend bulb life. Its inertness prevents the filament from reacting with the fill gas at high operating temperatures.
- Semiconductor fabrication — Ultra-high-purity argon is used in chemical vapour deposition, sputtering, and ion implantation during chip manufacturing, where even trace reactive gases would contaminate the silicon wafer.
- Wine preservation — Argon is pumped into partially consumed wine bottles to form a blanket above the liquid, displacing oxygen and preventing oxidation and spoilage. Its higher density compared to oxygen makes it particularly effective for this purpose.
- Fire suppression — Argon-based gaseous suppression systems extinguish fires in data centres, archives, and server rooms by reducing oxygen concentration below the level needed for combustion, leaving no residue that could damage sensitive equipment.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is argon used for?
Argon is used primarily as a shielding gas in arc welding to prevent atmospheric contamination of the weld pool. It is also used to fill incandescent and fluorescent light bulbs, in semiconductor fabrication as an inert purge gas, for wine preservation, and in fire suppression systems for archives and data centres.
Is argon dangerous to humans?
Argon is non-toxic and chemically inert, so it poses no chemical hazard. However, it is an asphyxiant — in confined spaces it can displace oxygen and cause unconsciousness or death through oxygen deprivation. It is colourless and odourless, so there is no sensory warning before dangerous concentrations build up.
Why is argon used in welding?
Argon is used as a shielding gas in TIG and MIG welding because it is chemically inert and prevents oxygen and nitrogen in the air from reacting with the hot molten metal. This produces cleaner, stronger welds with fewer defects. Argon’s density also means it blankets the weld area effectively at low flow rates.
Who discovered argon?
Argon was discovered in 1894 by Lord Rayleigh and William Ramsay. Rayleigh noticed that nitrogen extracted from air was slightly denser than nitrogen produced chemically, leading Ramsay to isolate the unknown component. They named it argon from the Greek word meaning ‘lazy’ or ‘inactive’, reflecting its chemical inertness.
Does argon form any chemical compounds?
Argon has a completely filled outer electron shell, giving it essentially zero chemical reactivity under normal conditions. Its oxidation state is always 0. Under extreme laboratory conditions, argon fluorohydride (HArF) was synthesised at very low temperatures in 2000, but no stable argon compounds exist at room temperature.