Argon (Ar)

• Element Symbol: Ar
• Atomic Number: 18
• Atomic Mass: 39.948
• Group # in Periodic Table: 18
• Group Name: Noble gases
• Period in Periodic Table: 3
• Block of Periodic Table: p-block
• Discovered by: Sir William Ramsay, Lord Rayleigh (1894)

Argon is a common, nonreactive element of the periodic table. It is one of the six naturally occurring noble gases. The other noble gases are helium, neon, krypton, xenon, and radon. Argon was the first of these gases to be discovered on Earth.

Argon is found in air. After nitrogen and oxygen, it is the third-most abundant gas in Earth’s atmosphere, making up almost 1 percent of Earth’s atmosphere. This translates to around 66 trillion tons of argon gas.

British scientist Henry Cavendish unearthed the first evidence of argon in 1785. To understand the composition of the atmosphere, Cavendish removed the oxygen from a sample of air. The air that remained was composed mainly of nitrogen gas. But Cavendish noted that a small fraction of the air in the sample was different and completely nonreactive. This discovery was the first indication that an inert new element existed in air.

In 1894 British scientists Lord Rayleigh and Sir William Ramsay continued investigations of atmospheric gases. Rayleigh discovered that atmospheric nitrogen was denser than the nitrogen that was produced during the breakdown of ammonia. This finding suggested that another gas was present in addition to the atmospheric nitrogen. To try to isolate this gas, Ramsay burned magnesium within a sample of atmospheric nitrogen. The nitrogen reacted to form solid magnesium nitride. An inert gas remained in the sample. The scientists named this gas argon. This name is from the Greek word argos, which means "lazy" or "idle." This idea reflects the nonreactive nature of argon gas. Rayleigh and Ramsay won a $10,000 prize for their discovery of this new element.

Physical Properties

Argon is a colorless, odorless gas in its standard state—that is, its state at 298 kelvins (K). When an electric current is passed through argon, it emits a blue glow. Argon gas has a density of 1.784 grams per liter (g/L) at 0 degrees Celsius (°C) and 1 atmosphere (atm). This makes argon gas denser than nitrogen or oxygen gas, the two other most common components of air. The melting point of argon is −189.3 °C. Below this temperature, argon is a crystalline solid. The boiling point of argon is −185.8 °C. When argon is a liquid, it is colorless. The specific heat of argon gas is 520.33 joules per kilogram-kelvin (J/kg·K). Its thermal conductivity is 0.01772 watts per meter-kelvin (W/m·K).

Chemical Properties

When argon is in a solid state, its crystal structure is face-centered cubic. However, argon is typically in a gas state. This element has a full valence electron shell. Therefore, it is chemically inert and nonreactive. Argon’s electron affinity is 0 kilojoules per mole (kJ/mol). Its oxidation state is 0. Argon’s ionization energies are 1520.6 kJ/mol, 2665.8 kJ/mol, 3931 kJ/mol, 5771 kJ/mol, 7238 kJ/mol, 8781 kJ/mol, 11,995 kJ/mol, 13,842 kJ/mol, 40,760 kJ/mol, and 46,186 kJ/mol. The electron configuration for argon is 1s²2s²2p⁶3s²3p⁶.

Argon has three main stable isotopes: argon-36, argon-38, and argon-40. Argon-36 and argon-38 are the most common argon isotopes in the universe. However, the argon-40 isotope is the most common one on Earth. Around 99.6 percent of the argon on Earth is argon-40.

Applications

Argon is around the twelfth-most abundant element in the universe. On Earth it is the most abundant noble gas. Nearly 1 percent of Earth’s atmosphere is argon. Most of this argon has been produced by the breakdown of the radioactive isotope potassium-40. Around 11 percent of potassium-40 atoms present in magma or rock decay to form argon-40 atoms. Some of these argon atoms enter the atmosphere as a gas.

The rate of decay of potassium-40 (⁴⁰K) to form argon-40 (⁴⁰Ar) can be used to date rock. This is called potassium-argon dating, or K-Ar dating. When ⁴⁰K breaks down to form ⁴⁰Ar, some of the argon remains trapped in the rock. Scientists can then compare the percentage of ⁴⁰Ar atoms to ⁴⁰K atoms to estimate when the rock crystallized.

Argon is a very important industrial element. The inert nature of argon makes it useful in a wide variety of ways. Its abundance makes argon relatively cheap to isolate and use. Argon can be isolated from liquid air. It is generally obtained during the production of liquid nitrogen and liquid oxygen.

Argon is used during the industrial production and manufacturing of metals and metal products. During steel production, argon and oxygen are added to molten steel. The oxygen reacts with the carbon in the steel, forming carbon dioxide as a waste product. The argon prevents the oxygen from reacting with chromium and other metallic elements in the steel. Argon prevents oxidation during titanium, uranium, and zirconium production as well. It also acts as a shield against oxygen during arc-welding of stainless steel and aluminum.

Argon is used in the manufacture of some lightbulbs. Incandescent bulbs are generally filled with argon or a mixture of argon and nitrogen. The argon surrounds the metal filament in the bulb and prevents it from oxidizing when it reaches high temperatures. Argon is also used along with some mercury in compact fluorescent bulbs. An electric current causes these elements to produce ultraviolet light. This light excites a fluorescent coating inside the bulb that then emits visible light.

The space between double-paned windows is often filled with argon. Argon is clear and nonreactive. It is also more of an insulator than plain air, so it gives the window energy-saving properties.

Cylinders of argon gas are sold in liquor stores and in some stores specializing in kitchenware. The gas can be added to a bottle of wine after it has been opened and partially used. The argon gas sits on top of the wine and prevents oxidation by shielding the wine from oxygen. Oxidation can greatly reduce the quality of the wine.

Glowing electric signs are often called neon signs, but many of these signs contain argon gas rather than neon. The gas is added to a clear tube that has been shaped into a letter, number, or other image or symbol. The tube is then sealed. An electric voltage that is applied to the tube produces a current, which causes the argon and any other gases in the tube to glow. The glow from argon is a clear blue.

Bibliography

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Gray, Theodore. The Elements: A Visual Exploration of Every Known Atom in the Universe. New York: Black Dog, 2009. Print.

Parsons, Paul, and Gail Dixon. The Periodic Table: A Visual Guide to the Elements. New York: Quercus, 2014. Print.

"Technical Data for Argon." The Photographic Periodic Table of the Elements. Element Collection, n.d. Web. 3 Sept. 2015.