If someone asked you to name the three most abundant gases in Earth's atmosphere, you might choose, in some order, oxygen, carbon dioxide and nitrogen. If so, you would be right – mostly. It's a little-known fact that behind nitrogen (N2) and oxygen (O2), the third-most plentiful gas is the noble gas argon, accounting for just under 1 percent of the atmosphere's unseen composition.
The six noble gases derive their name from the fact that, from a chemistry standpoint, these elements are aloof, even haughty: They don't react with other elements, so they don't become bonded to other atoms to form more complex compounds. Rather than rendering them useless in industry, however, this tendency to mind one's own atomic business is what makes some of these gases handy for specific purposes. Five major uses of argon, for example, include its placement in neon lights, its ability to help determine the age of very old substances, its use as an insulator in manufacturing metals, its role as a welding gas and its use in 3-D printing.
Noble Gas Basics
The six noble gases – helium, neon, argon, krypton, xenon and radon – occupy the rightmost column in the periodic table of the elements. (Any examination of a chemical element should be accompanied by a periodic table; see Resources for an interactive example.) The real-world implications of this is that noble gases have no shareable electrons. Rather like a puzzle box containing exactly the right number of pieces, argon and its five cousins does not have any subatomic shortages that need to be amended by donations from other elements, and it doesn't have any extras floating around to donate in turn. The formal term for this non-reactivity of noble gases is "inert."
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Like a completed puzzle, a noble gas is very stable chemically. This means that, compared to other elements, it is difficult to knock the outermost electrons from noble gases using a beam of energy. This means that these elements – the only elements to exist as gases at room temperature, the others all being liquids or solids – have what is called a high ionization energy.
Helium, with one proton and one neutron, is the second-most abundant element in the universe behind hydrogen, which contains only a proton. The giant, ongoing nuclear fusion reaction that is responsible for stars being the super-bright objects they are is no more than countless hydrogen atoms colliding to form helium atoms over a period of billions of years.
When electrical energy is passed through a noble gas, light is emitted. This is the basis for neon signs, which is a generic term for any such display created using a noble gas.
Properties of Argon
Argon, abbreviated Ar, is element number 18 on the periodic table, making it the third-lightest of the six noble gases behind helium (atomic number 2) and neon (number 10). As befits an element that flies under the chemical and physical radar unless provoked, it is colorless, odorless and tasteless. It has a molecular weight of 39.7 grams per mole (also known as daltons) in its most stable configuration. You may recall from other reading that most elements come in isotopes, which are versions of the same element with the different numbers of neutrons and thus different masses (the number of protons does not change or else the identity of the element itself would have to change). This has critical implications in one of the major uses of argon.
Uses of Argon
Neon Lights: As described, noble gases are handy for creating neon lights. Argon, along with neon and krypton, is used for this purpose. When electricity passes through the argon gas, it temporarily excites the outermost orbiting electrons and causes them to briefly jump to a higher "shell," or energy level. When the electron then returns to its accustomed energy level, it emits a photon – a massless packet of light.
Radioisotope Dating: Argon can be used along with potassium, or K, which is element number 19 on the periodic table, to date objects up to a staggering 4 billion years old. The process works like this:
Potassium ordinarily has 19 protons and 21 neutrons, giving it about the same atomic mass as argon (just under 40) but with a different composition of protons and neutrons. When a radioactive particle known as a beta-particle collides with potassium, it can convert one of the protons in the nucleus of potassium to a neutron, changing the atom itself to argon (18 protons, 22 neutrons). This occurs at a predictable and fixed rate over time, and very slowly. So if scientists examine a sample of, say, volcanic rock, they can compare the ratio of argon to potassium in the sample (which rises incrementally over time) to the ratio that would exist in a "brand-new" sample, and determine how old the rock is.
Note that this is distinct from "carbon dating," a term that is often wrongly used to refer generically to using radioactive decay methods to date old objects. Carbon dating, which is just a specific type of radioisotope dating, is useful only for objects known to be on the order of thousands of years old.
Shield Gas in Welding: Argon is used in the welding of specialty alloys as well as in the welding of automobile frames, mufflers and other automotive parts. It is called a shield gas because it does not react with whatever gases and metals are hovering in the vicinity of the metals being welded; it merely takes up space and prevents other, unwanted reactions from occurring nearby owing to reactive gases such as nitrogen and oxygen.
Heat Treating: As an inert gas, argon can be used used to provide an oxygen- and nitrogen-free setting for heat-treating processes.
3-D Printing: Argon is put to use in the burgeoning field of three-dimensional printing. During the rapid heating and cooling of the printing material, the gas will prevent oxidation of the metal and other reactions and can limit stress impact. Argon can also be mixed with other gases to create specialty blends as needed.
Metal Production: Similar to its role in welding, argon can be used in the synthesis of metals via other processes because it prevents oxidation (rusting) and displaces unwanted gases such as carbon monoxide.
Dangers of Argon
That argon is chemically inert does not, unfortunately, mean that it is free of potential health hazards. Argon gas can irritate the skin and the eyes on contact, and in its liquid form it can cause frostbite (there are relatively few uses of argon oil, and "argan oil," a common ingredient in cosmetics, is not even remotely the same as argon). High levels of argon gas in the air in a closed environment can displace oxygen and lead to respiratory problems ranging from mild to severe, depending on how much argon is present. This results in symptoms of suffocation including headache, dizziness, confusion, weakness and tremors at the milder end, and coma and even death in the most extreme cases.
In cases of known skin or eye exposure, rinsing and flushing with warm water is the preferred treatment. When argon has been inhaled, standard respiratory support, including oxygenation by mask, may be required to being blood oxygen levels back to normal; getting the affected person out of the argon-rich environment is of course necessary as well.