By Ron B. Davis Jr., Georgetown University
In 1892, Lord Raleigh observed that nitrogen from the air was different from nitrogen contained in chemical compounds. Previous samples of nitrogen contained a previously undetected element. And because this new element was so unreactive that it had escaped detection by chemical methods, Sir William Ramsay decided to name this element Argon, for a Greek word ‘argos’, meaning ‘idle’, ‘lazy’ or ‘inactive’.
New, Undiscovered Gas
On physicist Lord Raleigh’s discovery, the Scottish chemist, Sir William Ramsay, proposed a collaboration to investigate this unusual observation.
Ramsay remembered reading accounts of Henry Cavendish, who had chemically removed nitrogen and oxygen from samples of air, yet a small fraction of gas remained, just less than 1% of the original sample. Ramsay came to believe that this small fraction of the atmosphere was a new, undiscovered gas, with a density greater than nitrogen.
Ramsay proved to be correct. He proved his theory using liquified air—air that had been cooled to temperatures so low that the gases it contained condensed into liquids. As he slowly warmed his liquified air, he collected pure gases one by one as they reached their boiling points. In doing so, he managed to collect pure nitrogen from air that did, in fact, have the same density as nitrogen from other sources.
If we turn our attention to the periodic table, it is easy to see why Raleigh’s atmospheric nitrogen samples appeared a bit more dense.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Argon’s average atomic mass is reported to be about 40, while a dinitrogen molecule contains two nitrogen atoms at 14 amu each for a total of 28 amu. Hence, if we replace a small fraction of nitrogen molecules in a sample with heavier argon atoms, then the total mass of the system increases, increasing its density.
Argon was so different than all the other elements in its inertness. Raleigh wondered—could it be that argon is a member of a whole new group of lazy elements that simply don’t react with their surroundings?
Helium had already been noticed and named much earlier, back in 1868, but only in the corona of the Sun.
Because any such element was assumed to be molten metal, the astronomer Norman Lockyer gave it what had been a metal-style name—hel-IUM—making the name similar to sod-IUM, titan-IUM, and so on.
But for decades no one was able to find helium here on Earth. But Ramsay’s discovery of argon had scientists rethinking Lockyer’s assumption. Perhaps helium was related to Raleigh’s discovery of unreactive Argon gas.
This hunch was also proven right when Ramsay and a second group in Sweden both discovered helium emanating from methane gas associated with uranium ores here on Earth.
‘Distillation of Air’ Process
Over the next few years, Ramsay refined the liquid-air techniques he had used to isolate argon from air, studying argon more carefully.
In doing so, he identified neon, krypton and xenon all hiding in air as well, albeit in much lower concentrations than argon.
He accomplished this by cooling each sample of gas to the point at which it liquified, then slowly warming the sample, collecting gasses as they one-by-one reached their boiling points and vaporized. This is the ‘distillation of air’ process.
He discovered a gas in the air that boiled at an even lower temperature than nitrogen—and he called this ‘new’ gas neon. Additionally, distilling samples of purified argon left behind a heavier gas named krypton, for ‘hidden’.
When re-distilling purified krypton, Raleigh found it left behind yet another gas with an even higher boiling point—named ‘xenon’, for ‘stranger’.
A German chemist, Hugo Erdman, hearing about this group of new elements called them ‘noble gases’. Their lack of reactivity seemed similar to the already-known lack of reactivity of so-called ‘noble metals’.
Why Call Them ‘Noble’ Gases?
The octet rule gives us a sound argument for why group 18 is considered ‘noble’ elements. But why noble gases? What is it about this group that makes them all gaseous elements at room temperature?
Well, when it comes to nonmetals in particular, the phase behavior of a material—when and how it changes from solid to liquid to gas, depends largely on just how well an individual unit of that element sticks to others.
The ‘stickier’ an atom or molecule is, the easier is it to liquify from the gaseous state. Conversely, the less sticky atoms or molecules are, the wider the range of temperatures at which they have sufficient energy to fly apart and remain a gas.
Simply put, noble gases are some of the least sticky atoms on the table. And this gives them some of the lowest melting and boiling points of all.
When it comes to Argon gas, it is by far the most common of the noble gases in our atmosphere. This is mainly because it blends universal abundance with a density that allows it to be trapped by our atmosphere instead of floating into outer space like its smaller cousins, helium and neon can.
Argon’s abundance and air-like density have made it a popular choice as a fill gas anywhere an inert atmosphere is needed and density isn’t an issue. Argon works well for filling insulated windows and incandescent light bulbs because it does not tend to settle much over time.
Interestingly, argon, along with iodine and nickel, is one of those rare elements that bucks the trend of atomic number, increasing in lock-step with the increasing atomic mass. Argon’s atomic mass of 39.95 jumps ahead to be heavier than potassium at 39.10—that is, element 18 is heavier than element 19!
In conclusion, one can say that Argon is that noble gas which is always near the top of the list whenever ordinary air needs to be replaced with something less reactive. Thus, it is frequently called upon when inert atmospheres with densities similar to our own are needed.
Common Questions about Argon
Nitrogen isolated from our atmosphere had remained about 1% heavier in previous samples because scientists had not actually isolated pure nitrogen.
Argon works well for filling insulated windows and incandescent light bulbs because it does not tend to settle much over time.
When re-distilling purified krypton, Lord Raleigh found it left behind yet another gas with an even higher boiling point—named ‘xenon’.