Argon is element number 18 and has the atomic symbol Ar -- renamed in 1959 from its original atomic symbol, which was simply A. As you can see in the image above, argon gas produces a lovely bluish-purple colour when excited with electricity.

Argon is the third noble gas we've met so far, the others being helium and neon. Its name was derived from the Greek word for "lazy" or "the inactive one", because this element does not naturally undergo any chemical reactions. Argon's chemical inactivity results from having an outermost shell of electrons that is completely filled, so it isn't attracted to any other atoms (which is how chemical bonds are formed).

When I was a kid, I was first introduced to chemistry. In chemistry class, elements were characterised as either being "happy" or "unhappy" in their elemental form: "unhappy" elements formed bonds to create molecules, whereas "happy" elements remained single and mostly oblivious to all other elements. The noble gases are "happy" elements.

I sometimes wondered whether the "happy" elements were actually the zit-faced basement-dwelling computer-gamers of elemental world who couldn't be bothered to shower once in awhile before going out to find a mate. (Although, the noble gases are much prettier than most video gamers when stimulated with electricity.)

Ironically, "happy" argon is one of the noble gases used as lighting for "sin city" (Las Vegas). But besides that, what use does argon have? It is used in a number of uninteresting (to me) high-temperature industrial processes, where non-reactive substances are essential.

But argon does have an interesting story to tell astronomers and geologists. Argon constitutes 1.3 percent of earth's atmosphere by weight and 0.94 percent by volume. There are several stable isotopes (where the nucleus contains different numbers of neutrons) of argon on earth and each comes from a different source: argon-40 results from the decay of potassium-40 in rocks (nearly all argon found on earth is this isotope); argon-36 is produced directly by nuclear reactions in stars (most of the argon in solar winds is the argon-36 isotope); and argon-38, which is vanishingly rare on earth, appears to have either unknown or multiple sources.

Geologists use the ratio of argon-40 to argon-39 to identify the radiometric date for a geological event, such as the eruption and cooling of igneous rock and minerals. So-called argon-argon dating, this method is more accurate than potassium-argon (K/Ar) dating.

But perhaps the most exciting use of argon studies these days is in astronomy. Using several lines of evidence, astronomers can identify the elements present in planetary atmospheres. Relying on their knowledge of how these elements come about, they can make inferences and draw conclusions about the history and compositions of these planets. For example, the Martian atmosphere contains 1.6 percent argon-40 and 5 ppm of argon-36, Mercury's thin atmosphere is comprised of 70 percent argon (the relative abundances and identities of the isotopes is not clear), and Titan, the largest and most fascinating of Saturn's many moons, has argon-40 in its atmosphere. Based on these data, what conclusions can you make about the likely source for these argon isotopes, and what inferences can you make about these planetary bodies?

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You've already met these elements:

Chlorine: Cl, atomic number 17

Sulfur: S, atomic number 16

Phosphorus: P, atomic number 15

Silicon: Si, atomic number 14

Aluminium: Al, atomic number 13

Magnesium: Mg, atomic number 12

Sodium: Na, atomic number 11

Neon: Ne, atomic number 10

Fluorine: F, atomic number 9

Oxygen: O, atomic number 8

Nitrogen: N, atomic number 7

Carbon: C, atomic number 6

Boron: B, atomic number 5

Beryllium: Be, atomic number 4

Lithium: Li, atomic number 3

Helium: He, atomic number 2

Hydrogen: H, atomic number 1

Here's a wonderful interactive Periodic Table of the Elements that is just really really fun to play with!

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