Argon (Ar – Atomic number 18) is the third most abundant gas in the atmosphere, after nitrogen and oxygen, and on the increase – albeit slowly. Not really good for nothing – but good for doing nothing – one of the inert gases. Most of its uses rely on this fact – whether it is keeping wine fresh or allowing metals to be welded in an inert atmosphere – or protecting the filament of a light bulb.
I was going to say that Argon replace the vacuum that had been used to allow filaments in early light bulb to glow without burning away – but not sure it makes sense that a vacuum can be replaced. The word Argon comes from the Greek word αργον – meaning lazy – as it can’t be bothered to react. There are very few compounds of Argon – argon fluorohydride is one – but needs very low temperatures to survive (40 kelvin). (Just in case you are wondering – like I was – the root for the Argonauts was Argo – from the word meaning swift)
In the intro I mentioned the volume of Argon in the atmosphere is increasing (around 0.94% currently) and this is due to the decay of 40K to 40Ar by electron capture. This fact is also used for dating rocks – as the amount of Ar is indicative of the age or the rock.
Another good periodic table fact is that in early periodic tables by Mendelev, Potassium preceded Argon – as the order was first based on atomic mass – and even though Argon has one fewer protons than Potassium, the most abundant isotopes of Argon have more neutrons – and hence the relative atomic mass is higher for Argon. The nature of Argon compared to Potassium seemed to fit the other order better – and the periodic table was soon changed to follow atomic number.
The excellent interactive Periodic Table from the Royal Society of Chemistry is the one seen in the background of these photographs.