Argon is the third most abundant component of the Earth’s atmosphere, comprising almost 1% of it (by volume).
In fact, other noble gases (e.g. Neon, Helium, Krypton) are also major constituents of the atmosphere.
Why is there an apparent excess of these gases in the atmosphere? Relative to other elements, their concentration in the atmosphere does not mirror their concentration or abundance in the Solar System. They are overrepresented in the former. Furthermore, even among the noble gases themselves, there are (in my opinion) unexpected phenomena. For example, although Neon is 20 times more prevalent in the Solar System than Argon, its concentration is only 1/500th that of argon in the atmosphere.
I suppose one reason to account for the atmospheric “overabundance” of the noble gases is their inertness itself. In other words, unlike other substances which can participate in chemical reactions (and, perhaps, promote their removal from the atmosphere), the noble gases are stuck there. Still, even if this is the case, it doesn’t explain the relative overabundance of argon versus neon noted above.
This is a bit of a WAG, but I presume that it is due to their, “nobility.”
Other gases like carbon (carbon dioxide), nitrogen, oxygen, sulfer (sulfur dioxide), etc. get involved in some type of organic synthesis and used in biological processes on earth. The noble gasses just sit there and hang around in the atmosphere instead of getting fixed in cellulose, the nitrogen cycle, etc.
Helium and neon are underrepresented in the atmosphere, relative to their cosmic abundance, helium by a dramatic ratio. This is because they are light, with a relatively low escape velocity, and of course being noble don’t react to form compounds that might help retain them.
Krypton and xenon, while rare elements both in terms of cosmic abundance and in their role as constitutents of the atmosphere, are slightly more common in the atmosphere than in the universe at large. The same logic applies in reverse: they’re quite heavy for gases, and find it less easy to escape off the top of the atmosphere.
Argon is the oddball: about 1% of the atmosphere as a whole. And, oddly, almost completely a single isotope of the several stable argon nuclides: Argon-40. The reason here is the fact that one of the three isotopes of natural potassium, K-40, is radioactive, with a half-life of about a billion years. For the most part, it beta-decays into Ca-40, but a fraction (somewhat under 1%) of the time it reverse-beta-decays into Ar-40. Over 4.5 billion years, that has built up to constitute about 1% of the atmosphere.
Escape velocity as the phrase is commonly used is independent of mass. I think the proper statement is, because they are so light, random collisions with other molecules in the air will tend to impart more speed to them making it more likely they will reach escape velocity.
You’re correct, of course, and I misspoke myself. Lighter molecules (which in the case of noble gases are single atoms) in the high energy low pressure exosphere will preferentially pick up the energy to attain escape velocity in random interactions, while heaver ones will not. Escape velocity is the same for them all, but it’s far easier for helium to be knocked up to it than for xenon.