Argon is element 18 on the Periodic Table and it has an average atomic mass of 39.95 g/mol. Argon is a Noble gas. Noble gases are not very reactive since they have a very stable electron configuration. It is only about 1% of the air in the atmosphere but it is more abundant than any other element on the Periodic Table behind Nitrogen and Oxygen. There are three reasons why this gas is found in the atmosphere. One reason is because it is non-reactive. Like other noble gases, it is stable and resists combining with other elements. Once it gets in the atmosphere it is difficult to get rid of. Another reason it is found in the atmosphere is because it is relatively heavy. Argon is heavy enough that it tends to not be lost to space like for example Helium is lost. Helium is also a noble gas but it is very light and can over time travel to the upper atmosphere and escape into space. Third, there is a relative abundance of Argon that is emitted from the Earth as compared to other gases. This resupply with very little escaping the atmosphere or reacting with other elements helps maintain a high level of Argon as compared to most other gases.