Question

why is argon ar and not ar2?

Answer 1

@ganeshie8

Answer 2

Argon is not a diatomic element and therefore stands alone (ar instead of ar2)

Everything you need to know about diatomics:
http://chemistry.about.com/od/elementfacts/f/What-Are-The-Seven-Diatomic-Elements.htm

Answer 3

Molecules are formed because the atoms alone are not stable (in terms of electronic configurations, they do not have a full valence electron "shell") and so, they bond to each other to complete their "octet". In contrast, Argon (a noble) gas does have a full valence shell and does not require additional electrons to be stable (complete it's "octet") and thus exists as a monoatomic gas.