What does "[Ar]" Symbolize in Chemistry?

[Masschaos]

I've been going through some chemistry revision for my exams, and I've been noticing this symbol '[Ar]' pop up a lot. At first I thought it was the concentration of Argon atoms, but that doesn't make sense. It appears in some question in the following way.

Which ground state electronic configurations are incorrect?

a) Ca: [Ar] 4s²
b) Zn: [Ar] 2d¹⁰ 4s²
and so on.
What does it mean?

 

[Ygggdrasil]

It's short-hand for the electron configuration of Argon (1s2 2s2 2p6 3s2 3p6). Instead of writing out the configuration of the inner shell electrons, chemists usually shorten it by writing [Ar] instead.

 

[aroc91]

hemists usually shorten it by writing [Ar] instead.

Or whatever other noble gas is applicable.

 

[Masschaos]

Ahh! Thanks!