When writing a short hand electron configuration what does the square brackets mean?

Question

e.g. Calcium - [Ar] 4S^2

what does the square bracket around AR mean?

 

Thanks for you help!

Answer 1

The square bracket in short hand electron configuration always contains the noble gas preceding the element of interest, and is just an abbreviation for the full electron configuration of that noble gas.  Thus, in you example, [Ar]4s² means you have the electron configuration of argon plus an additional 4s² which would make the element calcium.

Answer 2

It means you add what is to the right of the brackets to the end of the electronic configuration of the element in the brackets.  In this case,

 

Ar is 1s²2s²2p⁶3p⁶.

 

Add at the end 4s² to get the electronic configuration of Ca:

 

1s²2s²2p⁶3p⁶4s².