The same way that ones before argon are determined. Find the noble gas before the element you are doing. This becomes your base for the electron configuration so you can start counting from that element number. Remember the subshells 1s 2s 2p 3s 3p 4s 3d 4p etc s=2, p=6, d=10, f=14 ......so just keep writing till you reach the number of the element. So eg Selenium (Se) would be- 1s(2) 2s(2) 2p(6) 3s(2) 3p(6) 4s(2) 3d(10) 4p(4) . All the numbers in the brackets add up to Selenium's Atomic number. A shorter way of writing it would be [Ar] 4s(2) 3d(10) 4p(4) as Arsenic is already at 18
