The only difference between writing a normal electron configuration and writing the electron configuration for an ion is that when writing an electron configuration for an ion, you have to remember to add or subtract electrons from your total. Remember if an ion has a positive (+) charge (cation) you must subtract electrons from your total. And, if the element is an anion (negative charge), you must add electrons to your total. Look at the example below:
Aluminum will lose three electrons when it forms an ion. So although a neutral atom of aluminum has 13 electrons, the ion of aluminum, Al3+ , has lost three electrons and only has 10. Thus, you should write the electron configuration for 10 electrons.
Answer: Al3+ : 1s22s22p6
Given : S2-
Sulfur will gain two electrons when it forms an ion. So although a neutral atom of sulfur has 16 electrons, the ion of sulfur, S2- , has gained two electrons. Therefore, you should write the electron configuration for 18 electrons.
Answer : S2- : 1s22s22p63s23p6