How would I write the configuration for ions? Would I alter the last subshell (e.g. 1s2 2s2 2p6 3s2 3p6 4s2 3d10: I change 3d10) or the shell with the n and l (1s2 2s2 2p6 3s2 3p6 4s2 3d10: I change 4s2)? Thanks.
If your ion has a negative charge, add one electron to the electron configuration. If your ion is positive, subtract one electron.
For example, take Na. The electron configuration for the atom is [Ne] 3s1. If you have Na- it becomes [Ne] 3s2 & Na+, just [Ne].
Another example, suppose you have Cl, the electron configuration is [Ne] 3s2 3p5. Cl- is [Ar] & Cl+ is [Ne] 3s2 3p4. Notice how you add & subtract from the orbital with the highest energy
You have to be careful with the transition metals. Take for example [B]Ti, its electron configuration is [Ar] 4s2 3d2. Ti- is [Ar] 4s2 3d3. But here is where it "can" be tricky. Ti+ is [Ar] 4s1 3d2. [/B] The d orbitals are written after the s, but the 4s orbitals are still of higher energy. So you remove from them first, before the d orbitals.