how do you identify spectator ions in redox equations? Chemistry question?

Question

how do you identify spectator ions in redox equations?
I understand everything about redox equations like oxidation and reduction how to balance them. Balance the electrons and balance them in acidic and basic solution. But how do you identify spectator ions? Like I know that when an element such as Na is attached to dichromate =
Na2Cr2O7 -2 we must eliminate the Na. I know this but how about this problem:
AuCl3 + NaI > Au + I2 + NaCl

my teacher eliminated all Cl and Na
so AU + I > Au + I2 (why??? eliminate Na and Cl is it because they dont change oxidation states in the reaction???)

why does this occur? ANY help please would be great

Answer 1

Na+ and Cl- are eliminated because they don't change in any way. They don't combine with something and precipitate out, they don't change in charge (oxidation state). So, like spectators in a ballpark, they just sit and watch the reaction without taking any part in it. Na+ containing compounds tend to be very soluble, you'll often find Na+ as a spectator.

When its an ion, chlorine only has one oxidation state (-1)
If it's not an ion, then its the gas Cl2 and has one at 0. Since AuCl3 is not Cl2 gas, each Cl is worth -1, and that stays the same in the whole problem.