What is a spectator ion in the reaction between ba(no3)2(aq) and (nh4)3po4(aq)?

the reaction mixture contains some ammonia, plus a lot of unreacted nitrogen and hydrogen. the mixture is cooled and compressed, causing the ammonia gas to condense into a liquid. the liquefied ammonia is separated and removed. the unreacted nitrogen and hydrogen are then recycled back into the reactor .

answer; 4 electrons;

4d^55s^1 ( third choice)