我想問什麼是多原子離子和什麼是單原子離子

nh3 ammonia, nh4 ammonium.

ammonium (NH4) is the ionized form of ammonia (NH3), i presume you know what ionized form is..... how does that happens you may ask,
well.. as your equation suggested (2)NH3 + H = NH4; it means that it happens when the water is acidic.