The term "ammonium" refers to a positively charged polyatomic ion, which is written as NH4+. It consists of one nitrogen atom (N) and four hydrogen atoms (H).
When discussing the relative atomic mass, it usually refers to the average mass of the isotopes of an element. However, since ammonium is an ion and not an element itself, it does not have a relative atomic mass. The concept of relative atomic mass is specific to individual elements and not ions or compounds.