#NH_4C_2H_3O_2#, is made of how many elements?
An element is simply a particular species of atom with one particular atomic number assigned to it. Some examples are
In ammonium acetate, all you have to do is pick out how many unique elements there are... and you can see there are nitrogen, hydrogen, carbon, and oxygen atoms in this compound.
Can you point to the letter that corresponds to each of the elements mentioned above? Can you thus count how many unique elements are in ammonium acetate?