Final answer:
Type III binary compounds, or ternary ionic compounds, are composed of three elements. The formula is written by placing the cation followed by the anion with charges, and swapping the charges as subscripts of the opposite ion.
Step-by-step explanation:
Type III binary compounds, also known as ternary ionic compounds, are composed of three elements. In a typical ternary ionic compound, there is still one type of cation and one type of anion involved, and both can be polyatomic ions. The formula of a binary ionic compound is written by placing the cation followed by the anion with charges. The charges are then swapped as subscripts of the opposite ion, and the subscripts are simplified to the simplest whole-number ratio.