Final answer:
To form binary ionic compounds such as CaS and CsF, we balance the positive and negative charges of the ions. For compounds with polyatomic ions like calcium nitrate, we use parentheses to denote multiple polyatomic ions and balance the overall charge.
Step-by-step explanation:
When combining ions to form binary ionic compounds, the goal is to create a compound that is electrically neutral. This means the total positive charge of the cations must equal the total negative charge of the anions. For example, to combine calcium ions (Ca2+) with sulfur ions (S2-), we need one of each ion so that the charges can cancel each other out, resulting in the formula CaS. Similarly, for cesium ions (Cs+) and fluorine ions (F-), we also need one of each to balance the charges, yielding the formula CsF.
Important Points about Polyatomic Ions
When dealing with polyatomic ions, if more than one is needed, we must enclose the polyatomic ion in parentheses and add the appropriate subscript outside the parentheses. For instance, combining calcium ions (Ca2+) with nitrate ions (NO3-) requires two nitrate ions to balance the charge of one calcium ion. The correct formula for calcium nitrate is thus Ca(NO3)2.