Final answer:
The subscripts in an ionic compound formula like CaCl2 are determined by balancing the charges of the ions involved. The crisscross method helps achieve this by crossing the numerical values of ion charges to the opposite ion, leading to a correct and electrically neutral formula.
Step-by-step explanation:
When writing the formula for an ionic compound like CaCl2, the subscripts are determined by ensuring the total positive charge equals the total negative charge. The subscript numbers indicate how many of each ion is needed. In the case of calcium chloride, calcium has a charge of +2 (Ca2+) and chlorine has a charge of -1 (Cl−). Using the crisscross method, the charges are crossed over to become the subscripts for the other ion, with signs omitted. Hence, one calcium ion is needed to balance two chloride ions, resulting in the chemical formula CaCl2.
The rules for writing the formulas of ionic compounds such as balancing overall charge, including polyatomic ions in parentheses, and the convention of writing the cation before the anion are crucial. For example, the compound between calcium ions and nitrate ions is Ca(NO3)2, not CaNO32 or CaN206. This highlights the importance of using parentheses when more than one polyatomic ion is needed to balance the charge.