Final answer:
To write the formula for an ionic compound, first determine the charge of the cation and anion, write the cation and then the anion, and use their charges to establish the correct subscripts for a neutral compound. Enclose polyatomic ions in parentheses if more than one ion is needed.
Step-by-step explanation:
Steps for Writing Formulas of Ionic Compounds
To write the formula for an ionic compound when you are given the names of its component ions, you would follow these steps:
Determine the formula for any polyatomic ion present.
Determine the charge on the ions that make up the compound. For monatomic ions consult the periodic table, for polyatomic ions recall the charge.
Write the cation first in the formula, then the anion.
Use the charges on the cation and anion to determine the correct subscripts to give a neutrally charged formula unit. If more than one of a particular polyatomic ion is needed to balance the charge, enclosure of the formula for the polyatomic ion in parentheses is necessary, and the numerical subscript is placed outside the parentheses (required).
For example, the compound name calcium nitrate consists of calcium ions (Ca2+) and nitrate ions (NO3-). To write its formula, write Ca2+ followed by NO3-. Since you need two nitrate ions to balance one calcium ion, the formula for calcium nitrate is Ca(NO3)2.