## Chemistry: The Science in Context (4th Edition)

Calcium forms the ion Ca$_{2+}$. The nitrate ion is given by the formula NO$_{3}^{-}$. Calcium has a +2 charge whereas nitrate has a -1 charge. For the compound between calcium and nitrate to have a neutral charge, the charges must equal. We need two negative charges to counter the +2 charge of calcium. To do this, we need to combine two nitrate ions for every calcium atom. The molecular formula for calcium nitrate is option (d) Ca(NO$_{3}$)$_{2}$.