# 3.1 Ionic and covalent compounds  (Page 4/13)

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In every ionic compound, the total number of positive charges of the cations equals the total number of negative charges of the anions. Thus, ionic compounds are electrically neutral overall, even though they contain positive and negative ions. We can use this observation to help us write the formula of an ionic compound. The formula of an ionic compound must have a ratio of ions such that the numbers of positive and negative charges are equal.

## Predicting the formula of an ionic compound

The gemstone sapphire ( [link] ) is mostly a compound of aluminum and oxygen that contains aluminum cations, Al 3+ , and oxygen anions, O 2− . What is the formula of this compound?

## Solution

Because the ionic compound must be electrically neutral, it must have the same number of positive and negative charges. Two aluminum ions, each with a charge of 3+, would give us six positive charges, and three oxide ions, each with a charge of 2−, would give us six negative charges. The formula would be Al 2 O 3 .

Predict the formula of the ionic compound formed between the sodium cation, Na + , and the sulfide anion, S 2− .

Na 2 S

Many ionic compounds contain polyatomic ions ( [link] ) as the cation, the anion, or both. As with simple ionic compounds, these compounds must also be electrically neutral, so their formulas can be predicted by treating the polyatomic ions as discrete units. We use parentheses in a formula to indicate a group of atoms that behave as a unit. For example, the formula for calcium phosphate, one of the minerals in our bones, is Ca 3 (PO 4 ) 2 . This formula indicates that there are three calcium ions (Ca 2+ ) for every two phosphate $\left({\text{PO}}_{4}{}^{3-}\right)$ groups. The ${\text{PO}}_{4}{}^{3-}$ groups are discrete units, each consisting of one phosphorus atom and four oxygen atoms, and having an overall charge of 3−. The compound is electrically neutral, and its formula shows a total count of three Ca, two P, and eight O atoms.

## Predicting the formula of a compound with a polyatomic anion

Baking powder contains calcium dihydrogen phosphate, an ionic compound composed of the ions Ca 2+ and ${\text{H}}_{2}{\text{PO}}_{4}{}^{-}.$ What is the formula of this compound?

## Solution

The positive and negative charges must balance, and this ionic compound must be electrically neutral. Thus, we must have two negative charges to balance the 2+ charge of the calcium ion. This requires a ratio of one Ca 2+ ion to two ${\text{H}}_{2}{\text{PO}}_{4}{}^{-}$ ions. We designate this by enclosing the formula for the dihydrogen phosphate ion in parentheses and adding a subscript 2. The formula is Ca(H 2 PO 4 ) 2 .

Predict the formula of the ionic compound formed between the lithium ion and the peroxide ion, ${\text{O}}_{2}{}^{2-}$ (Hint: Use the periodic table to predict the sign and the charge on the lithium ion.)

Li 2 O 2

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