Balancing the charges

The idea

In an ionic compound the positive and negative charges balance so that the overall structure is neutral, and the charges on the ions are not shown in its formula.

One calcium, how many chlorides?

A calcium ion has a charge of 2+. A chloride ion has a charge of 1−.

How many chloride ions do you think are needed to balance one calcium ion?

• Two
• One
• Three

Balance the charges

Choose the ions, then use the buttons to set how many of each. Make the net charge zero.

Neutral overall

In calcium chloride there are two chloride ions for each calcium ion. Two charges of 1− balance one charge of 2+. The charges are used to find the formula, but they are not written in it.

In an ionic compound the positive and negative charges balance so that the overall structure is neutral. The charges on the ions are not shown in its formula.

Notes

In an ionic compound positive and negative charges balance, so it is neutral. The charges are not shown in the formula.

Check yourself

How many chloride ions (Cl⁻) balance one calcium ion (Ca²⁺)?

Answer: 2

Ca²⁺ has a charge of 2+. Two Cl⁻ ions give 2−, so the charges balance.

Two Al³⁺ ions are present. What is their total positive charge?

Answer: 6

2 × 3 = 6, so the total charge is 6+.

How many O²⁻ ions are needed to balance the charge of 2 Al³⁺ ions?

Answer: 3

Total positive charge is 6+. Three O²⁻ ions give 6−, so they balance.

How is calcium chloride written?

• Ca²⁺Cl⁻₂, with the charges shown. The charges on the ions are not indicated in the formula of the compound.
• CaCl₂, without showing the charges — correct. Yes!
• Ca₂Cl. The charge of Ca, 2, becomes the subscript of Cl, and the charge of Cl, 1, becomes the subscript of Ca. The formula is CaCl₂.