Criss-cross for ionic compounds

The idea

Write the cation's symbol, then the anion's, with their charges beneath; cross over only the charge numbers as subscripts and divide by any common factor, since the formula gives the simplest ratio.

Magnesium and oxygen

Magnesium ions are Mg²⁺ and oxide ions are O²⁻. Crossing over the charges gives Mg₂O₂.

The formula gives the simplest ratio of the elements. What do you think the formula of magnesium oxide is?

• MgO
• Mg₂O₂
• Mg₂O

Cross the charges

Choose a cation and an anion. Only the numbers of the charges cross over.

The steps

Write the cation first, then the anion, with the charges under the symbols. Cross over only the numbers. The formula gives the simplest ratio, so divide the subscripts by any common factor.

Write the cation's symbol, then the anion's, with their charges beneath. Cross over only the charge numbers as subscripts and divide by any common factor, since the formula gives the simplest ratio.

Notes

Cross over only the charge numbers as subscripts, then divide by any common factor. The formula gives the simplest ratio: CaCl₂, Al₂O₃, MgO.

Check yourself

Ca has charge 2+ and Cl has charge 1−. What is the formula of calcium chloride?

Al has charge 3+ and O has charge 2−. What is the formula of aluminium oxide?

Crossing over the charges for Mg²⁺ and O²⁻ gives Mg₂O₂. Why is the formula written MgO?

Crossing over gives the subscripts 2 and 4. After dividing by the common factor 2, what is the smaller subscript?

Answer: 1

2 ÷ 2 = 1 and 4 ÷ 2 = 2, so the subscripts are 1 and 2.

• Ca₂Cl. The charge of Cl, 1, becomes the subscript of Ca, which is not written. The charge of Ca, 2, becomes the subscript of Cl.
• CaCl₂ — correct. Yes!
• CaCl. After crossing over, chlorine gets the subscript 2.
• Al₃O₂. The charge of Al, 3, becomes the subscript of oxygen. The charge of O, 2, becomes the subscript of aluminium.
• AlO. Crossing over gives Al₂O₃. The subscripts 2 and 3 have no common factor, so they stay.
• Al₂O₃ — correct. Yes!
• The subscripts are divided by the common factor 2 to give the simplest ratio — correct. Yes!
• The charges must be shown in the formula. The charges on the ions are not shown in a formula.
• Subscripts such as 2 are not allowed. Subscripts such as 2 are written, as in CaCl₂. In MgO the simplest ratio is 1 to 1, so the subscripts are 1.