1. Binary Ionic Compounds Formula Writing
Science 10
Chemical Reactions

2. Binary Ionic Compounds
A binary compound is any compound that contains only two different elements.
A binary ionic compound is a binary compound that consists of a metal ion and non-metal ion.

3. Binary Ionic Compounds – Formula Writing
When we write the formula for a binary ionic compound, we have to make sure that the charges on the positive ions and on the negative ions balance.
One way to make sure that the charges balance is to use the “cross-over” method.

4. “Cross-over” Method Write the formula of the ions beside each other.
Cross over the magnitude of the charge on the positive ion so that it becomes the subscript for the negative ion.
Cross over the magnitude of the charge on the negative ion to get the positive ion’s subscript.
Example: Be2+ P3-
Be3 P2

5. The “Three-Line” Method
Write the compound name.
Line 2
Write the symbols for the two ions separately.
Line 3
“Cross-over” the charges on the ions (ignoring the + and – signs) and write the final formula (remember to reduce the subscripts if possible).
Try This: potassium nitride