1. Ionic Compounds and Formula Units

2. An Ionic compound contains positive and negative ions
Even though that is the case, all ionic compounds are electrically neutral
In an ionic compound the total positive charge equals the total negative charge
This is the main idea to remember when writing formulas for ionic compounds

3. For ionic compounds, the correct formula is the simplest one.
A Formula unit- A chemical formula for an ionic compound. It is always reduced to the simplest form.
It is important to note that ionic compounds have formula units while molecular compounds have formulas.

4. Example
Calcium Chloride is an ionic compound. What is its formula?

5. How do you find the smallest whole number ratio
Consider Ca4Cl8…
Li6N2

6. Steps for Solving
Write the cation and anion symbols with ionic charges
Make the number in the positive charge into a subscript for the anion. Make the number in the negative charge into a subscript for the cation

7. If necessary reduce the resulting formulas to its smallest whole number ratio
Criss cross method will always create a formula unit that is neutral, but it may not be in the simplest form.