1. Here’s another example of writing the formula for an ionic compound.

2. We’re asked to write the formula for the compound lithium nitride Write the formula for the compound lithium nitride.

3. We find lithium on the periodic table. It is a metal and its ion is Li with a positive 1 charge. Write the formula for the compound lithium nitride.

4. We find an element that sounds like nitride but doesn’t end in –ide. Write the formula for the compound lithium nitride.

5. We see it is nitrogen, a non-metal whose ion has a negative 3 charge Write the formula for the compound lithium nitride.

6. So this is an ionic compound made up of the ions Li+ and N 3minus. N 3 minus is called the nitride ion. Write the formula for the compound lithium nitride. Li + N 3–

7. We can see that when we have one lithium ion with a charge of positive 1 and one nitride ion with a charge of negative 3, the charges do not add up to zero. Write the formula for the compound lithium nitride. Li + N 3– F–F– Li + +1–3 Total Positive Charge Total Negative Charge These DO NOT add up to zero

8. But if we add two more Li + ions (click) Write the formula for the compound lithium nitride. Li + N 3– F–F– Li + +1–3 Total Negative Charge

9. The total positive charge is now equal to 3 times positive 1 Write the formula for the compound lithium nitride. Li + N 3– F–F– Li + +1–3 Total Positive Charge Total Negative Charge

10. Which is Positive 3 Write the formula for the compound lithium nitride. Li + N 3– F–F– Li + +3–3 Total Positive Charge Total Negative Charge

11. So now, the positive and negative charges ARE balanced. Positive 3 and negative 3 do add up to zero Write the formula for the compound lithium nitride. Li + N 3– F–F– Li + +3–3 Total Positive Charge Total Negative Charge NOW these add up to zero

12. There are 3 lithium ions to one nitride ion, so the formula is (click) Li 3 N Write the formula for the compound lithium nitride. Li + N 3– F–F– Li + +3–3 Li 3 N

13. So that answers our question. The formula for lithium nitride is Li 3 N. We see that no charges are written in the final formula for a compound. If you have charges written in a final formula, make sure you erase them. Write the formula for the compound lithium nitride. Li + N 3– F–F– Li + +3–3 Li 3 N Formula: