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

2. In this example, we’re asked to write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +2–3 Ca 3 P 2 P 3– Write the formula for the compound calcium phosphide.

3. We find the elements on the periodic table. We see that calcium is a metal whose ion has a charge of positive 2 Write the formula for the compound calcium phosphide.

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

5. We see it is phosphorus, a non-metal whose ion has a negative 3 charge Write the formula for the compound calcium phosphide.

6. So this compound is made up of the ions Ca 2+ and P 3 minus Write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +2–3 Ca 3 P 2 P 3–

7. If we have one calcium ion and one phosphide ion, the total positive charge is positive 2 and the total negative charge is negative 3 Write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +2–3 Ca 3 P 2 P 3– Total Positive Charge Total Negative Charge

8. The positive and negative charges are not balanced. They do not add up to zero Write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +2–3 Ca 3 P 2 P 3– Total Positive Charge Total Negative Charge These DO NOT add up to zero

9. The lowest common multiple of 2 and 3 is 6, so if we add two more calcium ions (click) Write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +6–6 Ca 3 P 2 P 3–

10. The total positive charge is 3 times positive 2, which is Write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +6–6 Ca 3 P 2 P 3– Total Positive Charge

11. Positive 6 Write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +6–6 Ca 3 P 2 P 3– Total Positive Charge

12. In order to make the charges balance, we need to end up with a total negative charge of negative 6. Write the formula for the compound calcium phosphide. P 3– F–F– +6–6 Ca 3 P 2 P 3– Total Positive Charge Total Negative Charge Ca 2+

13. To get a total negative charge of negative 6, we add one more phosphide ion (click) Write the formula for the compound calcium phosphide. P 3– F–F– +6–6 Ca 3 P 2 P 3– Total Positive Charge Total Negative Charge Ca 2+

14. Negative 3 and negative 3 do add up to negative 6. Write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +6–6 Ca 3 P 2 P 3– Total Positive Charge Total Negative Charge

15. So when we have 3 calcium ions and 2 phosphide ions, the total positive and total negative charges are balanced. Positive 6 and negative 6 do add up to zero. Write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +6–6 Ca 3 P 2 P 3– Total Negative Charge Total Positive Charge NOW these add up to zero

16. There are three calcium ions for every 2 phosphide ions, Write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +6–6 P 3–

17. So the formula is Ca 3 P 2. Write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +6–6 P 3– Ca 3 P 2

18. We’ve now answered the question, the formula for calcium phosphide is Ca 3 P 2. Again, we see that no charges are written in the final formula for a compound. Write the formula for the compound calcium phosphide. Ca 2+ P 3– F–F– Ca 2+ +6–6 Ca 3 P 2 P 3– Formula: