Least Common Multiple Technique When
Given Name, Write the Formula

The rules to follow are:

• the total positive charges must equal the total negative charges.
• you cannot change the charges given to you.
• adjust the subscripts to equalize the charges.

Problem: write the formula for aluminum sulfide.

1. Write Al³⁺ and S²¯ right next to each other.
2. Select the least common multiple between the two charges (ignoring the signs). In this case it is 6. This is the amount of positive charge and negative charge you need for a correct formula.
3. In other words, you need sufficient aluminum atoms to make a +6 charge and you need sufficient sulfur atoms to make a -6 charge.
4. Obviously (I hope), you see you need two Al and three S, so the correct formula is Al₂S₃.

Problem: write the formula for radium oxide.

1. Write Ra²⁺ and O²¯ right next to each other.
2. The least common multiple between the two charges is 2.
3. This means that +2 is the total positive charge and -2 is the total negative charge needed.
4. One Ra and one O each provide the needed amount of charge, so the correct formula is RaO.