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.
- Write Al3+ and S2¯ right next to each other.
- 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.
- In other words, you need sufficient aluminum atoms to make a +6 charge and you need sufficient sulfur atoms to make a -6 charge.
- 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.
- Write Ra2+ and O2¯ right next to each other.
- The least common multiple between the two charges is 2.
- This means that +2 is the total positive charge and -2 is the total negative charge needed.
- One Ra and one O each provide the needed amount of charge, so the correct formula is RaO.