Earlier, we discussed isotopes and masses of elements. Let’s link these two together with something important called the Gram Formula Mass (GFM). The Gram Formula Mass of an element is the average mass of all the isotopes present. Here are the GFMs for 20 of the elements on the Periodic Table:

The problem with GFMs is that you are never told to use them for one element. Instead, you have to calculate them for a compound and then use them. For example, here’s how to calculate the GFM of water:

H₂O

Draw in arrows underneath the symbols and write the GFM multiplied by the number of the element at the end. Like this:

 Next, add up the two numbers, and you have the total GFM for the compound: