Answer:
Atomic mass is fractional because the weighted average between all masses of one element is used instead of the most common mass. This often comes up as a fraction.
Explanation:
If an element's most commonly found mass is 1, but there is another form with a mass of 2 that appears 25% of the time, then the atomic mass will be 1.25. 3/4ths will have a mass of one. There are three atoms with a mass of one. 1/4 will have a mass of 2. There is one atom with a mass of 2. Weigh these masses together to get a combined mass of 5. But there were only 4 atoms in total. Divide the combined mass by the number of atoms to get the atomic mass. In this case, it's 5/4, or an atomic mass of 1.25.
