What makes atomic mass fractional ?Explain with eg

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.