Why some elements have atomic mass in fraction?
This is because some atoms have isotopes i.e. atoms of same atomic number and different mass number. So we take average mass of each isotope of that particuar atom.We dont apply mean method to calculate average mass. There is a different formula for that. It is given on last page of your 4th chapter, science ncert book, class 9th. Averages do not always give result in whole number. So mass of those atoms are in decimals or fractions.