(i) Atomic masses of most of the elements have a value that is not a whole number but is fractional, this is because of the occurrence of isotopes of that element in the nature.
(ii) Isotopes are those elements which have the same atomic number but have different mass.
(iii) There can be two or more isotope same element because they all have the same number of protons.
(iv) The atomic masses for any elements are taken as the average of masses of all of its commonly occurring or stable isotopes which comes out to be a fractional number in most cases.
(v) It is called the relative atomic mass. Example: there are two stable isotopes of chlorine (Cl) which are 1735Cl and 1737Cl the isotopes have atomic mass of approximately 35 and 37 respectively. The relative atomic mass is calculated by formula,
(vi) Relative atomic mass = (percent isotopic composition × atomic mass of isotope 1) + (percent isotopic composition × atomic mass of isotope 2) + …..so on.
For chlorine relative atomic mass is =
(75/100)×35 + (25/100)×37 = 35.5u
Hence the atomic masses are fractional for most of the elements due to its isotopes.