Some elements hav A. fractions of protons

B. fractions of neutrons

C. isotopes

D. valence electrons

The atomic mass of some elements are in fractions because they exist as a mixture of isotopes of different masses. The atomic mass arises in fraction because of this mixture.

Mass =

Note- Isotopes are atoms of the same element having a same atomic number but different atomic masses due to the presence of different numbers of neutrons.

Ex- Carbon-12 and Carbon-14 are both isotopes of carbon, one with 6 neutrons and one with 8 neutrons but both have 6 protons.