Question

explain why the atomic masses of many elements are in fractions and not whole numbers.

Answer 1

By using mass values of the sub atomic particles mass of an atom can be determined. Generally mass these sub atomic particles is considered as the rest of particles are very unstable and posses negligible mass. The term, relative mass has come from by comparing the mass of particular particle with 1/12th of mass of the carbon atom. Mass of an atom is mainly due to mass values of proton and neutron. For example relative atomic mass of nitrogen is 14.105. It comes by the summation of 7- protons and 7- neutrons. Relative atomic mass of Nitrogen = 7 *(relative mass of proton) + 7 *(relative mass of neutron)                                      = 7 (1.007) + 7 (1.008)                                      = 7 .049 + 7.056                                     = 14.105amu

Thus for most of the elements the calculation of mass value will come in fractions.

Answer 2

This is because they are the Average Atomic Mass of that element.
When an element has isotopes , the mass of every isotope and it's percentage of existence in nature is taken into consideration and the Average Atomic Mass is calculated .
Isotopes are forms of an element which have same atomic number but different mass numbers due to the differences in the number of neutrons in their nucleus.