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**Hint:**We know that it is asked why the atomic mass of most of the elements is fractional. At the macroscopic level, most of the elements exist as a mixture of isotopes which though may be radioactive, affects the mass of the given atom. One the other hand, the mass numbers of the atoms are always whole numbers.

**Complete answer:**

We know that the atomic mass of an atom depends upon the atomic mass of each of its isotopes. Due to this mixture of the isotopes, the atomic mass is fractional. The atomic mass of an atom is also known as the isotopic mass of the atom. If you closely look at the periodic table, you can see that the atomic masses or the isotopic masses of the elements are not whole numbers. They may be close to a whole number.

But, they are not whole numbers. The majority of elements exist as a mixture of isotopes of different masses. Because of this combination, fractional atomic masses arise. The atomic mass is the average of atomic masses of all the isotopes of an element. Hence, this difference or variation of isotopic mass and mass number of a specific isotope of an atom is expressed in terms of a quantity known as packing fraction. The value of packing fraction is equal to the difference of atomic mass and mass number whole divided by the mass numbers

**Note:**

Remember that the negative or positive value of the packing fraction has further meaning to it too. The negative sign of the packing fraction implies that some part of the mass gets converted into energy via Einstein's mass energy relation. This energy is known as the binding energy known normally, and the mass which gets converted into the binding energy is known as mass defect.

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