The atomic mass of an element does not represent the actual mass of an atom due to the following reasons:

1. Introduction:
The atomic mass of an element is a weighted average of the masses of its isotopes. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei. Therefore, the atomic mass represents the average mass of all the isotopes of an element found in nature.

2. Isotopes:
Isotopes have different masses due to the varying number of neutrons in their nuclei. While the number of protons (which determines the element's identity) remains constant, the number of neutrons can differ. These isotopes of an element have similar chemical properties but differ in their atomic masses.

3. Abundance of Isotopes:
The atomic mass takes into account the abundance of each isotope in nature. The abundance refers to the percentage of each isotope found in a sample of the element. Isotopes with higher abundances contribute more to the atomic mass calculation than those with lower abundances.

4. Calculation of Atomic Mass:
To calculate the atomic mass, the mass of each isotope is multiplied by its abundance and then summed up. For example, if an element has two isotopes with masses of 10 amu and 12 amu and respective abundances of 70% and 30%, the atomic mass would be calculated as follows:

Atomic mass = (10 amu x 0.7) + (12 amu x 0.3) = 10.6 amu

5. Varied Atomic Masses:
Due to the presence of isotopes, the atomic mass of an element can differ from the mass of a specific atom of that element. The atomic mass is an average value that considers the different masses and abundances of isotopes found in nature. Therefore, the atomic mass is not a whole number and may not correspond to the mass of any individual atom.

6. Atomic Mass on the Periodic Table:
The atomic mass listed on the periodic table is a rounded value that represents the average mass of all the isotopes of an element found in nature. It is expressed in atomic mass units (amu), where one amu is defined as 1/12th the mass of a carbon-12 atom.