Explain how the atomic mass of an element is determined and why the resulting value is not a whole number.?

The classical answer will be that it is the average of isotopes.

However, Aluminum has a single isotope and its atomic mass is not a whole number.

The answer is that the atomic mass of a proton or a neutron are nearly a whole number but only nearly. Only C12 is defined as a whole number.

No other isotopes have whole number masses due to the different mass of neutrons and protons, as well as mass change due to binding energy.

So the classical answer is a really serious mistake that a lot of teachers and young pupils make.

The atomic mass of an element is the number of protons plus the number of neutrons. For a single atom the atomic mass is always a whole number. But most elements have isotopes - forms of the element that have the same number of protons but which differ in the number of neutrons. This means that for most elements, not all of the atoms have the same atomic mass. Let's look at a hypothetical example. Let's say we had two forms of the exact same element. One had an atomic mass of 80 and one had an atomic mass of 79. If the two forms were equally common, the element as a whole would be said to have an atomic mass of 79.5 , halfway between 79 and 80 and not a whole number. Simply put, imagine you took two tests. You made 79 on one and 80 on the other. Your average grade would be 79.5 . The atomic mass given in the Periodic Table for an element is the average atomic mass of all the isotopes of that element. One more thing. Imagine that half of your class made 79 on a test and the other half made 80. The class average would be 79.5 . However, if the majority of the class made 80 while only a few made 79 the average would still be a decimal mixed number between 79 and 80 but in this case it would be much closer to 80. Not only do atoms of the same element usually have different forms but some of those forms are more common than others. The atomic mass given in the Periodic Table is an average of the atomic masses of all the different isotopes of that element corrected for the frequency of their occurence in nature,

The only isotope with atomic mass that is a whole number is Carbon-12 which is exactly 12.0000 grams. This is where other atoms' atomic masses are based. The atomic mass of an element is calculated by taking the average of the atomic masses of all the chemical element's isotopes as found in a particular environment, weighted by isotopic abundance.

Its determined my taking the average weight of all the known isotopes of an element by percent abundance. It's not usually a whole number because its an average.