Carbon-12 is the most common isotope of carbon, and has an atomic mass of 12 daltons. A mole of carbon in naturally occurring coal, however, weighs slightly more than 12 grams. Why?
A) The atomic mass does not include the mass of electrons.
B) Some carbon atoms in nature have an extra proton.
C) Some carbon atoms in nature have more neutrons.
D) Some carbon atoms in nature have a different valence electron distribution.
E) Some carbon atoms in nature have undergone radioactive decay.

Although the atomic mass unit of the carbon atom is 12, this does not play down the fact that there are other isotopes of the element carbon, for example carbon-14.

For the atomic mass unit of carbon to be 12, this shows that the carbon-12 has the highest amount of abundance in nature. Hence, we are saying that the relative high amount of abundance of the carbon-12 makes it have a greater overall effect when calculating the relative atomic mass unit.

Now, one mole of the carbon in coal weighs more than the supposed 12g, this is due to the presence of another isotope of the element carbon in that particular coal sample. Isotopes are only different in terms of the number of neutrons, they contain the same number of protons as no single element can have different atomic number.

Thus, the difference between the elements is simply that one contains more neutrons than the standard carbon 12 which translates to it having more weight