The natural abundance for 12C and 14C are 75% and 25% respectivaly.calculate the avarage atomic mass for carbon
Answers
The atomic mass is an experimental number determined from all of the naturally occuring isotopes of an element. As we saw in our lesson on atomic structure, not all atoms of an element are identical. For example, hydrogen has three different isotopes that occur in nature – 1H, 2H, 3H. So when you look on the periodic table and see that it has an atomic mass of 1.01 amu, that is the average of the masses of all three isotopes, not just one of them.
Notice that the units were listed as amu, which stands for atomic mass units. This unit is based off the mass of the isotope 12C (carbon-12). Carbon-12 was chosen as the basis for all of the masses on the periodic table and has been defined to be exactly 12 amu. So all of the other masses on the period table are relative to the mass of carbon-12. In that case, then why is the atomic mass of carbon on the periodic table not exactly 12 amu? Because not all carbon in nature is 12C! Most of it is 12C, some of it is 13C, and a very tiny amount is 14C. When these are averaged together you get the average atomic mass shown on the periodic table of 12.01 amu.
Since the abundances are not equal, we cannot do a typical simple average where we just add them up and divide by three. Instead, we need to perform a weighted average. The formula to calculate the average atomic mass is:
average atomic mass = ∑(relative abundance x mass of isotope)