cant grasp this concept, help!

Use the concepts of relative abundance and relative weight to explain why carbon has an atomic mass of 12.011 amu when there are three isotopes of carbon weighing 12 amu, 13 amu and 14 amu. Wouldn't the average of these three be 13, instead of 12.011?

Question

cant grasp this concept, help!

Use the concepts of relative abundance and relative weight to explain why carbon has an atomic mass of 12.011 amu when there are three isotopes of carbon weighing 12 amu, 13 amu and 14 amu. Wouldn't the average of these three be 13, instead of 12.011?

anonymous · Answer

No. The Atomic mass of an element given on the periodic table is the sum of the each isotopic mass multiplied by it's relative abundance on Earth. Carbon 12 is the most common isotope, then Carbon 13, and Carbon 14 is the rarest. The relative abundance of each on Earth is: Carbon 12: 98.93%   Carbon 13: 1.07%   Carbon 14: .0000000001%

The total is approximately 100% and the percentages are obviously approximate and not exact. 

From the relative abundances of each isotope, we just multiply it by the isotopes atomic mass and add up all of that:

Carbon Average Atomic Mass = 12(0.9893) + 13(0.0107) + 14(.000000000001)
                                           =  12.0107 
                                           =  12.011 (Rounded to 3rd decimal place)


@missy101 

Also, this isn't a Chemistry section but I answered your question anyway =]

anonymous · Answer

oh oops! hehe. well thank you SO much!