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The atomic mass of an element is the average mass of all the isotopes of that element, taking into consideration the relative abundance of each element. That is, if an element has two naturally occurring isotopes with masses m1 and m2 and each has a relative abundance of x% and y% (where, x +y =100%), then the atomic mass of the element can be calculated as:
Atomic mass = (m1x + m2y)/(x + y)
For example, carbon has three naturally occurring isotopes: carbon -12, carbon- 13 and carbon- 14. C-12 makes up for almost 99% of all the carbon, C-13 accounts for rest 1%, with occurrence of C-14 less than 1 part in a billion. Thus, the atomic mass of carbon would be slightly more than 12 (its actually 12.0107 gm per mole).
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