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The average Atomic Mass of carbon is 12.011. This is a weighted average based on three forms (isotopes) of carbon with different natural abundances. Carbon 12 has an atomic mass of 12.00 (carbon 12 is a special case because its mass is defined to be exactly 12 atomic mass units) and a natural abundance of 0.9893. So nearly 99% of all carbon is carbon 12. But there are two heavier isotopes too, carbon 13 and carbon 14.

Carbon 13 has an atomic mass of 13.00335 and a natural abundance of 0.0107 (about 1.1%).

Carbon 14 has an atomic mass of 14.003 and an abundance of 0.000000000001 (1 part in a trillion). Because it is so rare, carbon 14 makes an infinitesimal contribution to the average atomic mass.

To calculate the weighted average, sum the products of the abundance X atomic mass for each isotope:

(0.9893)*(12) + (0.0107)*(13.00335) + (0.000000000001) * (14.003) =12.01074.

Round 12.01074 to 12.011, which is the accepted value.

In other units, the atomic mass of carbon is :

1. 12.0107 daltons

2. 0.0120107 kilodaltons

3. 1.99442 x 10-23 grams

4. 1.99442 x 10-26 kilograms

5. 11.1879 GeVc-2

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Q: What is the atomic mass of carbon?
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