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Relative atomic masses of atoms of all chemical elements are numbers without units, being the value of proportion compared to $\frac{1}{12}^\text{th}$ the mass of the carbon atom.

But the relative atomic mass of carbon is never 12! Instead, it is 12.01 or, more accurately, 12.011.

Why is this so?

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up vote 10 down vote accepted

Simply because the atomic mass is defined as 1/12 of the mass of 12C. Others isotopes of carbon (13C mostly, with an abundance of 1.1% approximately) account for an average atomic mass slightly above 12.

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Yes, the mass of an element on the period table is the weighted average atomic mass over all naturally occurring isotopes. It is why some elements, like chlorine (35.45) have masses that are even less integer-like. – Ben Norris Dec 16 '12 at 23:15

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