Why is the mass in amu of a carbon-12 atom reported as 12.011 in the periodic table of the elements?

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Chemistry

Stone

7 May, 10:36

Why is the mass in amu of a carbon-12 atom reported as 12.011 in the periodic table of the elements?

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Lennon Zamora · Answer 1

The mass of an element listed in the Periodic Table is the weighted average of all its naturally occurring isotopes.

Naturally occurring carbon is about

99 % carbon-12 (12.000 u) + 1 % carbon-13 (13.003 u).

That extra carbon-13 makes the average atomic mass greater than 12.000 u.