Carbon-12 is the most common isotope of carbon and has a mass number of 12. however, the average atomic mass of carbon found on a periodic table is slightly more than 12 daltons. why?

Carbon has 3 isotopes in nature: carbon-12, carbon-13 and carbon-14.

The average atomic mass of carbon, which is the one found in the periodic table, can be calculated as follows:
(average mass of 12C x its percentage of abundance) + 
(average mass of 13C x its percentage of abundance) + 
(average mass of 14C x its percentage of abundance) 

Since carbon-12 is the most abundant one, therefore, the average mass is closest to 12, however, other isotopes (carbons having more than 12 neutrons)  are also taken into consideration.

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