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.

Explanation:

Isotope refers to each of two or more forms of the same element that contains the same number of protons but a different number of neutrons. Isotopes differ with respect to relative atomic mass.

Carbon is a non-metal having atomic number 6 and has two main forms diamond and graphite. It also occurs in impure forms charcoal, soot, and coal.

Carbon-12, Carbon-13, Carbon-14 are naturally occurring isotopes of carbon.

Among these, Carbon-12 is the most common isotope of carbon such that it has a mass number of 12. However, the average atomic mass of carbon found on a periodic table is slightly more than 12 Daltons.