Question

what is the difference between the mass number and the average atomic mass of an atom on the periodic table of elements

Answer 1

Carbon has three common isotopes: C-12 (stable ), C-13 (stable) ,and C-14 (unstable).

The 12, 13 and 14 are the mass numbers, i.e. the sum of protons and neutrons in each species. Carbon always has 6 protons (by definition), and so the neutron number in C-12 is 6, in C-13 it is 7, and in C-14: 8.

Some amount of C-13 and 14 are present in most carbon samples, but C-12 is much more common (99%).
Consequently, the average atomic mass of carbon is only slightly greater than the mass number of C-12, the tiny portions of C-13&14 increase the average mass to 12.01.


The slightly greater mass of the neutrons to the proton, and binding energy also feature in the average mass (but not mass number). These are not significant in the case of carbon.