what is the atomic weight of carbon?
Carbon 13 has an atomic mass of 13.00335 and a natural abundance of 0.0107 (about 1.1%). Carbon 14 has an atomic mass of 14.003 and an abundance of 0.000000000001 (1 part in a trillion). Because it is so rare, carbon 14 makes an infinitesimal contribution to the average atomic mass.