Final answer:
The atomic mass of carbon, 12.01 amu, on the periodic table is the weighted average of the masses of carbon-12 and carbon-13, the two stable isotopes of carbon, reflecting their natural abundance. Carbon-12 has a mass of 12 amu, while carbon-13 has a mass of 13.003355 amu.
Step-by-step explanation:
The atomic mass of carbon is listed as 12.01 atomic mass units (amu) on the periodic table because this value represents the weighted average of the masses of carbon's naturally occurring isotopes. Carbon is mostly made up of isotopes carbon-12 (12C) and carbon-13 (13C), with a distribution of approximately 98.89% and 1.11% respectively. To calculate the average atomic mass, the mass of carbon-12 (12 amu) is multiplied by its abundance, and the mass of carbon-13 (13.003355 amu) is multiplied by its abundance, and these are summed to give the average atomic mass as follows:
(0.9889 × 12 amu) + (0.0111 × 13.003355 amu) = 12.01 amu
Since carbon-12 is the reference for all atomic mass calculations and its mass number is taken as exactly 12 amu, this is why the atomic mass of carbon is close to, but slightly above, 12 amu on the periodic table. This value correlates to the molar mass, where the mass of one mole of carbon atoms (6.022 × 1023 atoms) is 12.01 grams (g/mol).