The molar mass of carbon-12 (C-12) is indeed 12.011 grams per mole (g/mol) as listed in the periodic table. This slight difference from the whole number 12 is due to the presence of electrons.
The molar mass of an element is the mass of one mole of atoms of that element. In the case of carbon-12, it has 6 protons and 6 neutrons in its nucleus, giving it a mass of 12 atomic mass units (amu). However, the molar mass takes into account not only the mass of the protons and neutrons in the nucleus but also the mass of the electrons that surround the nucleus.
While the mass of an electron is very small compared to the masses of protons and neutrons, it is still considered in the molar mass calculation. Electrons contribute to the overall mass of an atom, and when considering a large number of atoms (one mole), their collective mass becomes significant.
The atomic mass unit (amu) is based on the unified atomic mass unit (u), which is defined as 1/12th the mass of a carbon-12 atom. Therefore, the molar mass of carbon-12 is calculated based on the mass of a single carbon-12 atom, taking into account the mass of its electrons.
Hence, the molar mass of carbon-12 is accurately represented as 12.011 g/mol in the periodic table, reflecting the contribution of electrons to the overall mass of the atom.