Atoms that make up the elements of the periodic table are so small, that providing their individual mass becomes cumbersome. For example, a single carbon atom has a rough mass of 1.99 x 10-23 grams.
Instead, the molecular weight of elements is measured based on a mole (mol) of atoms. A mole is a count of atoms (like a dozen) and is equal to 6.022 x 1023 atoms. When a mole of carbon atoms is massed, the result is 12.011g which is a more manageable number to work with.
For carbon, the number 12.011 can be written as 12.011 grams per mole (g/mol) or 12.011 atomic mass units (amu). Therefore, 1 mole of carbon equals 12.011 grams.
