Why does the atomic mass of an atom equal the weight of 1 mol of that atom in grams?

Question

Natalie Anton · Accepted Answer

And you might just as well ask why there are 12 eggs in a dozen eggs......

A dozen is a convenient number for counting, because we can use our thumb to mark out a dozen with the tips and joints of the four fingers on each hand. #"Dozens, Bakers' dozens, Botany Bay dozens"# are thus all convenient units to measure a particular thing. On the other hand, a mole contains #6.022140xx10^23# individual items. And thus a mole of #""^12C# represents #6.022140xx10^23# individual #""^12C# atoms. Why should we use such an absurdly large number? Because #6.022140xx10^23# individual #""^12C# atoms have a mass of #12*g# precisely. The mole is this the link between the (sub!) micro world of atoms, and molecules, to the macro world of grams, and litres, what chemists can measure out in a laboratory. And in every chemistry exam you should be given a copy of the Periodic Table, which tells you the molar mass of each known element, as well as its atomic number. It is up to you to learn how to use the Table effectively. Good luck. I have said much the same thing here, and in the links. Note that on the Table, the atomic mass of carbon is quoted as #12.011*g#....why does it diverge from #12#? This is because of the existence of the #""^13C# isotope, which contains an extra nuclear neutron. This raises the atomic mass slightly.

Jose Ayala · Answer

undefined The atomic mass of an atom equals the weight of 1 mole of that atom in grams because the atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom. Since 1 mole of any substance contains Avogadro's number of atoms (6.022 x 10^23), the mass of 1 mole of atoms in grams is numerically equal to the atomic mass of the atom in atomic mass units (amu).