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Second, 12 is an important number in the English system of weights and measures, so the definition of a dozen as 12 things makes sense.
The early versions of the atomic weight scale were established by scientists who had no knowledge of the electron, proton, or neutron.
When these were discovered in the late 19th and early 20th centuries, it turned out that the mass of an atom on the atomic weight scale was very nearly the same as the number of protons in its nucleus.
Many chemists prefer to use the term molar mass for the mass of a mole of substance.
In this course, we will use the phrase Formula Weight for both situations. The importance of the mole concept can be summed up as follows: any statement that can be made about the number of atoms of an element in a molecule or formula unit of a substance can also be made about the number of moles of an element in a mole of the substance. Convert moles of S to mass of S using the atomic weight.
It is nearly analogous to defining a dozen as the mass of a substance that contains the same number of fundamental units as are contained in 733 g of Grade A large eggs.
This definition completely obscures the utility of the dozen: that it is 12 things! The mole is the same kind of unit as the dozen -- a certain number of things. First, the number of things in a mole is so huge that we cannot identify with it in the way that we can identify with 12 things.
The atomic weight scale defines the masses of atoms relative to the mass of an atom of C, which is assigned a mass of exactly 12.000 atomic mass units (amu).
The number 12 is chosen so that the least massive atom, hydrogen, has a mass of about 1 (actually 1.008) on the scale.
Because the atomic weight scale is numerically preserved in the definition of gram atomic weights, the mass of 1 gram-atomic weight of any element could be immediately determined as the atomic weight in grams.
Thus 1 gram-atomic weight of sulfur weighs 32.06 g; 1 gram-atomic weight of hydrogen weighs 1.008 g, and so on.