A “mole” is just a number. There are many wordy definitions of a mole to be found on the Internet, such as Wikipedia’s definition:
The mole is defined as the amount of substance of a system that contains as many “elementary entities” (e.g. atoms, molecules, ions, electrons) as there are atoms in 12 g of carbon-12 (12C). A mole has 6.0221415 × 1023 atoms or molecules of the pure substance being measured. A mole will possess mass approximately equal to the substance’s molecular/atomic weight in grams. Because of this, one can measure the number of moles in a pure substance by weighing it and comparing the result to its molecular/atomic weight.
In reality, of course, no substance is truly “pure,” as found in the technical definition of a mole, but that’s the basic idea. For example, a mole of hydrogen atoms will have a mass of 1.00794, even though the atomic weight of hydrogen is 1, exactly. The reason for this is that hydrogen exists in nature in a couple of isotopes, which have different masses. Because the actual atomic weight is so much closer to 1 than it is to 2, for instance, means that hydrogen atoms with a mass of 1 amu (atomic mass units) are much more common in the natural world than hydrogen atoms with a mass of 2 amu.
This means that a “mole” (or 6.022 × 1023 atoms) of hydrogen in nature will have a mass of approximately 1.00794 g. A mole of “pure” hydrogen atoms (having a mass of 1 amu) would have a mass of 1 g, exactly, though, by definition and on the balance. So it is with all elements: a mole of that atom would have a mass of the atomic weight in grams. For example, a mole of gold atoms would have a mass of about 196.97 g.
That’s a lot of atoms, and a mole is an extremely big number. However, in terms of our world, it’s really not that big. For example, a strand of human hair has about a million carbon atoms across its diameter. That is, one strand of the girl’s hair (indicated by the red arrows) has a diameter of about a million atoms of carbon. Since a mole is a lot bigger, a strand of human hair has a diameter of about one quintillionth of a mole.
