The relative masses of the atoms of different elements can be expressed in terms of
their atomic masses, which indicate how massive one atom is compared to another. The unit
on this scale is called an atomic mass unit ().
The reference element has been chosen to be the most abundant isotope of carbon,
called carbon-12, and its atomic mass unit is defined exactly twelve atomic mass units,
or . Thus, one atomic mass unit is exactly one-twelfth
the mass of a carbon-12 atom. This is how masses of atoms are expresses ; in atomic mass
To express the number of atoms or molecules, the gram-mole or simply the mole is used.
One mole of a substance contains as many particles (atoms or molecules) as there are
atoms in 12 grams of the isotope carbon-12. Experiment shows that 12 grams of carbon-12
contain atoms. This number is called the
Avogadro's number , , after the Italian
scientist Amedeo Avogadro (1776 ~ 1856).
Just as the meter is the SI base unit for length, the mole is the SI base unit for
expressing "the amount of substance". Thus, one mole of atomic sulfur contain
sulfur atoms. One mole of water contains
and so on.
It is important that a mole of one substance does not have the mass as a mole of
another substance. For example, an aluminum atom is more massive than a carbon-12
atom by the ratio of their atomic masses, .
Since Avogadro's number (1 mole) of carbon-12 atoms has a mass of 12 grams, then
Avogadro's number of aluminum atoms must have a mass of the following.