a mole is a unit of measurement used in chemistry to describe or to express amounts of a chemical substance. avogadro’s number is a historical term closely related to the avogadro constant. the avogadro constant is named after the early 19th-century italian scientist amedeo avogadro.national mole day is observed annually by chemists and chemistry students on october 23.

this celebration takes place between 6:02 am and 6:02 pm, making the date 6:02 10/23 in the american style of writing dates. the time and date are derived from avogadro’s number, which is approximately 6.02×10^23, defining the number of particles (atoms or molecules) in one mole of substance, one of the seven base si units.

national mole day originated in the early 1980s in an article from the science teacher. prairie du chien, wisconsin high school chemistry teacher, was inspired by the article and founded the national mole day foundation on may 15, 1991.

the mole is the unit of amount in chemistry. it provides a bridge between the atom and the macroscopic amounts of material that we work with in the laboratory. it allows the chemist to weigh out amounts of two substances, say iron and sulfur, such that equal numbers of atoms of iron and sulfur are obtained. a mole of a substance is defined as:

the mass of substance containing the same number of fundamental units as there are atoms in exactly 12.000 g of 12c.

fundamental units may be atoms, molecules, or formula units, depending on the substance concerned. at present, our best estimate of the number of atoms in 12.000 g of 12c is 6.022 x 1023, a huge number of atoms. this is obviously a very important quantity. for historical reasons, it is called avogadro's number, and is given the symbol na.

unfortunately, the clumsy definition of the mole obscures its utility. 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! similarly, a mole is na things. the mole is the same kind of unit as the dozen -- a certain number of things. but it differs from the dozen in a couple of ways. 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. 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. however, the choice of the unusual number, 6.022 x 1023, as the number of things in a mole seems odd. why is this number chosen? would it not make more sense to define a mole as 1.0 x 1023 things, a nice (albeit large) integer that everyone can easily remember? to understand why the particular number, 6.022 x 1023 is used, it is necessary to resurrect an older, in some ways more sensible and useful, definition of the mole, which is grounded in the atomic weight scale addressed above.

the atomic weight scale defines the masses of atoms relative to the mass of an atom of 12c, 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. the atomic mass unit is a very tiny unit of mass appropriate to the scale of single atoms. originally, of course, chemists had no idea of its value in laboratory-sized units like the gram. 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. this is a very useful correspondence, but it was discovered only after the weight scale had been in use for a long time.

in their desire to be able to count atoms by weighing, chemists gradually developed the concept of the "gram-atomic weight", which was defined in exact correspondence with the atomic weight scale:

1 atom of 12c weighs 12.000 amu

1 gram-atomic weight of 12c weighs 12.000 g

thus the gram-atomic weight of an element was defined as the atomic weight of the element, expressed in grams. 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. analogous terms, such as gram-molecular weight for the molecular weight of a compound expressed in grams, were similarly used. however, having to use a different term depending on whether elements or compounds were being discussed was awkward and inconvenient. for this reason, the term "mole" was adopted to signify the atomic, molecular, or formula weight of a pure substance expressed in grams.