The History of the Mole

The mole’s beginning is traced back to the most basic units of matter, atoms. Since atoms are extremely small, scientists have perplexed a great deal over finding a way to measure atomic mass. They struggled to find a universal container that had the capability to mass atoms. Measuring the mass of one atom on a balance in grams is not practical, because the result in grams is an amount too small to be of any use in the laboratory. Fortunately, a new idea came about. If two or more same amounts of different substances are compared, the resulting ratios can be used to deduce atomic mass. These atomic mass ratios always remain the same. For example, one atom of Helium has 1/12 the mass of one atom of Carbon-12. In this way, Helium has an atomic mass of one unit, whereas the isotope Carbon-12 has an atomic mass of 12 units. Carbon-12 is the standard for atomic mass. Eventually, a mass spectrometer was invented to determine a single atom’s mass even more accurately. It measured atomic mass by comparing the time and distance an atom traveled when knocked off its course. The invention of the mass spectrometer led to extremely accurate values for single atomic mass.

Now that the mass of a single atom could be figured, the quest for a universal container continued. Knowing how much one atom weighs but not how many atoms exist is not very helpful to the chemist. A universal container must have the capacity to contain the same number of molecules under the same conditions of pressure, temperature and volume. Such conditions made the search for a universal container very difficult.

However, an Italian scientist by the name of Avogrado finally solved the dilemma of a container. The answer to this scientific puzzle was in the form of a number, Avogrado’s number. Avogrado’s number, 6.02 x 10²³, is known as the mole and an accepted SI standard for finding atomic mass. In a mole, the number of atoms remains the same for al substances. The mole is used to find atomic mass through this relationship: one mole of Carbon-12 has 6.02 x 10²³ atoms and a mass of 12 grams. Likewise, one mole of the element oxygen, which has an atomic mass of 16u, would consist of the same number of atoms and have a mass of 16g. This makes the mass of oxygen greater than the mass of carbon. The mole of any element will always have 6.02 x 10²³ atoms. Moles can be used in conversions with molecules, ions, atoms, formula units, and grams. Clearly, the mole is a highly valuable tool to the chemist.

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