23 October 18

October 23 (10 - 23) is always a great day to be a chemist. We celebrate "Mole Day" today because 10 - 23 represents the exponential part of Avogadro's Number (6.02 times 10 to the 23rd power), one of the most important fundamental constants in chemistry. It counts the number of things in ONE MOLE of a substance. (The word came from the German use of "Mol" as a unit for counting molecules. It has nothing to do with the small animal, although cute pictures of moles show up in Mole Day decorations and T-shirts.) A little pile of 12 grams of carbon, for example, has that many atoms of carbon, and a few teaspoons (18 milliliters) of water has that many molecules of H-2-O. If you gather 56 ordinary paper clips, you have 6.02-times-ten-to-the-23rd-power atoms of iron. If you ever took a chemistry class, you undoubtedly ran into calculations using the "Ideal Gas equation," PV = nRT, where "n" is the number of moles. My periodic table tie shows the symbols, atomic numbers and atomic weights for many elements. (The atomic weight for an element equals one mole of it.)