23 October 20

 It's Mole Day! It's a special day for chemists worldwide, celebrated annually on October 23 (10-23), because those digits are the most important part of Avogadro's Number, 6.02 x 10²³. Yes, that's an absolutely huge, unfathomable number. It counts the number of "things" in a mole of a substance, such as the number of molecules in 18 grams of water, the number of molecules in 342 grams of sugar, and the number of atoms of iron in two large nails (56 grams). The mole concept is at the foundation of stoichiometry (the part of chemistry that shows how many atoms are in a molecule and how those atoms get rearranged during chemical reactions). To get one mole of water molecules you need two moles of hydrogen atoms and one mole of oxygen atoms, but since elemental oxygen exists as O₂, and elemental hydrogen exists as H₂,  the reaction is ordinarily shown as combining one mole of hydrogen (molecules) with one-half mole of oxygen (molecules) to produce one mole of water (molecules).

The periodic table (like the one on my tie) shows the "atomic weight" of each of the elements, which is the measure in grams for 6.02 x 10²³ atoms (one mole) of each one.