The first key, the atomic theory, was discovered by John Dalton at the dawn of the 19th century. The renowned physicist Richard Feynman felt that the atomic theory was so important that he said, “If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis that all things are made of atoms—little particles that move around in perpetual motion.”
These are the 92 (naturally occurring) elements that are the fundamental building blocks of all the matter in the universe. However, almost everything in the universe is a compound; a combination of different kinds of elements. Thus, the second key to modern chemistry was the discovery that each compound was a collection of identical molecules. For example, a batch of pure water is made of lots and lots of identical H2O molecules.
But just how many molecules? Getting the bookkeeping right so that we could predict the result of chemical reactions proved to be a major roadblock to the advancement of chemistry. The Italian chemist Amadeo Avogadro proposed that at the same temperature and pressure equal volumes of different gases contained the same number of molecules. This hypothesis was largely unappreciated when it was first announced, but it enabled chemists to deduce the structure of molecules by measuring volumes at the start and finish of a chemical reaction. Avogadro’s number is defined to be the number of atoms in 12 grams of carbon, and is approximately six followed by 23 zeroes. (It’s also the number of molecules in a mole, a unit of measurement that chemists use to express the amount of a substance.)
via Popular Mechanics.