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The mole^a concept and Avogadro’s number are discussed as sought by Albert Einstein in his PhD thesis of 1905. Einstein would probably have regarded the metric system of units based on centimetre-gram-second (cgs) preferable to today’s SI system and specifically he would have rejected a recent SI suggestion to redefine Avogadro’s constant as based on a nonatomistic continuum description of matter. He would probably also have preferred keeping a dualistic definition of mole able of bookkeeping both mass and number of particles: we advocate that here and call it the ‘Einstein Definition’ and as Avogadro’s number we shall adopt an integer, the cube of 84446888 as suggested by Fox and Hill, providing also a definition of the kilogram based on the atomic mass of the carbon 12 isotope.

Einstein was the first to explain the microscopic movements of pollen grains reported by Robert Brown in 1828 and his explanation that the particles move as a result of an unequal number of water molecules bumping into them from opposite sides was what finally made the scientific world accept the atom theory in its modern shape. In a cosmic diffusion analogy, pollen or bacterial spores moving randomly in outer space driven by the solar winds between solar systems can be envisaged. Applying Einstein’s diffusion theory, one can argue that life might have emerged from far outside of our planet from billions of solar systems, though not from outside of our Milky Way galaxy. As a curiosity we note that the number of solar systems (stars) in the Universe has been estimated to be of the order of Avogadro’s number.

Etymology: “Mole” was derived from French Molécule (extremely small particle) or diminutive from Greek $\mu o\lambda o\sigma$ (molos= substance). In analogy with atom, derived from Greek atomos or atemnein, uncleavable, we may thus interpret molecule as the smallest part that a substance can be divided into without losing its chemical character.

“What do you want to be when you grow up?” is a common question posed to children, and answers such as firefighter, policeman, athlete, doctor, or teacher are probably just as common. Some, like Oliver Sacks, recall an early fascination with metals, the periodic table, and chemical reactions that planted the seeds for the later pursuit of the natural sciences or medicine (neurology in his case). We are familiar with memories of chemists that include their first chemistry set, followed by complaints by parents over strange smells and close calls due to particularly exothermic reactions. For others, including myself, a future in research remained more obscure until a later period in adolescence or perhaps even the undergraduate years. Rather than seeking out a field, the field finds you. In actuality, teachers and mentors with expertise in and enthusiasm for a field exert a force that charts a path toward scientific research throughout life. Here, I stress the importance of terrific teachers and mentors from high school onwards to the undergraduate, Ph.D., and postdoc years for setting me on a track (and on occasion preventing me from derailing) to research in structural chemistry and molecular mechanism using crystallography as the main tool.