Basic metrology: It's elementary

We are now well into 2019, the International Year of the Periodic Table (IYPT), celebrating the 150th anniversary of pioneering work of Dmitri I. Mendeleev (1834-1907). He not only made sense of the zoo of chemical elements that were known in 1869 but also predicted the existence of several elements that were unknown at the time, as well as their expected properties. Mendeleev took advantage of advances that had been made in the measurement of atomic weights (relative atomic masses is the preferred term in our century) of the elements to guide his thinking, which was indisputably bold and prescient.