The scientific foundation for nuclear physics was established in the 19th and early 20th centuries. At the beginning of the 19th century, and English schoolmaster, John Dalton had collected the insights of laboratory research into the nature of matter. Using the theory that matter consisted of infinitely small atoms, and referring to collections of atoms as elements, he observed from the laboratory experiments, that atoms/elements combined in nature to form molecules/compunds. He observed that the elements always combined in fixed ratios that never varied. This was an important basis for the study of chemistry.
Around mid-century, the Russian scientist, Mendeleev categorized the known elements in accordance with their atomic weight and other characteristics. As he identified elements with their characteristics, he discovered that he could create a table based on those similarities. When he did so, he placed the known elements in the table in their appropriate places and observed that there were blank squares. He theorized that there were elements in nature, yet to be discovered, whose characteristics would place them in the blanks. This hypothesis, when known to other scientists, motivated them to measure the characteristics of new elements that they discovered. As they did so, they, indeed, were able to fill in the blank spaces one by one. The periodic table of the elements, an essential tool of chemistry, was created.