Dmitri Mendeleev, the Russian chemist credited with the development of the periodic table, did not directly determine atomic numbers or weights for elements as we understand them today. Instead, he arranged the elements in the periodic table based on their chemical and physical properties, which allowed him to predict the existence and properties of undiscovered elements.
During Mendeleev's time in the late 19th century, the concept of atomic number was not yet fully understood. However, scientists had recognized the existence of different elements and had determined their relative atomic weights. Mendeleev noticed patterns in the properties of elements when they were arranged according to their atomic weights.
Mendeleev's key insight was that elements with similar chemical properties occurred at regular intervals when ordered by increasing atomic weight. He used this observation to construct his periodic table, where he arranged the known elements in horizontal rows (periods) and vertical columns (groups) based on their similarities. Mendeleev left gaps in his table for undiscovered elements, accurately predicting their properties based on the patterns and trends he observed among the known elements.
Mendeleev's periodic table was later refined and expanded as our understanding of atomic structure improved. With the discovery of the proton by Ernest Rutherford in 1911 and the subsequent identification of atomic numbers, it became clear that elements were better arranged by their atomic numbers rather than atomic weights. This led to the modern form of the periodic table, where elements are ordered by increasing atomic number.
In summary, while Mendeleev did not directly determine atomic numbers and weights, his arrangement of elements based on atomic weights and observed patterns laid the foundation for the development of the modern periodic table and contributed to our understanding of the periodicity of chemical elements.