Dmitri Mendeleev, the creator of the periodic table, developed his table based on the patterns and properties of elements known at the time. When it came to determining the atomic masses for the elements, Mendeleev used various sources of experimental data, including chemical analyses and measurements of specific heat capacities.
Mendeleev arranged the elements in order of increasing atomic mass and noticed that certain properties repeated periodically. He used this periodicity to organize the elements into groups and periods, forming the foundation of the modern periodic table.
It's important to note that the methods and data available to Mendeleev during the late 19th century were not as precise as the techniques used today. Therefore, his atomic mass values were approximate and sometimes contained errors. Nevertheless, Mendeleev's periodic table provided a valuable framework for organizing the elements and predicting the properties of undiscovered elements, earning him recognition as the "father of the periodic table."