Final answer:
Mendeleev's periodic table was organized based on atomic mass and included gaps to predict undiscovered elements, which differed from the modern periodic table organized by atomic number. His predictions and the method of organization using cards were groundbreaking, setting the stage for future advancements that led to our understanding of the periodic properties of elements.
Step-by-step explanation:
Dmitri Mendeleev's periodic table looked different from the current version primarily due to his organization of the elements based on atomic mass rather than atomic number. Mendeleev's version, published in 1869, made it possible to predict properties of elements that had not yet been discovered, leaving intentional gaps for elements such as scandium, gallium, germanium, and technetium. These gaps were key indicators of Mendeleev's insights into chemical periodicity. On the other hand, the modern periodic table is an arrangement of the elements in order of their atomic numbers, which was later shown by H.G.J. Moseley to be the true determinant of periodic properties.
Mendeleev used a set of cards to physically move elements around until he found patterns that led to his version of the periodic table. His method allowed for a flexible arrangement that included undiscovered elements, demonstrating his innovative approach. It was this method and theorization that set his periodic table apart from earlier attempts like that of Julius Lothar Meyer, who also organized elements by atomic mass but did not leave gaps for undiscovered elements.
It's important to note that the recognition of the correlation between electronic structure and chemical properties led to the periodic table's current form, which emphasizes repeating patterns known as periodicity. This transformation was built upon Mendeleev's groundwork, adapting as new scientific discoveries were made regarding atomic structure.