Final answer:
Mendeleev organized the periodic table by atomic masses but shifted some elements to align with similar properties. This led to the modern understanding that atomic numbers, not masses, determine the periodic grouping of elements.
Step-by-step explanation:
Mendeleev's periodic table was an impressive achievement in chemistry because it organized elements according to their atomic masses and similar properties. When developing his periodic table, Mendeleev encountered a few anomalies where the sequence of atomic mass did not coincide with the grouping of elements that had similar chemical behaviors. This was notably the case for tellurium (Te) and iodine (I), where Mendeleev switched their order based on their chemical properties rather than their atomic mass. Tellurium has a higher atomic mass than iodine, yet it was placed before iodine to keep it with elements like sulfur and selenium, which had properties dissimilar to iodine. Iodine's placement with elements such as fluorine (F), chlorine (Cl), and bromine (Br) made more sense given their shared characteristics. Mendeleev's insight laid the foundation for what is now recognized as the Modern Periodic Table, where elements are arranged in order of their atomic numbers, reaffirming that the properties of elements are periodic functions of their atomic numbers.