What did Mendeleev finally insert into his periodic table to make the arrangement of elements make sense in their locations?

• A. Electron configurations
• B. Group charges
• C. Gaps for undiscovered elements
• D. Atomic numbers

Answer: (C)

Verified step by step guidance

1. Understand that Mendeleev was organizing elements based on their properties and atomic masses, but he noticed some inconsistencies in the order when strictly following atomic mass.
2. Recognize that to resolve these inconsistencies, Mendeleev predicted the existence of elements that had not yet been discovered and left intentional gaps in his periodic table for these missing elements.
3. These gaps allowed Mendeleev to maintain the pattern of chemical properties across the table, anticipating that future discoveries would fill these spaces and confirm his arrangement.
4. Note that Mendeleev did not use electron configurations or atomic numbers, as these concepts were developed later; his method was primarily based on observed chemical properties and atomic masses.
5. Therefore, the key innovation Mendeleev made was inserting gaps for undiscovered elements, which made the periodic table's arrangement logical and predictive.