What major challenge did Dmitri Mendeleev face when organizing the elements into his periodic table?

• A. He was unable to identify any repeating patterns among the elements.
• B. He could not distinguish between metals and nonmetals.
• C. Some elements did not fit the order based on atomic mass and their properties.
• D. All elements had already been discovered, leaving no gaps in the table.

Answer: (C)

Verified step by step guidance

1. Understand the historical context: Dmitri Mendeleev was one of the first chemists to organize elements into a periodic table based on their properties and atomic masses.
2. Recognize that Mendeleev arranged elements in order of increasing atomic mass and noticed recurring patterns in their chemical properties, which led to the concept of periodicity.
3. Identify the major challenge: some elements did not fit neatly into the order when strictly following atomic mass, as their chemical properties suggested a different placement.
4. Mendeleev resolved this by sometimes placing elements out of strict atomic mass order to group elements with similar properties together, predicting the existence of undiscovered elements to fill gaps.
5. This challenge highlighted the limitations of using atomic mass alone and eventually led to the modern periodic table being organized by atomic number, which resolved these inconsistencies.