Which significant change did Glenn Seaborg make to Mendeleev's periodic table?

• A. He grouped the transition metals with the alkali metals.
• B. He rearranged the noble gases to the beginning of each period.
• C. He added the actinide series as a separate row below the lanthanides.
• D. He removed the halogens from the main table.

Answer: (C)

Verified step by step guidance

1. Understand the historical context: Mendeleev's periodic table was originally arranged by increasing atomic mass and grouped elements with similar properties into columns.
2. Recognize that Glenn Seaborg's major contribution was related to the placement of the actinide series, which was not clearly identified in Mendeleev's table.
3. Seaborg proposed that the actinides form a separate series, similar to the lanthanides, and placed them as a distinct row below the lanthanide series in the periodic table.
4. This rearrangement helped clarify the structure of the periodic table by properly grouping the actinides and explaining their chemical behavior.
5. Therefore, the significant change Seaborg made was adding the actinide series as a separate row below the lanthanides, improving the organization and understanding of the periodic table.