What is the most likely reason that Mendeleev placed tellurium before iodine in his periodic table?

• A. He prioritized arranging elements by atomic mass rather than atomic number.
• B. He was unaware of the existence of iodine at the time.
• C. He wanted to group elements with similar physical properties together.
• D. He believed tellurium was more chemically reactive than iodine.

Answer: (A)

Verified step by step guidance

1. Understand that Mendeleev's periodic table was primarily organized based on atomic masses and chemical properties, as the concept of atomic number was not yet discovered.
2. Recognize that tellurium has a higher atomic mass than iodine, but Mendeleev placed tellurium before iodine to maintain the grouping of elements with similar chemical properties in the same columns.
3. Note that Mendeleev prioritized chemical properties and periodic trends over strict increasing atomic mass, which sometimes led to elements being placed out of order by mass to preserve group similarities.
4. Realize that the modern periodic table is arranged by atomic number, which resolves the apparent discrepancy in the order of tellurium and iodine.
5. Conclude that the most likely reason Mendeleev placed tellurium before iodine was his focus on atomic mass and chemical properties rather than atomic number, which was unknown at the time.