Which action by Dmitri Mendeleev set him apart in the development of the periodic table?

• A. He predicted the existence and properties of undiscovered elements.
• B. He discovered all of the noble gases.
• C. He classified elements based on their chemical reactivity alone.
• D. He arranged elements strictly by atomic number.

Answer: (A)

Verified step by step guidance

1. Understand the historical context of Dmitri Mendeleev's work on the periodic table, which was developed before the discovery of atomic number and noble gases.
2. Recognize that Mendeleev arranged elements primarily by increasing atomic mass and grouped them by similar chemical properties, which was innovative at the time.
3. Note that Mendeleev left gaps in his periodic table where no known elements fit, hypothesizing that these gaps corresponded to undiscovered elements.
4. Appreciate that Mendeleev predicted the properties of these missing elements based on trends in the table, which was a key action that distinguished his work.
5. Conclude that the unique and defining action by Mendeleev was his prediction of the existence and properties of elements not yet discovered, rather than discovering noble gases or arranging by atomic number.