Why did Mendeleev find that the periodicity of the atomic numbers did not work in his original periodic table?

• A. He arranged elements by atomic number, which always matched their chemical properties.
• B. He classified elements based on their reactivity with water.
• C. He grouped elements solely by their physical state at room temperature.
• D. He arranged elements by atomic mass, which sometimes placed elements with different properties together.

Answer: (D)

Verified step by step guidance

1. Understand that Mendeleev's original periodic table was arranged based on increasing atomic mass, not atomic number.
2. Recognize that atomic mass does not always increase uniformly due to the presence of isotopes and variations in nuclear structure.
3. Note that arranging elements strictly by atomic mass sometimes placed elements with different chemical properties next to each other, causing inconsistencies in the periodic trends.
4. Realize that Mendeleev corrected some of these inconsistencies by leaving gaps and predicting properties of undiscovered elements, showing the importance of chemical properties in his arrangement.
5. Learn that the modern periodic table is arranged by atomic number (number of protons), which resolves these inconsistencies and aligns perfectly with periodic chemical properties.