Which observation or conclusion helped scientists develop the first periodic table?

• A. All elements have the same number of protons.
• B. The atomic number of elements increases by two for each successive element.
• C. Elements can be classified solely by their physical state at room temperature.
• D. Elements show periodic patterns in their chemical properties when arranged by increasing atomic mass.

Answer: (D)

Verified step by step guidance

1. Understand that the development of the first periodic table was based on recognizing patterns in element properties rather than atomic structure, which was not fully known at the time.
2. Recall that Dmitri Mendeleev arranged elements in order of increasing atomic mass and noticed that certain chemical properties repeated periodically.
3. Identify that this periodic repetition of chemical properties allowed Mendeleev to group elements with similar behaviors into columns, which formed the basis of the periodic table.
4. Recognize that the key observation was the periodicity of chemical properties, not the number of protons or physical states of elements.
5. Conclude that the correct insight leading to the first periodic table was that elements show periodic patterns in their chemical properties when arranged by increasing atomic mass.