What is Dmitri Mendeleev known for?

• A. Discovered the electron
• B. Proposed the quantum model
• C. Developed the modern atomic theory
• D. Built the periodic table

Answer: (D)

The main idea here is recognizing a scientist who organized elements to reveal patterns in their properties. Dmitri Mendeleev is known for building the periodic table, arranging elements in a way that showed repeating (periodic) trends as you move across the table. He ordered elements by increasing atomic mass and grouped them so that elements with similar properties lined up in the same columns. When gaps appeared, he predicted that undiscovered elements would fit there and even estimated some of their properties. This approach created a powerful tool for understanding chemical behavior and sparked the discovery of several elements later on.

Discovered the electron is associated with J. J. Thomson, not Mendeleev. Proposing a quantum model is linked to the development of quantum mechanics (later scientists), not the organization of the periodic table. Developing the modern atomic theory is a broader achievement built over time by multiple scientists, not just Mendeleev. So the statement about building the periodic table best captures Mendeleev’s landmark contribution.