When Mendeleev started his work, how many elements were known at that time?
Show answer & explanation
Correct answer: D
Explanation
The correct answer is option 4 i.e. 63.
When Dmitri Mendeleev began his work on the periodic table in the mid-19th century, around 63 elements were known. His pioneering arrangement of these elements based on their atomic mass and chemical properties laid the groundwork for the modern periodic table.