Dmitri Mendeleev developed the first periodic table to allow for elements yet to be discovered. Why did he leave gaps where others did not?
During the early 1800s, scientists attempted to understand the chemical elements by sorting them. Which property of the elements was used to order them?
Periodic tables have been around for a couple of hundred years, and some of them look very different from the one we have today. What is it about periodic tables that makes them periodic?