How did Mendeleev know that there must be undiscovered elements and how did he take this into account?

Mendeleev also arranged the elements known at the time in order of relative atomic mass relative atomic mass: The relative atomic mass is the number of times heavier an atom is compared to one twelth of a carbon-12 atom., but he did some other things that made his table much more successful. read more

Best Answer: He set up a table of all known elements based on their atomic weight and properties, and found that there were holes in his table. He correctly assumed that these gaps represented elements yet undiscovered. read more

He was also able to work out the atomic mass of the missing elements, and so predict their properties. And when they were discovered, Mendeleev turned out to be right. For example, he predicted the properties of an undiscovered element that should fit below aluminium in his table. read more