Are there undiscovered elements or is there a specific limiting reason?

Winkler realized that the sample was 75% silver, 18% sulfur, and 7% of an undiscovered element.

Mendeleev had also predicted the properties of these undiscovered elements.

Mendeleev gave temporary names for the undiscovered elements.

The incompleteness of a table he drew up in 1864 alluded to the possible existence of additional, undiscovered elements.

The discoveries hint strongly at a landscape of undiscovered elements.

He first had the idea that wolframite might contain a previously undiscovered element (tungsten).

It left gaps where undiscovered elements allegedly remained to be inserted.

Mendeleev left blanks in the table where he believed as yet undiscovered elements would one day fit.

To explain the gaps, Mendeleev said that the gaps were due to undiscovered elements.