This is Mendeleev's Periodic Table from 1869. Explain why Mendeleev left gaps in his version of the Periodic Table.

Mendeleev left gaps in his version of the periodic table to place elements that were not known at that time. Mendeleev arranged the elements with the increase of their atomic masses and with the repetition of similar properties. On the basis of these chemical and physical properties of the elements next to a gap, Mendeleev could predict the properties of the undiscovered elements. Hence, Mendeleev left spaces for the undiscovered elements. For example, gallium, germanium and scandium were not known during that time but all these elements were discovered later and could be placed in the gaps predicted for them.