Why did Mendeleev leave some gaps in his periodic table of elements? Explain your answer with an example.

In order to make sure that elements having similar properties fell in the same vertical column or group, Mendeleev left some gaps in his periodic table. These gaps were left for subsequent inclusion of elements not known at that time.

For example,

(i) Eka-aluminium (means one place below aluminium in the group); its atomic mass and chemical properties are quite similar to those of the element gallium discovered later in 1876.

(ii) Properties of eka-silicon are the properties of germanium.