What type of oxide would Eka-aluminium form?

In Mendeleev's Periodic Table,the elements were arranged in the increasing order of their atomic masses. However,cobalt with an atomic mass of $58.93 \ amu$ was placed before nickel having an atomic mass of $58.71 \ amu$. Give the reason for the same.

Name any two elements which were discovered later after Mendeleev's periodic table.

What were the criteria used by $Mendeleev$ in creating his Periodic Table?

Mendeleev predicted the existence of certain elements not known at that time and named two of them as Eka-silicon and Eka-aluminium.
$(a)$ Name the elements which have taken the place of these elements.
$(b)$ Mention the group and the period of these elements in the Modern Periodic Table.

Write the formulae of chlorides of Eka-silicon and Eka-aluminium,the elements predicted by Mendeleev.