Three elements $B, Si$ and $Ge$ are:

Arrange the following elements in increasing order of their atomic radii.
$(a)$ $Li, Be, F, N$
$(b)$ $Cl, At, Br, I$

In Mendeleev's Periodic Table,gaps were left for elements to be discovered later. Which of the following elements found a place in the periodic table later?

Which of the given elements $A$,$B$,$C$,$D$ and $E$ with atomic numbers $2$,$3$,$7$,$10$ and $30$ respectively belong to the same period?

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.

Define Newlands' law of Octaves.