Name any two pairs of elements which were adjusted by Newlands in the same slot.

Which among the following elements has the largest atomic radii?

"Hydrogen occupies a unique position in the Modern Periodic Table". Justify the statement.

An element $X$ belongs to the $3rd$ period and group $13$ of the Modern Periodic Table.
$(a)$ Determine the valence electrons and the valency of $X$.
$(b)$ Determine the molecular formula of the compound formed when $X$ reacts with an element $Y$ (atomic number $= 8$).
$(c)$ Write the name and formula of the compound formed when $X$ combines with chlorine.

Which of the following sets of elements is written in the order of their increasing metallic character?

State the reason for the following:
$(a)$ The elements of the same group have similar chemical properties.
$(b)$ The elements of the same period have different properties.