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

Write two reasons responsible for the late discovery of noble gases.

Name the following:
$(a)$ $A$ metal that can be cut with a knife.
$(b)$ $A$ metal that is liquid at room temperature.
$(c)$ $A$ metal that has $3$ valence electrons.

State whether the following statements are true or false: In the Modern Periodic Table,a zig-zag line separates metals from non-metals. The borderline elements—boron,silicon,germanium,arsenic,antimony,tellurium,and polonium—are intermediate in properties and are called metalloids or semimetals.

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.

Two elements $X$ and $Y$ have atomic numbers $11$ and $16$ respectively.
$(a)$ Write the electronic configuration of both.
$(b)$ Which type of bond will they form?
$(c)$ Write the formula of the compound formed by their combination (in terms of $X$ and $Y$).