The element with atomic number $18$ belongs to which group?

Identify and name the metals from the following elements whose electronic configurations are given below:
$(a)$ $2, 8, 2$
$(b)$ $2, 8, 1$
$(c)$ $2, 8, 7$
$(d)$ $2, 1$

The three elements $A$,$B$,and $C$ with similar properties have atomic masses $X$,$Y$,and $Z$ respectively. The mass of $Y$ is approximately equal to the average mass of $X$ and $Z$. What is such an arrangement of elements called? Give one example of such a set of elements.

An element $X$ (atomic number $17$) reacts with an element $Y$ (atomic number $20$) to form a divalent halide.
$(a)$ Where in the periodic table are elements $X$ and $Y$ placed?
$(b)$ Classify $X$ and $Y$ as metal$(s)$,non-metal$(s)$,or metalloid$(s)$.

Write true or false for the statement: The term atomic size refers to the distance between the centre of the nucleus and the penultimate shell of an isolated atom.

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