What is a metalloid? Name any two of them.

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?

An element $X$ belongs to the $3rd$ period and group $16$ of the Modern Periodic Table.
$(a)$ Determine the number of valence electrons and the valency of $X$.
$(b)$ Write the molecular formula of the compound formed when $X$ reacts with hydrogen and draw its electron dot structure.
$(c)$ Name the element $X$ and state whether it is metallic or non-metallic.

An element $A$ has an atomic number $16$. To which group and period does it belong?

An element $X$ has a mass number of $35$ and the number of neutrons is $18$. Determine the atomic number and electronic configuration of $X$. Also, identify the group number, period number, and valency of $X$.

Define Newlands' law of Octaves.