It was assumed by Newlands that only :

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.

If an element $X$ is placed in group $14$,what will be the formula and the nature of bonding of its chloride?

Describe the basic character of the oxides of third period elements across the period from left to right.

When Mendeleev started his work,the number of known elements were:

Which of the following statements about the Modern Periodic Table is correct?