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

An element is placed in $2^{nd}$ Group and $3^{rd}$ Period of the Periodic Table. It burns in the presence of oxygen to form a basic oxide.
$(a)$ Write a balanced chemical equation when this oxide is dissolved in water.
$(b)$ Draw the electron dot structure for the formation of this oxide.

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.

Write true or false for the statement: It was assumed by Newlands that only $63$ elements existed in nature and no more elements would be discovered in the future.

The electronic configuration of an element $X$ is $2, 8, 8, 2$. To which $(a)$ period and $(b)$ group of the modern periodic table does $X$ belong? State its valency. Justify your answer in each case.

Taking the example of an element of atomic number $16$, explain how the electronic configuration of the atom of an element relates to its position in the modern periodic table and how valency of an element is calculated on the basis of its atomic number.