The atomic numbers of three elements $A, B$,and $C$ are $12, 18$,and $20$ respectively. State,giving reason,which two elements will show similar properties.

Give an account of the process adopted by Mendeleev for the classification of elements. How did he arrive at Periodic Law?

The element with atomic number $14$ is hard and forms acidic oxide and a covalent halide. To which of the following categories does the element belong?

The electrons in the atoms of four elements $A, B, C$ and $D$ are distributed in three shells having $1, 3, 5$ and $7$ electrons in the outermost shell respectively. State the period in which these elements can be placed in the modern periodic table. Write the electronic configuration of the atoms of $A$ and $D$ and the molecular formula of the compound formed when $A$ and $D$ combine.

The element with atomic number $14$ is hard and forms acidic oxide and a covalent halide. To which of the following categories does the element belong?

Give two reasons to explain how the metallic character of elements varies: $(a)$ in a group,$(b)$ in a period.