Why are lithium (atomic number $3$) and potassium (atomic number $19$) placed in group $1$? What will be the atomic numbers of the first two elements in group $2$?

Name any two elements which were discovered later after Mendeleev's periodic table.

Four elements $P, Q, R$ and $S$ have atomic numbers $12, 13, 14$ and $15$ respectively.
Answer the following questions giving reasons:
$(i)$ What is the valency of $Q$?
$(ii)$ Classify these elements as metals and non-metals.
$(iii)$ Which of these elements will form the most basic oxide?

''Elements in Periodic Table show periodicity of properties.'' List any four such properties.

The element $X$ has the following electron configuration $2, 8, 8, 2$. It means that it belongs to:

Which of the following sets of elements is written in the order of their increasing metallic character?