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

Two elements $P$ and $Q$ belong to the same period of the modern periodic table and are in Group $1$ and Group $2$ respectively. Compare their following characteristics in tabular form:
$(a)$ The number of electrons in their atoms
$(b)$ The sizes of their atoms
$(c)$ Their metallic characters
$(d)$ Their tendencies to lose electrons
$(e)$ The formula of their oxides
$(f)$ The formula of their chlorides

Define Newlands' law of Octaves.

Which one of the following elements exhibits the maximum number of valence electrons?

Write true or false for the statement: In Mendeleev's periodic table,elements were arranged in the order of their increasing atomic masses and it was also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties.

$(a)$ Name the element with atomic number $17$.
$(b)$ To which period does it belong?
$(c)$ To which group does it belong?
$(d)$ Write its electronic configuration.