State whether the following statement is true or false: $Na, Mg, Li, Al, Si, F, P$ and $Cl$ belong to the third period of the Modern periodic table.

$(a)$ State Modern Periodic Law.
$(b)$ Elements $A, B, C$ and $D$ have atomic numbers $1, 8, 11$ and $19$ respectively. Choose the odd element and give reason for your answer.

In the Modern Periodic Table,which are the metals among the first ten elements?

In the Modern Periodic Table,calcium (atomic number $20$) is surrounded by elements with atomic numbers $12$,$19$,$21$ and $38$. Which of these have physical and chemical properties resembling calcium?

An element $X$ belongs to the $3rd$ period and group $13$ of the Modern Periodic Table.
$(a)$ Determine the valence electrons and the valency of $X$.
$(b)$ Determine the molecular formula of the compound formed when $X$ reacts with an element $Y$ (atomic number $= 8$).
$(c)$ Write the name and formula of the compound formed when $X$ combines with chlorine.

$(i)$ State Modern Periodic Law.
$(ii)$ How many :
$(a)$ groups and
$(b)$ periods are present in the modern periodic table?
$(iii)$ State how the problem of placing:
$(a)$ hydrogen,
$(b)$ isotopes of an element has been solved in this periodic table.