In Mendeleev's Periodic Table,gaps were left for elements to be discovered later. Which of the following elements found a place in the periodic table later?

On moving from left to right in a period in the periodic table,the size of the atom.

The position of some elements $A, B, C, D, E, F$ and $G$ in the Modern Periodic Table is given as under:

Group $\to$	$16$	$17$	$18$
Period $1$			$A$
Period $2$	$B$	$C$	$D$
Period $3$	$E$	$F$	$G$

$(a)$ In which group are inert elements placed?
$(b)$ What type of ions would $B, C, E,$ and $F$ form?
$(c)$ Which element would have chemical properties similar to $C$?
$(d)$ How many shells would $A$ have?
$(e)$ What is the similarity between $A$ and $D$?
$(f)$ Identify the most abundant element in the earth's crust.

Given below are four elements with their atomic numbers:

Element	Atomic Number
$A$	$16$
$B$	$11$
$C$	$3$
$D$	$14$

$(a)$ Identify the elements which belong to the same group of the Modern Periodic Table.
$(b)$ Arrange the given elements in decreasing order of atomic size.
$(c)$ Write the formula of the oxide of $B$.
$(d)$ Which of the above elements is a metalloid?

State the reason for the following:
$(a)$ The elements of the same group have similar chemical properties.
$(b)$ The elements of the same period have different properties.

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.