With the help of the following reactions,arrange the metals $A$,$B$,$D$,and $E$ in decreasing order of their reactivity:
$(I)$ $B + ANO_3 \longrightarrow BNO_3 + A$
$(II)$ $A + HCl \longrightarrow ACl + \frac{1}{2} H_2$
$(III)$ $D + ECl \longrightarrow DCl + E$
$(IV)$ $D + HNO_3 \longrightarrow$ $H_2$ gas is not evolved

Arrange the following oxides in the increasing order of their basic nature: $(a) Al_2O_3, (b) K_2O, (c) P_2O_5, (d) MgO$.

$Mg^{2+}$ is . . . . . . than $Al^{3+}$.

The first $(\Delta_{i} H_{1})$ and the second $(\Delta_{i} H_{2})$ ionization enthalpies (in $kJ \ mol^{-1}$) and the $(\Delta_{eg} H)$ electron gain enthalpy (in $kJ \ mol^{-1}$) of a few elements are given below:

Elements	$\Delta_{i} H_{1}$	$\Delta_{i} H_{2}$	$\Delta_{eg} H$
$I$	$520$	$7300$	$-60$
$II$	$419$	$3051$	$-48$
$III$	$1681$	$3374$	$-328$
$IV$	$1008$	$1846$	$-295$
$V$	$2372$	$5251$	$+48$
$VI$	$738$	$1451$	$-40$

Which of the above elements is likely to be:
$(a)$ the least reactive element.
$(b)$ the most reactive metal.
$(c)$ the most reactive non-metal.
$(d)$ the least reactive non-metal.
$(e)$ the metal which can form a stable binary halide of the formula $MX_{2}$ $(X = \text{halogen})$.
$(f)$ the metal which can form a predominantly stable covalent halide of the formula $MX$ $(X = \text{halogen})$?

As the atomic number increases in a group,the atomic density .........

How many elements in the periodic table are gases? Provide their positions in the periodic table.